Method for quantifying amyloid beta protofibril

Abstract

Disclosed herein are immunoassay methods of measuring amyloid β protofibril levels in biological samples and diagnostic and therapeutic uses thereof. Methods disclosed herein use a single molecule counting instrument for detection. Methods disclosed herein may detect amyloid β protofibril at femtomolar concentrations and selectively measure protofibril as compared to amyloid β monomers.

Description

[0001] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 METHOD FOR QUANTIFYING AMYLOID BETA PROTOFIBRIL RELATED APPLICATIONS The present application claims priority to U.S. Provisional Patent Application No. 63 / 660,323 filed June 14, 2024, the entire contents of which are hereby incorporated by reference for all purposes. SEQUENCE LISTING This application contains a Sequence Listing named 08061_6066-00000_SL.xml, which was created December 18, 2024, and is 31,432 bytes in size. The Sequence Listing has been filed electronically in XML format and is hereby incorporated by reference in its entirety. The present disclosure relates to methods of measuring amyloid β protofibril levels. Alzheimer’s disease (“AD”) is a progressive, neurodegenerative disorder of unknown etiology and the most common form of dementia among older people. In 2006, there were 26.6 million cases of AD in the world (range: 11.4-59.4 million) (Brookmeyer, R., et al., Forecasting the global burden of Alzheimer’s Disease. Alzheimer Dement.2007; 3:186-91), while there were more than 5 million people in the United States reportedly living with AD (Alzheimer’s Association, Alzheimer’s Association report, 2010 Alzheimer’s disease facts and figures. Alzheimer Dement.2010;6:158-94). By the year 2050, the worldwide prevalence of AD is predicted to grow to 106.8 million (range: 47.2-221.2 million), while in the United States alone the prevalence is estimated to be 11 to 16 million. (Brookmeyer, supra, and 2010 Alzheimer’s disease facts and figures, supra). Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 The disease generally involves a global decline of cognitive function that progresses slowly and leaves end-stage subjects bedridden. AD subjects typically survive for only 3 to 10 years after symptom onset, although extremes of 2 and 20 years are known. (Hebert, L.E., et al., Alzheimer disease in the U.S. population: prevalence estimates using the 2000 census. Arch Neurol.2003; 60:1119-1122.) AD is the seventh leading cause of all deaths in the United States and the fifth leading cause of death in Americans older than the age of 65 years, despite the fact that mortality due to AD is greatly underestimated because death certificates rarely attribute the cause of death to AD. (Alzheimer’s Association. Alzheimer’s Association report. 2010 Alzheimer’s disease facts and figures. Alzheimer Dement.2010; 6:158-94.) AD represents a significant economic burden across industrialized countries with a substantial impact on healthcare systems and the public purse as well as on subjects and their families. In the United States alone, total payments for 2010 were estimated at $172 billion, including $123 billion for Medicare and Medicaid. Histologically, the disease is characterized by neuritic plaques, found primarily in the association cortex, limbic system, and basal ganglia. The major constituent of these plaques is amyloid beta (Aβ) peptide. Aβ exists in various conformational states - monomers, oligomers, protofibrils, and insoluble fibrils. Some details of the mechanistic relationship between onset of Alzheimer’s disease and Aβ production are unknown. The “amyloid hypothesis” proposes that amyloid β (Aβ) peptides play a central role in the pathogenesis of AD. Specifically, it is hypothesized that neurodegeneration in AD may be caused by deposition of Aβ plaques in brain tissue due to an imbalance between Aβ production and Aβ clearance, leading to formation of neurotoxic neurofibrillary tangles. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Aβ peptides generally exist in a dynamic continuum of conformational states such that species tend to progress from monomeric Aβ, to soluble Aβ assemblies that include a range of low molecular weight oligomers to higher molecular weight protofibrils, and finally to insoluble fibrils (plaques). The protofibril form of Aβ is one of the largest soluble forms of Aβ and may play a leading role in AD. Development of therapies that selectively target Aβ protofibrils have led to the generation of humanized anti-Aβ protofibril antibodies, which are highly selective for the protofibril form of Aβ. As an example, Lecanemab, a recombinant humanized immunoglobulin gamma 1 (IgG1) monoclonal antibody directed against aggregated soluble and insoluble forms of amyloid beta, was recently approved for the treatment of Alzheimer’s disease. U.S. Patent No.8,025,878 (incorporated herein by reference in its entirety). Treatment with lecanemab may be initiated in patients with mild cognitive impairment or mild dementia stage of disease. Targeting soluble Aβ protofibrils, e.g., in these patients, may provide therapeutic benefit. A need remains, however, for measuring Aβ (particularly Aβ protofibrils) more accurately in patients. The detection and quantification of protofibrils in biological samples has been limited due to the need for a selective method of detection for the protofibril form of Aβ over other forms of Aβ (such as monomers) and the low concentration of protofibrils in biological samples. In some instances, the concentration of protofibrils is below the low limit of detection for an assay and may be in the femtomolar range. Provided herein is a method of quantifying Aβ protofibrils in biological samples. The assay may be sensitive to detect femtomolar concentrations of Aβ protofibrils. The assay may be selective for Aβ protofibrils compared to Aβ monomers. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, the presence of a therapeutic anti-Aβ protofibril antibody (e.g., lecanemab or an antibody comprising its CDRs or variable regions, or an antibody binding the same epitope as or competing for binding with lecanemab) in a biological sample surprisingly does not interfere with the detection of Aβ protofibrils in an Aβ protofibril assay using the same antibody (e.g. the lecanemab as the anti-Aβ protofibril detection antibody as well as for clinical treatment). SUMMARY In some aspects, the present disclosure provides a method of treating a subject having or suspected of having or at risk for developing Alzheimer’s disease (AD) or mild cognitive impairment (MCI) with an anti-Aβ protofibril antibody, comprising: obtaining a first biological sample from the subject prior to receiving the anti-Aβ protofibril antibody (i.e., a baseline sample); obtaining subsequent samples after treatment with the anti-Aβ protofibril antibody; measuring concentrations of Aβ protofibrils in the first and the subsequent samples; and continuing to administer the anti-Aβ protofibril antibody (e.g., lecanemab) if a change, e.g., an increase in Aβ protofibrils is detected between the first and subsequent samples; wherein the concentrations of Aβ protofibrils in the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample. In some embodiments, the present disclosure provides for a method of treating a subject having or suspected of having or at risk for developing AD or MCI with an anti-Aβ protofibril antibody comprising obtaining a first biological sample from the subject prior to receiving the anti-Aβ protofibril antibody (i.e., a baseline sample); obtaining a biological sample from the subject prior to receiving the anti-Aβ protofibril antibody; measuring a concentration of Aβ protofibrils in the sample; and administering the anti-Aβ protofibril antibody if there is an elevated Aβ protofibril level relative to a control; wherein the concentrations of Aβ protofibrils in the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample. In some embodiments, the present disclosure provides for a method monitoring a patient treated with an anti-Aβ protofibril antibody comprising obtaining a first biological sample from the subject prior to receiving the anti-Aβ protofibril antibody and a second biological sample from the subject after receiving the anti-Aβ protofibril antibody; and measuring concentrations of Aβ protofibrils in the samples; wherein the concentrations of Aβ protofibrils in Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and wherein a change, e.g., an increase, in the Aβ protofibril level in the second sample relative to the first sample indicates treatment efficacy. In some embodiments, the present disclosure provides for a method of diagnosing a subject with AD comprising obtaining a biological sample from the subject; and measuring a concentration of Aβ protofibrils in the sample; wherein the concentrations of Aβ protofibrils in the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 instrument; and quantifying Aβ protofibrils in the sample; and wherein an elevated Aβ protofibril level relative to a control indicates that the subject has AD. In some embodiments, the present disclosure provides for a method of determining an AD clinical stage of a subject comprising obtaining a biological sample from the subject; and measuring a concentration of Aβ protofibrils in the sample; wherein the concentrations of Aβ protofibrils in the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and wherein the Aβ protofibril level indicates the AD clinical stage of the subject. In some embodiments, the present disclosure provides for a method of determining an AD clinical stage of a subject comprising obtaining a biological sample from the subject; and measuring a concentration of Aβ protofibrils in the sample; wherein the concentrations of Aβ protofibrils in the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and wherein an elevated Aβ protofibril level relative to a control indicates that the subject is suitable for treatment with an anti-Aβ protofibril antibody. In some embodiments, the present disclosure provides for a method detecting progression of AD comprising obtaining a first biological sample from a subject and a second biological sample from the subject at a later period in time; and measuring concentrations of Aβ protofibrils in the samples; wherein the concentrations of Aβ protofibrils in the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and wherein a change, e.g., an increase, in the Aβ protofibril level in the second sample relative to the first sample indicates AD progression. In some embodiments, the change in Aβ protofibrils between the first and subsequent samples is used to determine the treatment dose of anti-Aβ protofibril antibody. In some embodiments, the subject is administered the same or increased dose of the anti-Aβ protofibril Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 antibody in a subsequent dose if an increase in Aβ protofibrils between the first and a subsequent sample is detected. In some embodiments, the subject is administered a maintenance dose of the anti-Aβ protofibril antibody if an increase in Aβ protofibrils between the first and a subsequent sample is detected. In some embodiments, the subsequent samples are obtained after 12-months of treatment with the anti-Aβ protofibril antibody. In some embodiments, the subsequent samples are obtained after 18-months of treatment with the anti-Aβ protofibril antibody. In some embodiments, the subsequent samples are obtained after 12-months and 18-months of treatment with the anti-Aβ protofibril antibody. In some embodiments, the rate of change in Aβ protofibril levels from baseline to 12 months is measured in a subject. In some embodiments, the rate of change in Aβ protofibril levels from baseline to 18 months is measured in a subject. In some embodiments, the methods disclosed herein further comprise comparing the Aβ protofibril level relative to a control, wherein the anti-Aβ protofibril antibody is continued to be administered if the Aβ protofibril level or the rate of change in the Aβ protofibril level is increased relative to the control. In some embodiments, the control is a subject with AD or MCI who has not been treated with an anti-Aβ protofibril antibody. In some embodiments, the control is a subject with AD or MCI who has been treated with a placebo. In some embodiments, the control comprises an Aβ protofibril measurement in a biological sample obtained from a subject without AD, an average Aβ protofibril measurement in biological samples obtained from a group of age-matched subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of young healthy subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of amyloid negative subjects with mild cognitive impairment, or an average Aβ protofibril measurement in biological samples obtained from a group of amyloid positive subjects without Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 MCI. In some embodiments, the control Aβ protofibril measurement is a baseline measurement in a baseline sample obtained from the subject 6-18 months prior to the biological sample. In some embodiments, the anti-Aβ protofibril capture antibody comprises a heavy chain variable region comprising SEQ ID NO: 7 and a light chain variable region comprising SEQ ID NO: 8. In some embodiments, the anti-Aβ protofibril capture antibody comprises a heavy chain region comprising SEQ ID NO: 9 and a light chain region comprising SEQ ID NO: 10. In some embodiments, the anti-Aβ protofibril capture antibody comprises a heavy chain constant region comprising SEQ ID NO: 11 and a light chain constant region comprising SEQ ID NO: 12. In some embodiments, the anti-Aβ protofibril capture antibody comprises lecanemab. In some embodiments, the method does not involve a SIMOA assay, e.g., a SIMOA assay as reported in Wilson DH, et al., “The SIMOA HD-1 Analyzer: A Novel Fully Automated Digital Immunoassay Analyzer with Single-Molecule Sensitivity and Multiplexing” J Lab Autom 2016, Vol.21(4) 533–547. In some embodiments, the present disclosure provides a method wherein contacting the biological sample with the capture antibody forms a first immune complex and contacting the first immune complex with the detection antibody forms a second immune complex. In some embodiments, the present disclosure provides a method wherein the label on the anti-Aβ detection antibody comprises a tag, wherein the tag comprises a protein tag, a fluorescent tag, a quantum dot tag, an aptamer tag, an oligonucleotide tag, a SULFO-TAG, or a biotin tag. In some embodiments, the present disclosure provides a method wherein anti-Aβ detection antibody binds a region of Aβ that does not overlap a region bound by lecanemab, and / or wherein the anti-Aβ detection antibody does not compete for binding to Aβ protofibrils Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 with lecanemab. In some embodiments, the anti-Aβ detection antibody binds to an N-terminus of an Aβ, e.g. Aβ1-5. In some embodiments, the anti-Aβ detection antibody comprises heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 13 (HCDR1), SEQ ID NO: 14 (HCDR2), and SEQ ID NO: 15 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO:16 (LCDR1), SEQ ID NO:17 (LCDR2), and SEQ ID NO: 18 LCDR3. In some embodiments, the anti-Aβ detection antibody comprises a heavy chain variable region comprising SEQ ID NO: 19 and a light chain variable region comprising SEQ ID NO: 20. In some embodiments, the anti-Aβ detection antibody comprises 3D6. In some embodiments, the present disclosure provides a method wherein the anti-Aβ detection antibody comprises lecanemab. In some embodiments, the present disclosure provides a method wherein the biological sample comprises a body fluid or tissue. In some embodiments, the present disclosure provides a method wherein the biological sample comprises cerebrospinal fluid, whole blood, plasma, serum, brain tissue homogenate, or brain tissue lysate. In some embodiments, the biological sample is a brain homogenate. In some embodiments, the biological sample is stored at -80°C prior to contacting the sample with the capture antibody. In some embodiments, the method does not comprise repetitive freeze-thaw cycles of the biological sample. In some embodiments, the method further comprises thawing the frozen biological sample and gently mixing and / or vortexing. In some embodiments, the biological sample is prepared and stored in low protein-binding tubes. In some embodiments, the present disclosure provides a method wherein the biological sample is at a concentration 0.1-10 mg of protein per mL of sample. In some embodiments, the biological sample is at a concentration of 1 mg of protein per mL of sample. In some Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 embodiments, the present disclosure provides a method wherein the biological sample is diluted 1-fold to 4-fold. In some embodiments, the biological sample is diluted 1-fold, 2-fold, 3-fold, or 4-fold. In some embodiments, the biological sample is diluted 2-fold. In some embodiments, the biological sample is diluted in a sample buffer. In some embodiments, the present disclosure provides a method wherein the biological sample is a CSF sample and optionally the CSF sample is not diluted. In some embodiments, the CSF sample is diluted. In some embodiments, the CSF sample is diluted 1-fold to 4-fold. In some embodiments, the CSF sample is diluted 1-fold, 2- fold, 3-fold, or 4-fold. In some embodiments, the CSF sample is diluted 2-fold. In some embodiments, the CSF sample is diluted in a sample buffer. In some embodiments, the present disclosure provides a method wherein the biological sample is a blood sample and optionally the blood sample is not diluted. In some embodiments, the blood sample is diluted. In some embodiments, the blood sample is diluted 1-fold to 4-fold. In some embodiments, the blood sample is diluted 1-fold, 2-fold, 3-fold, or 4-fold. In some embodiments, the blood sample is diluted 2-fold. In some embodiments, the blood sample is diluted in a sample buffer. In some embodiments, the sample buffer for diluting the biological sample comprises a buffering agent, NaCl (e.g.150mM NaCl), a blocking agent (e.g. bovine serum albumin), and a detergent (e.g. non-ionic detergent, e.g. Tween 20). In some embodiments, the sample buffer for diluting the biological sample comprises a Good’s buffer, NaCl (e.g.150mM NaCl), a blocking agent (e.g. bovine serum albumin), and a detergent (e.g. non-ionic detergent, e.g. Tween 20). In some embodiments, the sample buffer comprises a blocking agent (e.g. bovine serum albumin) and a detergent (e.g. non-ionic detergent, e.g. Tween 20) in buffered saline (e.g. phosphate- buffered saline (PBS), and tris-buffered saline (TBS)). In some embodiment, the sample buffer Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 comprises 0.5% BSA and 0.05% Tween 20 in PBS. In some embodiments, the sample buffer is a commercially available dilution buffer or blocking buffer for immunoassay. In some embodiments, the sample buffer is Discovery Standard Diluent (Cat#02-0560-00. EMD Millipore). In some embodiments, the present disclosure provides a method wherein the anti-Aβ protofibril capture antibody is immobilized on a surface. In some embodiments, the surface is a plate, a bead, or a particle, optionally a magnetic bead or particle. In some embodiments, the particle is a tosylactivated magnetic bead. In some embodiments, the present disclosure provides a method wherein the capture antibody is conjugated to the bead at a concentration of 10-40 µg of antibody per mg of bead. In some embodiments, the present disclosure provides a method wherein the capture antibody is conjugated to the bead at a concentration of 20 µg of antibody per mg of bead. In some embodiments, the capture antibody is conjugated to the bead at a concentration of 20 µg antibody per mg of tosylactivated magnetic bead. In some embodiments, the present disclosure provides a method wherein the tag is a fluorescent tag, and wherein the signal from the fluorescent tag is measured by the single molecule counting instrument. In some embodiments, the single molecule counting instrument is an SMCxPRO instrument. In some embodiments, the present disclosure provides a method further comprising washing the sample to remove unbound materials. In some embodiments, the present disclosure provides a method wherein the capture antibody is conjugated to the bead at a concentration of 10-40 µg antibody per mg of bead. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, the present disclosure provides a method, wherein the capture antibody is conjugated to the bead at a concentration of 20 µg antibody per mg of bead. In some embodiments, the present disclosure provides a method wherein the capture antibody is conjugated to the bead at a concentration of 20 µg antibody per mg of tosylactivated magnetic bead. In some embodiments, the present disclosure provides a method wherein the biological sample comprises Aβ protofibrils in an amount of at least about 0.01 picomolar, e.g., about 0.01 picomolar to about 2 picomolar. In some embodiments, the present disclosure provides a method wherein the method has a sensitivity for Aβ protofibrils of about 0.003 picomolar. In some embodiments, the present disclosure provides a method wherein the method has a lower limit of quantification of about 0.01 picomolar of Aβ protofibrils. In some embodiments, the present disclosure provides a method wherein the method provides a selectivity for an Aβ protofibril of 1,000,000-fold or greater than for an Aβ monomer. In some embodiments, the present disclosure provides a method wherein the quantity of Aβ protofibril is determined relative to a control. In some embodiments, the present disclosure provides a method wherein an Aβ protofibril standard curve is used to determine the Aβ protofibril quantity. In some embodiments, the present disclosure provides a method wherein the Aβ protofibril standard curve is prepared from Aβ1-42 by size exclusion chromatography. In some embodiments, the present disclosure provides a method wherein the Aβ protofibril standard curve is prepared in low protein-binding tubes and stored at -80C. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, the present disclosure provides a method wherein the method provides a S / N ratio greater than 100, e.g., at concentrations of Aβ protofibril of about 1 pM. In some embodiments, the protofibrils are measured in a sample from a human subject who has or is suspected of having Alzheimer’s disease (AD), preclinical AD, and / or mild cognitive impairment (MCI). In some embodiments, the human subject received treatment with lecanemab prior to or at the time of the biological sample collection. In some embodiments, the sample comprises a CSF or plasma sample. In some embodiments, the method further comprises measuring one or more additional plasma or CSF biomarkers and / or one or more cognitive measurements of AD or MCI. In some embodiments, the method further comprises administering the anti-Aβ protofibril antibody to the subject. In some embodiments, the anti-Aβ protofibril antibody is lecanemab. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several non-limiting embodiments of the invention and together with the description, serve to explain the principles of the disclosure. Fig.1. Detection of Aβ1-42 protofibril on SMCxPRO. Correlation between the concentration of Aβ1-42 protofibril and response measured by single molecule counting on an SMCxPRO instrument. Fig.2. Selectivity to PF, compared to monomer Aβ. Correlation between the response measurements on the SMCxPRO single molecule counting instrument and concentration of Aβ protofibril (closed circle), Aβ1-16 monomer (closed triangle) or Aβ1-40 monomer (open circle). Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Fig.3. Dilution linearity. Correlation between protofibril concentration and dilution factor of protofibril sample. Fig.4. BAN2401 interference. Concentration of protofibrils in samples with or without the addition of 500 ng / mL BAN2401. Fig.5. Measurement of brain homogenate. Protofibril concentrations in brain homogenates of patients with Alzheimer’s disease (AD; n=6 patients) compared to brain homogenates from controls (n=6 individuals). Figs.6A-B. CSF measurement. Fig.6(A) The concentration of Aβ protofibril in CSF samples (open circles) was determined relative to a standard curve (closed circles) of Aβ1-42 protofibril. Fig.6(B) The concentration of protofibril in CSF from patients with Alzheimer’s disease (AD; n=8) compared to controls (n=8). Fig.7. Evaluation of anti-Aβ detection antibodies by sandwich ELISA. Correlation between optical density (O.D.) at 450-650 nm relative to Aβ1-42 protofibril concentration for indicated detection antibodies in combination with BAN2401 capture antibody. Figs.8A-B. Comparison of antibody pairs BAN2401 / 3D6 and BAN2401 / BAN2401 on Meso Scale Discovery Electrochemiluminescence (MSD-ECL) platform. Fig.8(A) Correlation of the ECL signal (logarithmic scale) and PF standard. Fig.8(B) Correlation of ECL signal (linear scale) and PF standard. Fig.9. Comparison of antibody pairs, BAN2401 / 3D6 and BAN2401 / BAN2401 on SMCxPro. The S / N ratio detected with BAN2401 as the capture antibody and 3D6 as the detection antibody compared to BAN2401 as the capture antibody and BAN2401 as the detection antibody for the detection of 0.1 pM PF or 1 pM PF. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Figs.10A-B. Comparison of SMCxPRO to SIMOA for detection of PF with BAN2401 / 3D6. Fig.10(A) Correlation of the S / N ratio (logarithmic scale) to PF concentration on SMCxPRO compared to SIMOA. Fig.10(B) Correlation of the S / N ratio (linear scale) to PF concentration on SMCxPRO compared to SIMOA. Fig.11. Measurement of Amyloid β protofibril levels on tosylactivated magnetic beads in CSF samples relative to blank samples on SMCxPRO. Fig.12(A) shows measurement of 0.003 and 0.01 pM of PF standard versus blanks after coating tosylactivated beads with 40 µg, 20 µg, or 10 µg of BAN2401. Fig.12(B) shows measurement of PFs in CSF following coating of 1 mg of tosylactivated beads with 10 µg, 20 µg, or 40 µg of BAN2401. Figs.13A-B. Correlation of CSF PF and AD Biomarkers at baseline. Fig.13(A) shows the correlation of CSF PF with brain amyloid plaque at baseline. Fig.13(B) shows the correlation of CSF PF with neurogranin at baseline. Shading shows the 95% confidence interval. Figs.14A-B. shows the change in CSF PF in lecanemab or placebo treated subjects from 0-18 months. Fig.14(A) shows the CSF PF in lecanemab or placebo treated subjects at 0, 12, and 18 months. Fig.14(B) shows the CSF PF in subjects treated with lecanemab that remained amyloid positive (> / =30 centiloids at 18 months, “LEC A+”) subjects treated with lecanemab that achieved amyloid negativity (<30 centiloids at 18 months, “LEC A-”) and subjects that received placebo. DETAILED DESCRIPTION Definitions In order to better understand the disclosure, certain definitions are provided first. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 As used herein, the singular terms “a,” “an,” and “the” include the plural reference unless the context clearly indicates otherwise. The phrase “and / or,” as used herein, means “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Thus, as a non-limiting example, “A and / or B,” when used in conjunction with open- ended language such as “comprising” can refer, in some embodiments, to A only (optionally including elements other than B); in other embodiments, to B only (optionally including elements other than A); in yet other embodiments, to both A and B (optionally including other elements); etc. As used herein, “at least one” means one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and / or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 When a number is recited, either alone or as part of a numerical range, it should be understood that the numerical value can vary above and below the stated value by a variance of 10% of the stated value. When a range of values is listed herein, it is intended to encompass each value and sub-range within that range. For example, “2.5 mg / kg to 10 mg / kg” is intended to encompass, for example, 2.5 mg / kg, 3 mg / kg, 3.5 mg / kg, 4 mg / kg, 4.5 mg / kg, 5 mg / kg, 5.5 mg / kg, 6 mg / kg, 6.5 mg / kg, 7 mg / kg, 7.5 mg / kg, 8 mg / kg, 8.5 mg / kg, 9 mg / kg, 9.5 mg / kg, 10 mg / kg, 2.5 mg / kg to 3 mg / kg, 2.5 mg / kg to 4.5 mg / kg, 3 mg / kg to 4.5 mg / kg, 4.5 mg / kg to 8 mg / kg, 2.5 mg / kg to 9 mg / kg, and so forth. As used herein, the term “affinity” refers to the strength of the sum total of noncovalent interactions between a single binding site of a molecule (e.g., an antibody) and its binding partner (e.g., an antigen). The affinity of a molecule X for its partner Y can generally be represented by the equilibrium dissociation constant (KD) (or its inverse equilibrium association constant, KA). Affinity can be measured by common methods known in the art. See, for example, Pope M.E., Soste M.V., Eyford B.A., Anderson N.L., Pearson T.W., (2009) J. Immunol. Methods.341(1-2):86-96. As used herein, “competitive binding” refers to the relationship between two molecules which bind to the same site of a third molecule, for example two antibodies that bind to the same epitope of a protein. In such an instance, a first antibody or antigen-binding fragment may bind to an epitope of a protein (e.g., an epitope on an Aβ protofibril) in a manner sufficiently similar to the binding of a second antibody or antigen-binding fragment, such that the binding of the first antibody or antigen-binding fragment with its epitope is detectably decreased in the presence of the second antibody or antigen-binding fragment compared to the binding of the first antibody or antigen-binding fragment in the absence of the second antibody or antigen-binding fragment. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 The alternative, where the binding of the second antibody to its epitope is also detectably decreased in the presence of the first antibody, can, but need not be the case. That is, a first antibody can inhibit the binding of a second antibody to its epitope without that second antibody inhibiting the binding of the first antibody to its respective epitope. “Competitive binding” may be determined in an assay in which the immunoglobulin / antibody / binding fragment being evaluated inhibits specific binding of a reference antibody to a common antigen, such as Amyloid β (e.g., an Aβ sequence of Table 9), e.g., as measured by competitive radioimmunoassay (RIA), enzyme immunoassay (EIA), or a sandwich ELISA assay. In preferred embodiments, a sandwich ELISA is used. In some embodiments, an assay involves the use of purified antigen bound to a solid surface or expressed on a cell surface, an unlabeled test immunoglobulin and a labeled reference immunoglobulin. Competitive inhibition may be measured by determining the amount of label bound to the solid surface or cells in the presence of the test immunoglobulin. In some embodiments, the test immunoglobulin is present in excess. In some embodiments, when a competing antibody is present in excess, it will inhibit specific binding of a reference antibody (e.g. radiolabeled antibody) to a common antigen by a detectable amount, e.g., at least 50% or more. As used herein “specifically binds” or “specific binding” in reference to an antibody means that the antibody binds to its target antigen or epitope with greater affinity than it does to structurally different antigen(s) or epitope(s). Specific binding refers, in some embodiments, to binding that is measurably different from a non-specific interaction. Specific binding can be measured, for example, by determining binding of a molecule compared to binding of a control molecule, which generally is a molecule of similar structure that does not have binding activity, or by competition assay with a control molecule that shares similar binding affinity but is Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 unlabeled. In this case, specific binding is indicated if the binding of the labeled target to a probe is competitively inhibited by excess unlabeled target. Binding kinetic properties may also be measured using surface plasmon resonance (SPR). For example, a molecule used in an assay disclosed herein may have a KDfor the target of at least about 10−6M, alternatively at least about 10−7M, alternatively at least about 10−8M, alternatively at least about 10−9M, alternatively at least about 10−10M, alternatively at least about 10−11M, alternatively at least about 10−12M, alternatively at least about 10−13M, alternatively at least about 10−14M, or greater. As used herein, “Amyloid β peptide(s)” may be used interchangeably with “Aβ peptide(s)”, “Amyloid β monomer(s)” and “Aβ monomer(s)”, and refers to polypeptides, whether folded into a three-dimensional protein, unfolded and / or misfolded, resulting from the cleavage of the Amyloid Precursor Protein at various cleavage sites. Amyloid β 1-42 (Aβ1-42, Aβ(1-42), or Aβ1-42) refers to a 42 amino acid amyloid β monomer (Table 9, SEQ ID NO: 25). Amyloid β 1-40 (Aβ1-40 or Aβ1-40) refers to a 40 amino acid amyloid β monomer (Table 9, SEQ ID NO: 26). In some embodiments, an amyloid β monomer may be present intracellularly. In some embodiments, an amyloid β monomer may be extracellular, for example in the intracellular space or blood. In some embodiments, an amyloid β monomer may exist in biological samples, for example in the blood or CSF. As used herein, “Amyloid Precursor Protein (APP)” is a transmembrane protein comprising a 770 amino acid polypeptide (Table 9, SEQ ID NO: 29) with an extracellular domain, which may be cleaved by proteases to generate an Aβ peptide and / or protein. In some embodiments, the Aβ peptide comprises amino acids 1-42 (Table 9, SEQ ID NO: 25). In some embodiments, the Aβ peptide comprises amino acids 1-40 (Table 9, SEQ ID NO: 26). Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 As used herein, “the Aβ aggregation process” refers to the dynamic continuum of Aβ three-dimensional conformational states such that species of Aβ tend to progress from monomeric Aβ, to soluble Aβ assemblies that include a range of low molecular weight oligomers to higher molecular weight protofibrils, and finally to insoluble fibrils and plaques. In some embodiments, the Aβ aggregates may take organized 3D conformational states (such as folded soluble oligomers or Aβ protofibrils), which may be associated with pathological processes. In some embodiments, during the Aβ aggregation process the Aβ polypeptide molecules may bind together in a disorganized state associated with misfolding or abnormal folding of Aβ polypeptides producing Aβ aggregates. In some embodiments, the Aβ aggregates may be the result of misfolding and exposure of hydrophobic residues. In some embodiments, the Aβ aggregates may undergo a nucleation step during which aggregation proceeds slowly until the Aβ aggregate reaches a size or number of Aβ monomers so that the addition of new Aβ monomers to the aggregate occurs more rapidly. As used herein, “Amyloid β oligomer” is used interchangeably with “oligomer” or “Aβ oligomer”. As used herein amyloid β oligomer refers to a molecule comprising two or more Aβ monomers. In some embodiments, an amyloid β oligomer may be intracellular. In some embodiments, an amyloid β oligomer may be extracellular, for example in the interstitium also known as the intracellular space. In some embodiments, an amyloid β oligomer may exist in biological samples, for example in the blood or CSF. As used herein, “Amyloid β protofibrils,” “Aβ protofibrils,” “Aβ protofibril,” “protofibril(s)” and “PF” are used interchangeably. Aβ protofibril refers to soluble Aβ aggregate species. In some embodiments, molecular weight of Aβ protofibril may be ≥75kDa, ≥80kDa, ≥100kDa or 75-5000 kDa. In some embodiments, protofibrils may be curved linear structures Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 with a diameter of about 5 nm and a length of up to 200 nm (Englund et al. “Sensitive ELISA detection of amyloid-B protofibrils in biological samples” J of Neurochemistry 2007; 103, 334- 345). In some embodiments, PF have a diameter of about 3 nm and range in length from 60 nm to 220 nm. (Dubnovitsky et al. “Amyloid B Protofibrils: Size Morphology and Synaptotoxicity of an Engineered Mimic" PLoS one 2013; 8(7): e66101) As used herein, “Amyloid β fibril” or “Aβ fibril” refers to an insoluble aggregation of Aβ molecules and is the major structural component of Amyloid β plaques. As used herein, “Amyloid β plaque” refers to a deposit of insoluble aggregated Aβ protein molecules. An Amyloid β plaque is comprised in large part of Amyloid β fibrils. The term “antibody” refers to an immunoglobulin, whether genetically engineered, natural, or wholly or partially synthetically or recombinantly produced. Intact antibodies typically comprise a heavy chain and a light chain, each comprised of a variable region forming the binding pocket for an antigen and a constant region that contributes to effector function. The antibody, by virtue of its chosen heavy chain, can be a member of any immunoglobulin class and subclass, including any of the human classes: IgG, IgM, IgA, IgD, and IgE, or a derivative or fragment thereof. Likewise, the light chain of the antibody may derive from any species, such as a human kappa (κ) or lambda (λ) light chain, determined based on the amino acid sequences of the constant region. The basic antibody structural unit typically comprises a tetramer. In various embodiments, the tetramer comprises two identical pairs of polypeptide chains, each pair having one “light” (about 25 kDa) and one “heavy” chain (about 50-70 kDa). The amino-terminal (also referred to as the N-terminus) portion of each chain includes a variable region of about 100 to 110 or more amino acids primarily responsible for antigen recognition. The carboxy-terminal Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 (also referred to as the C-terminus) portion of each chain defines a constant region primarily responsible for effector function. Human light chains are classified as kappa and lambda light chains. Heavy chains are classified as mu, delta, gamma, alpha, or epsilon, and define the antibody’s isotype as IgM, IgD, IgA, and IgE, respectively. Within light and heavy chains, the variable and constant regions are joined by a “J” region of about 12 or more amino acids, with the heavy chain also including a “D” region of about 10 or more amino acids. See generally, Fundamental Immunology Ch 7. (Paul, W., 2nded. Raven Press, N.Y. (1989)) (incorporated by reference in its entirety for all purposes). The variable regions of each light / heavy chain pair form the antibody binding site. Thus, an intact antibody typically has two binding sites. Except in bifunctional or bispecific antibodies, the two binding sites are the same. The chains all exhibit the same general structure of relatively conserved framework regions (FR) joined by three hypervariable regions, also called complementarity determining regions or CDRs. The CDRs from the two chains of each pair are aligned by the framework regions, enabling binding to a specific epitope. From N-terminal to C-terminal, both light and heavy chains comprise the domains FR1, CDR1, FR2, CDR2, FR3, CDR3, and FR4. The assignment of amino acids to each domain may be done in accordance with the IMGT numbering system. Alternative definitions are also known to know of ordinary skill in the art. See, e.g., Kabat Sequences of Proteins of Immunological Interest (National Institutes of Health, Bethesda, Md. (1987 and 1991)), or Clothia & Lesk J. Mol. Biol.196:901-917 (1987); Clothia et al. Nature 342:878-883 (1989). An “antibody fragment” or “antigen binding fragment” comprises a portion of a full- length antibody, generally at least the antigen binding portion / domain or the variable region thereof and preferably one retaining some or all of the antigen binding properties of the full- length antibody. The term antibody fragment is a subset of the term antibody discussed above. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Examples of antibody fragments or antigen binding fragments include: Fab, Fab’, F(ab’)2, Fd, scFv, (scFv)2, scFv-Fc, Fv fragment, diabodies, single-chain antibody molecules, immunotoxins, and multi-specific antibodies formed from antibody fragments. In addition, antibody fragments may comprise single chain polypeptides having the characteristics of a VH chain capable of being able to assemble together with a VL chain to form a functional antigen binding pocket and thereby providing the property of binding to Aβ protofibril. The terms also comprise fragments that per se are not able to provide effector functions (e.g., Antibody-dependent cell-mediate cytotoxicity (“ADCC”) or complement dependent cytotoxicity (“CDC”) but provide this function after being combined with the appropriate antibody constant region(s). The term “identity” or “sequence identity” refers to the homology between at least two sequences (e.g., amino acid sequences), and may be quantified as a percentage. The percent identity between two sequences may be assessed using a mathematical algorithm. Suitable algorithms are known in the art. For example, the percent identity between two amino acid sequences can be determined using methods such as those described in Computational Molecular Biology, Lesk, A. M., ed., Oxford University Press, New York, 1988 and Sequence Analysis in Molecular Biology, von Heinje, G., Academic Press, 1987; each of which is incorporated herein by reference in its entirety. Techniques for determining identity are codified in publicly available computer programs. Computer software to determine homology between two sequences include, but are not limited to, BLASTP, BLASTN, and FASTA Altschul, S. F. et al., J. Molecular Biol., 215, 403 (1990)). Preferably, BLAST alignment may be used unless specified otherwise. The term “chimeric antibody” refers to a monoclonal antibody comprising all or part of a variable region from one source or species and at least a portion of a constant region derived from a different source or species, usually prepared by recombinant DNA techniques. In some Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 embodiments, chimeric antibodies comprise a murine variable region and a human constant region. Such murine / human chimeric antibodies may be produced by expressing immunoglobulin genes comprising DNA segments encoding murine immunoglobulin variable regions and DNA segments encoding human immunoglobulin constant regions. Other forms of “chimeric antibodies” may be those in which the class or subclass has been modified or changed from that of the original antibody. Such “chimeric” antibodies are also referred to as “class- switched antibodies.” Methods for producing chimeric antibodies involve conventional recombinant DNA and gene transfection techniques now known in the art. See, e.g., Morrison, S. L., et al., Proc. Natl. Acad Sci. USA 81 (1984) 6851-6855; U.S. Pat. Nos.5,202,238 and 5,204,244. The term “conjugated,” as used herein, refers to a bond or chemical moiety formed from a chemical reaction between a functional group of a first molecule (e.g., an antibody) with a functional group of a second molecule (e.g., a detectable signor or therapeutic agent or drug). Such bonds include, but are not limited to, covalent linkages and non-covalent linkages, while such chemical moieties include, but are not limited to, esters, carbonates, imines phosphate esters, hydrazones, acetals, orthoesters, peptide linkages, and oligonucleotide linkages. As used herein, the term “immune complex” refers to an antibody or other antigen binding fragment bound with its target antigen. In some embodiments, an immune complex comprises the target with the capture antibody and / or detection antibody. In some embodiments, the target bound to the capture antibody may be called the “first immune complex” and / or “immune complex I”. In some embodiments, an immune complex comprises the target bound to both the capture antibody and the detection antibody. In some embodiments, the target bound to Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 both the capture and detection antibody may be called the “second immune complex” and / or “immune complex II”. The term “epitope” refers to a site on an antigen to which an immunoglobulin or antibody (or antigen binding fragment thereof) can specifically bind. Thus, an epitope on Aβ is the site on Aβ molecule where an immunoglobulin or antibody (or antigen binding fragment thereof) can specifically bind. An epitope, as the term is used herein, can be formed from either contiguous amino acids or noncontiguous amino acids juxtaposed by tertiary folding of a protein. An epitope may include additional stretches of amino acids anywhere in the antigen, e.g., 1, 2, 3, 4, 5, 10, 15, 20, or more amino acids on the N-terminus and / or C-terminus of a peptide, or at a region in the center of the peptide. When a polypeptide is considered in a linearized sequence, the sequence may be divided into sections. Amino acids are numbered within a protein beginning at the N-terminus identified by a terminal free amino group and proceeds to the C-terminus identified by a terminal free carboxylic acid group. An epitope may be located in a region closer to the N-terminus (e.g. N-terminal epitope) or the C-terminus (e.g. C-terminal epitope), or may be localized somewhere in the middle. Epitopes may be linear or conformational, i.e., the epitope bound by an antibody may form from the three-dimensional folding of an antigen and comprise more than one discrete section of the linear peptide. Epitopes formed from contiguous amino acids are typically retained on exposure to denaturing solvents whereas epitopes formed by tertiary folding are typically lost on treatment with denaturing solvents. An epitope may include at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 amino acids, often in a unique spatial conformation. Methods of determining spatial conformation of epitopes include, for example, alanine scanning, x-ray crystallography, and 2-dimensional nuclear magnetic resonance. See, Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 e.g., Epitope Mapping Protocols in Methods in Molecular Biology, Vol.66, G. E. Morris, Ed. (1996). Exemplary methods are discussed and used herein for the disclosed antibodies. An epitope on Aβ, as discussed herein, is identified with reference to amino acid residues of a reference Aβ protein. For example, the epitope of 3D6 is Aβ1-5(SEQ ID NO.: 28). One of skill in the art could readily identify corresponding amino acid residues on other Aβ isoforms and fragments, e.g., via alignment programs such as BLAST®. Unless otherwise indicated, a reference to a particular amino acid residue on Aβ and should be understood to encompass the corresponding positions on other Aβ isoforms and fragments. As used herein, the term “analyte” may be used to describe the molecule quantified by an assay. The terms “analyte” and “target” may be used interchangeably. In some embodiments, an analyte is a polypeptide or protein. In some embodiments, the analyte may be a biomarker. In some embodiments, the analyte levels may be determined as an exact concentration. In some embodiments, a concentration may be determined using linear regression from a standard curve. In some embodiments, the analyte levels may be determined as a relative concentration compared to other samples. In some embodiments, the analyte may be Aβ protofibrils. As used herein, a “biological sample” is any portion of tissue or fluid obtained or isolated from a living or once living being. In some embodiments, the biological sample is collected from a living or deceased human being. In some embodiments, biological samples include whole blood or blood-derived fluid, such as serum or plasma. In some embodiments, biological samples include cerebrospinal fluid (CSF) or brain tissue. In some embodiments, the biological sample is handled to protect the integrity of the sample and prevent degradation of the sample associated with removal of the sample from the living or deceased being and / or Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 processing of the sample following isolation (e.g., avoid freeze thaw cycles). In some embodiments, the biological sample may contain analyte(s) and / or biomarker(s). As used herein, the term “blood sample” or “blood” refers to a sample of blood, including serum and / or blood plasma from a human subject. In some embodiments, the blood sample is serum from a human subject. In some embodiments, the blood sample is plasma from a human subject. In some embodiments, subjects are required to fast if possible before collection of the blood. In other embodiments and / or at other time points, subjects do not require fasting. As used herein, a “control sample” which may be used interchangeably with “control” refers to a biological sample that is distinct from samples being evaluated. In some embodiments, a control sample refers to a biological sample that is distinct from samples being evaluated following an intervention or treatment. In some embodiments, a control sample may be obtained from a healthy subject. In some embodiments, a healthy subject may be a cognitively unimpaired subject or a cognitively normal subject. In some embodiments, a control sample may be obtained from a healthy age and / or gender matched subject. In some embodiments, a control sample may be obtained from a subject who does not have a diagnosis of Alzheimer’s disease. In some embodiments, a control may be obtained from a subject with co-morbidities that are not associated with AD. In some embodiments, a control sample may be obtained from a subject with a diagnosis along the spectrum of AD disease including, for example, preclinical AD or mild cognitive impairment. In some embodiments, a control sample may be a baseline sample collected from a subject prior to initiation of any treatment. In some embodiments, a control sample may be a baseline sample collected from a subject prior to initiation of anti-amyloid (e.g. anti- Aβ) treatment. In some embodiments, a control sample may be a sample collected from a subject after initiation of a treatment. In some embodiments, a control sample may be a post Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 treatment sample collected from a subject e.g.3 months, 6 months, 9 months, 12 months, 15 months, 18 months, 24 months, 30 months, 36 months after initiation of treatment. In some embodiments, a control sample may be a post-treatment sample collected from a subject who showed efficacy according to at least one efficacy measurement or who did not show efficacy. Such efficacy may be cognitive improvement, slowed cognitive decline, maintained cognitive function, or less accumulation or removal of brain amyloid, e.g. brain amyloid plaque, e.g., as determined using any of the cognitive scores disclosed herein, or by methods such as brain imaging. In some embodiments, a control sample may be a post treatment sample collected from a subject who is amyloid negative, amyloid positive, or has a specified level of brain amyloid, e.g. as determined by amyloid PET or blood-based biomarker(s) reflecting brain amyloid pathology. The anti-Aβ treatment may be by an anti Aβ antibody such as an anti-Aβ protofibril antibody, e.g. lecanemab. In some embodiments, a control may include a sample or multiple samples from a group of subjects. As used herein, the term “elevated Aβ protofibril levels” refers to the level of Aβ protofibril in a sample that is increased relative to a control, e.g. a sample from a healthy subject or “control sample” as defined above. In some embodiments, the term “elevated Aβ protofibril levels” refers to the level of Aβ protofibril in a sample that is increased relative to the average level of Aβ protofibril detected in samples from two or more healthy subjects. As used herein, the term “sandwich immunoassay” refers to a method of determining the concentration of an analyte comprising a capture antibody, e.g., one immobilized on a surface and a detection antibody conjugated to a tag that can be detected by an instrument. More particularly, a sandwich immunoassay may use two antibodies, which may bind to different sites on an analyte. The capture antibody, which is highly specific for the analyte, is typically attached Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 to a solid surface. The analyte is then added, followed by the addition of a second antibody referred to as the detection antibody. The detection antibody may bind to the analyte at different site than the capture antibody. As a result, the antigen is “sandwiched” between the two antibodies. In some embodiments, the concentration of the analyte may be determined relative to a standard curve. In some embodiments, the concentration of the analyte may be determined relative to other samples. In some embodiments, the concentration of the analyte may be determined relative to samples designated “control(s)”. In some embodiments, a sandwich immunoassay may comprise an enzyme-linked immunosorbent assay (ELISA). In some embodiments, a sandwich immunoassay may comprise measuring the response from the detection antibody by single molecule counting. As used herein, the term “level(s)” may be used interchangeably with “value(s)” and “concentration(s)” and refers to the amount of an analyte or biomarker. In some embodiments, the level is an absolute concentration determined using linear regression and interpolating experimental values from a standard curve. In some embodiments, the level is a relative amount of something compared to other samples (e.g. a fold change relative to control samples). In some embodiments, the level may be a numerical value obtained as an output from an instrument capable of detecting fluorescence. As used herein, the term “response” used in a context of measurement, detection or quantification refers to the numerical output of single molecule counting instrument during the quantification of the signal from the detection antibody. In some embodiments, the response is a measure of the signal, for example a fluorescent signal, from the tag, for example a fluorescent tag, on the detection antibody. In some embodiments, there is a positive correlation between the response and the levels of signal from the detection antibody. In some embodiments, there is a Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 positive correlation between the response and the levels of PF. In some embodiments, the response indicates the relationship between the signal of the detection antibody and the level of the analyte. In some embodiments, the response is a measure of the signal, for example a fluorescent signal from the tag on the detection antibody. The response may be generated mathematically in part through the integration of the intensity of a fluorescence signal into a numerical measurement or value. In some embodiments, the level of an analyte (e.g. Aβ PF) may be reported as a numerical value “response” generated in part through the integration of the intensity of a fluorescence signal by a digital ELISA instrument. In some embodiments, the level may be a “response” generated in part through the integration of the intensity of a fluorescence signal by a single molecule counting instrument (e.g. SMCxPRO). As used herein, the term “single molecule counting’ is a method of quantifying individual molecules in a sample. In some embodiments, individual molecules are counted digitally. In some embodiments, single molecule counting quantifies a signal from a tag on a detection antibody. In some embodiments, single molecule counting quantifies a fluorescent signal from a tag on a detection antibody. In some embodiments, the readout from single molecule counting is a response corresponding to a number. In some embodiments, the response is positively correlated with the analyte concentration such that the higher the number associated with a response the higher the concentration of the analyte. In some embodiments, the instrument used for single molecule counting is an SMCxPRO. In some preferred embodiments, protofibril concentrations are determined by single molecule counting on an SMCxPRO. As used herein, the term “lower limit of quantification (LLoQ)” may refer to the lowest concentration accurately measured by an assay. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 As used herein, the term “upper limit of quantification (ULoQ)” may refer to the highest concentration accurately measured by an assay. As used herein, the term “selectivity” maybe used interchangeably with “analytical selectivity” and refers to the ability of an assay to differentiate and quantify the target compared to other components in a sample. For example, a typical biological sample comprising cerebrospinal fluid may include different forms of amyloid β peptide, tau protein, or forms of phosphorylated tau protein. Selectivity is a measure of the ability of the bioanalytical method described herein to differentiate these and other components that may be present in the biological sample from the target biomarker of the method. Interference attributable to an extraneous biomarker can be investigated by spiking the sample with the extraneous biomarker and noting its effect on measurement. See, e.g., Examples 4 and 5 herein. In some embodiments, the selectivity of the protofibril assay may be determined using a standard curve of Aβ1-42 PF compared to standard curves of Aβ1-40and Aβ1-16monomers. As used herein, the term “sensitivity” maybe used interchangeably with “analytical sensitivity” and refers to the smallest amount of substance in a sample that can be accurately measured. In some embodiments, the sensitivity of an assay may be determined by the thresholds set by the LLoQ. In some embodiments, the sensitivity of an assay may be determined by the thresholds set by the LLoQ and the ULoQ. As used herein, the term “signal-to-noise ratio” is used interchangeably with “S / N ratio”, “S / N”, or “SNR”. The S / N ratio is a measure that compares the desired signal level to the level of background noise, i.e., signal generated non-specifically. As used herein, the term “cognitive test,” includes digital, computerized, and / or conventional (e.g., pen and paper) methods to detect early cognitive changes that may signal Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 mild cognitive impairment and / or a risk for developing dementia. These may be used to identify or monitor the treatment of a subject herein, e.g., in combination with the protofibril assay discussed below. Such tests, for example, may screen for cognitive impairment, and potentially identify individuals with MCI. Tests may use artificial intelligence to analyze cognitive test results to determine whether a case of mild cognitive impairment will escalate into Alzheimer’s within a year. In some embodiments, cognitive tests may include: Mini-Mental State Examination (MMSE), Alzheimer’s Disease Assessment Scale-Cognitive (ADAS-Cog), clinical dementia rating - sum of boxes (CDR-SB), Alzheimer’s Disease Composite Score (ADCOMS), Alzheimer's Disease Cooperative Study-Activities of Daily Living Scale for Mild Cognitive Impairment (ADCS MCI-ADL), “ARIA” refers to amyloid-related imaging abnormality (ARIA), amyloid related imaging abnormality edema / effusion (ARIA-E), amyloid related imaging abnormality hemorrhage (ARIA-H). As used herein, “MMSE” refers to the Mini-Mental State Examination, a cognitive instrument commonly used for screening purposes, but also often measured longitudinally in AD clinical trials having a 30 point scale with higher scores indicating less impairment and lower scores indicating more impairment, ranging from 0 (most impaired) to 30 (no impairment). In some embodiments, seven items measuring orientation to time and place, registration, recall, attention, language, and drawing may be assessed as part of the MMSE score. (Folstein, M.F. et al., “Mini-mental state. A practical method for grading the cognitive state of patients for the clinician.” J. Psychiatr. Res.1975;12:189-98.) As used herein, “ADAS-Cog” refers to Alzheimer’s Disease Assessment Scale- Cognitive. The ADAS-Cog is a widely used cognitive scale in Alzheimer's disease trials having a structured scale that evaluates memory (word recall, delayed word recall, and word Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 recognition), reasoning (following commands), language (naming, comprehension), orientation, ideational praxis (placing letter in envelope) and constructional praxis (copying geometric designs). (Rosen, W.G. et al., “A new rating scale for Alzheimer’s disease.” Am. J. Psychiatry 1984; 141:1356-64.) Ratings of spoken language, language comprehension, word finding difficulty, ability to remember test instructions, maze, and number cancellation may also be obtained. In some embodiments, ADAS-Cog refers to the use of the Alzheimer Disease Assessment Scale-Cognitive Subscale14 (ADAS-Cog14). In some embodiments, a modified version may be used herein and is scored from 0 to 90 points with a score of 0 indicating no impairment, and a score of 90 indicating maximum impairment. In some embodiments, the ADAS–Cog14 tasks include memory (word recall, delayed word recall, and word recognition), reasoning (following commands), language (naming, comprehension), orientation, ideational praxis (placing letter in envelope), constructional praxis (copying geometric designs), spoken language, language comprehension, word finding difficulty, ability to remember test instructions, maze, and number cancellation (Rosen et al, 1984). As used herein, “CDR-SB” refers to clinical dementia rating - sum of boxes. The CDR is a clinical scale that describes 5 degrees of impairment in performance on each of 6 categories of function including memory, orientation, judgment and problem solving, community affairs, home and hobbies, and personal care. (Berg, L. et al., “Mild senile dementia of the Alzheimer type: 2. Longitudinal assessment.” Ann. Neurol.1988; 23:477-84.) The ratings of degree of impairment obtained on each of the 6 categories of function are synthesized into 1 global rating of dementia CDR score (ranging from 0 to 3). A sum of boxes score provides an additional measure of change where each category has a maximum possible score of 3 points and the total score is a sum of the category scores giving a total possible score of 0 to 18 with higher scores Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 indicating more impairment. The global score may be used as a clinical measure of severity of dementia. As used herein, “ADCOMS” refers to Alzheimer’s Disease Composite Score, a composite clinical score based on an analysis of four ADAS-Cog items (delayed word recall, orientation, word recognition, and word finding difficulty), two Mini Mental State Examination (MMSE) items (orientation to time, and drawing), and all six CDR-SB items (personal care, community affairs, home and hobbies, memory, orientation, and judgment and problem solving), as discussed in the Examples and in Wang, J. et al., “ADCOMS: a composite clinical outcome for prodromal Alzheimer’s disease trials.” J. Neurol. Neurosurg. Psychiatry. 2016; 87:993-999. ADCOMS was developed to be particularly sensitive to disease progression during early stages of AD (i.e., preclinical AD or early AD). In some embodiments, ADCOMS can be calculated using the following formula: 12 7 6 ^^^^ =where Ai(t), Bi (t) and from ADAS-cog, reversed MMSE scores, and CDR-SB, respectively (Wang, J. et al., “ADCOMS: a composite clinical outcome for prodromal Alzheimer’s disease trials). ADCOMS is particularly sensitive to disease progression during early stages of AD, i.e., prodromal and mild AD. As used herein, “ADCS MCI-ADL” refers to the Alzheimer's Disease Cooperative Study-Activities of Daily Living Scale for Mild Cognitive Impairment (ADCS MCI-ADL). The ADCS MCI-ADL is a clinical scale that assess the competence level of a patient at six basic activities of daily living. Additional examples are discussed in Kreutzer J.S., DeLuca J., Caplan B. (eds) Encyclopedia of Clinical Neuropsychology. Springer, New York, NY. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 As used herein, “modified iADRS” or “iADRS” refers to a composite tool that combines scores from the ADAS Cog14 (all items) and the ADCS MCI-ADL (all items). The modified iADRS score can be used to evaluate disease progression: Modified iADRS score = [-1(ADAS-cog14) +90] + ADCS MCI-ADL. In some embodiments, the subject has “elevated amyloid plaque” which can also be referred to as “elevated amyloid” or “intermediate amyloid plaque.” As one of ordinary skill in the art will recognize, amyloid levels from amyloid PET can be reported using the Centiloid method in “centiloid” units (CL). (Klunk WE et al. The Centiloid Project: standardizing quantitative amyloid plaque estimation by PET. Alzheimer’s Dement.2015; 11:1–15 e1–4). The Centiloid method measures a tracer on a scale of 0 CL to 100 CL, where 0 is deemed the anchor- point and represents the mean in young healthy controls and 100 CL represents the mean amyloid burden present in subjects with mild to moderate severity dementia due to AD. (Id.) As is known to one of ordinary skill in the art, centiloid thresholds may vary, for example may be refined, based on new or additional scientific information. An elevated level of amyloid can be set relative to a baseline threshold in a healthy control determined according to methods known to a POSA. For example, a centiloid value of 32.5 can be used as a threshold value for “elevated amyloid plaque,” and an “intermediate amyloid plaque” level refers to an Aβ amyloid PET in the range of 20-32.5 CL (e.g., 30 CL). In another example, a centiloid value of 40 can be used as a threshold value for “elevated amyloid plaque,” and an “intermediate amyloid plaque” level refers to an Aβ amyloid PET in the range of 20-40 CL. Patients with “preclinical AD,” as described herein, are cognitively normal individuals with intermediate or elevated levels of amyloid plaque in the brain and can be identified by asymptomatic stages with or without memory complaints and emerging episodic memory and Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 executive function deficits. Subjects with preclinical AD may have a clinical diagnosis or meet clinical criteria for preclinical AD or be suspected of having preclinical AD based on symptoms or clinical presentation. “Early AD” or “early Alzheimer’s disease,” as used herein, is a continuum of AD severity from mild cognitive impairment due to AD – intermediate likelihood to mild Alzheimer’s disease dementia. Subjects with early AD include subjects with mild Alzheimer’s disease dementia as defined herein and subjects with mild cognitive impairment (MCI) due to AD – intermediate likelihood as defined herein. Subjects with early AD may have a clinical diagnosis or meet clinical criteria for early AD or be suspected of having early AD based on symptoms or clinical presentation. Subjects with “mild Alzheimer’s disease dementia,” as used herein, are subjects who meet the NIA-AA core clinical criteria for probable Alzheimer’s disease dementia in McKhann, G.M. et al., “The diagnosis of dementia due to Alzheimer’s disease: Recommendations from the National Institute on Aging – Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer’s disease.” Alzheimer Dement.2011; 7:263-9. Also included herein are subjects who have a clinical dementia rating score of 0.5 to 1.0 and a Memory Box score of 0.5 or greater at screening and baseline and subjects that exhibit change in the score on the Wechsler Memory Scale-Revised Logical Memory subscale II (WMS-R LM II). Subjects with “MCI due to AD – intermediate likelihood,” as used herein are those identified as such in accordance with the NIA-AA core clinical criteria for mild cognitive impairment due to Alzheimer’s disease – intermediate likelihood (see McKhann supra). For example, subjects may be symptomatic but not demented AD subjects with evidence of brain amyloid pathology as measured by the ADCOMS Composite Clinical Score defined herein. Also Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 included are subjects who have a clinical dementia rating score of 0.5 and a Memory Box score of 0.5 or greater at screening and baseline. Furthermore, subjects who report a history of subjective memory decline with gradual onset and slow progression over the last 1 year before screening, which is corroborated by an informant, are also included herein. Memory decline and / or episodic memory impairment can be assessed in a subject by change in the score on the Wechsler Memory Scale-Revised Logical Memory subscale II (WMS-R LM II). As used herein, subjects having “intact cognition” refer to subjects having a score of greater than 27 on the MMSE after education adjustment and a global clinical dementia rating equal to 0. The terms “BAN2401” and “lecanemab” are interchangeably used and refer to the anti-Aβ protofibril antibody comprising the sequences of CDRs, variable regions and full chains shown in Tables 1-3. Unless otherwise specified by context, the terms “BAN2401” and “lecanemab” respectively encompass any antibodies having the same primary amino acid sequences of BAN2401 or lecanemab or functional variants thereof, e.g., any biosimilars thereof. “Biosimilar” is a biotherapeutic product that is similar in terms of quality, safety, and efficacy to an already licensed reference biotherapeutic product, for example, defined in WHO guidelines (Guidelines on evaluation of similar Biotherapeutic Products (SBPs), Annex 2, Technical Report Series No.977, 2009) or in FDA Biosimilars Guidances. Aβ Protofibril Assay Without being bound by theory, the detection and quantification of protofibrils in biological samples has been limited due to the requirement to have a selective method of detection for the protofibril form of Aβ over other forms of Aβ (such as monomers) and the low Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 concentration of protofibrils in biological samples. In some instances, the concentration of protofibrils is below the low limit of detection for an assay and may be in the femtomolar range. Provided herein is a method of quantifying Aβ protofibrils in biological samples. The assay may be sensitive to detect femtomolar concentrations of Aβ protofibrils. The assay may be selective for Aβ protofibrils compared to Aβ monomers. In some embodiments, the assay comprises obtaining a sample from a patient (e.g., a CSF sample or a blood sample) and contacting it with a capture antibody that selectively binds Aβ protofibrils. In some embodiments, the capture antibody is directly immobilized on a solid surface. In other embodiments, the capture antibody is immobilized in a subsequent step (e.g., using a second anti-capture antibody on a solid surface, or by using biotin-streptavidin system with a biotin-conjugated capture antibody and streptavidin on the surface to which the capture antibody will be immobilized, or by similar methods). In some embodiments, the sample is contacted with a detection antibody. The detection and capture antibodies may be incubated at the same time, or they may be incubated in separate steps (e.g., with intervening purification, washing and / or transfer, steps, etc.). Capture Antibody In various embodiments of the protofibril assay, a capture antibody is used that selectively binds to the epitope of Aβ protofibrils. The capture antibody may be immobilized on a surface. The capture antibody may be used in an assay to determine the concentration of the target Aβ protofibrils. In some embodiments, immobilization of the capture antibody on a surface facilitates target retrieval and washing to promote assay sensitivity and selectivity. In Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 some embodiments, the capture antibody is selective for Aβ protofibrils, in some preferred embodiments, the capture antibody may be BAN2401 (also referred to herein as lecanemab). In some embodiments, the assay comprises an anti-protofibril antibody for the capture antibody and an antibody that does not bind the same epitope as the capture antibody for the detection antibody. In some embodiments, the capture antibody binds the C-terminus of Aβ protofibrils and the detection antibody binds an N-terminus. In some embodiments, the capture antibody is BAN2401 or comprises the CDRs or variable regions from BAN2401, or binds the same epitope as BAN2401 (for CDR, variable region, and full chain sequences, see Tables 1-3). In some embodiments, the protofibril assay, e.g., using BAN2401 as the capture antibody is capable of accurate determination of the protofibril concentration to the femtomolar level. In some embodiments, the assay is capable of doing so in the presence of therapeutic BAN2401, e.g., in samples from patients being treated therapeutically with BAN2401. In various embodiments, quantification of protofibrils uses an immobilized capture antibody that preferentially binds protofibrils, such as anti-protofibril antibody. In some embodiments, the capture antibody is an anti-Aβ protofibril antibody that selectively binds Aβ protofibrils. In some embodiments, the capture antibody binds to an epitope of Aβ protofibrils with a greater affinity and lower dissociation constant (KD) in comparison to other proteins. In some embodiments, the anti-Aβ protofibril antibody binds the same epitope and / or competes for binding with BAN2401. In some embodiments, the capture antibody binds to an epitope of Aβ protofibrils with a greater affinity and lower dissociation constant (KD) than to other forms of Aβ (e.g. oligomers, monomers). In some embodiments, the capture antibody comprises the CDRs (Table 1; SEQ ID NOs.: 1, 2, 3, 4, 5, and 6) and / or variable regions of BAN2401 (Table 2; SEQ ID NOs.: 7 and 8). In some embodiments, the capture antibody competes for binding with, Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 and / or binds the same or similar epitope (e.g., in the C-terminal region of Aβ) as BAN2401. In some embodiments, the capture antibody competes for binding with BAN2401. In some embodiments, the capture antibody comprises BAN2401. In some embodiments, the capture antibody comprises the complementarity determining regions of Table 1. In some embodiments, the capture antibody comprises heavy chain complementarity determining regions (CDRs) (i.e., HCDR1, HCDR2, and HCDR3) comprising amino acid sequences of SEQ ID NOs: 1, 2, and 3 and light chain CDRs (i.e., LCDR1, LCDR2 and LCDR3) comprising amino acid sequences of SEQ ID NOs: 4, 5, and 6. In some embodiments, the capture antibody may have HCDR1, HDCR2, HCDR3 and LCDR1, LCDR2, LCDR3 amino acid sequences that have 80% to 100% sequence identity to the sequences of Table 1. In some embodiments, the capture antibody may have HCDR1, HDCR2, HCDR3, LCDR1, LCDR2, and LCDR3 amino acid sequences that have 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to the sequences of Table 1. In some embodiments, the capture antibody comprises the heavy chain variable region and light chain variable region of Table 2. In some embodiments, the amino acid sequence of the heavy chain variable region of the capture antibody has 80-100% sequence identity to SEQ ID NO: 7. In some embodiments, the amino acid sequence of the heavy chain variable region of the capture antibody has 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO: 7. In some embodiments, the amino acid sequence of the light chain variable region of the capture antibody has 80-100% sequence identity to SEQ ID NO: 8. In some embodiments, the amino acid sequence of the light chain variable region of the capture antibody has 80%, 81%, 82%, 83%, Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO: 8. In some embodiments, the heavy chain variable region of the capture antibody comprises an amino acid sequence of SEQ ID NO: 7 and the light chain variable region of the capture antibody comprises an amino acid sequence of SEQ ID NO: 8. In some embodiments, the capture antibody comprises the heavy chain and the light chain amino acid sequences of Table 3. In some embodiments, the amino acid sequence of the heavy chain of the capture antibody has 80-100% sequence identity to SEQ ID NO: 9. In some embodiments, the amino acid sequence of the heavy chain of the capture antibody has 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO: 9. In some embodiments, the amino acid sequence of the light chain of the capture antibody has 80-100% sequence identity to SEQ ID NO: 10. In some embodiments, the amino acid sequence of the light chain of the capture antibody has 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO: 10. In some embodiments, the heavy chain of the capture antibody comprises an amino acid sequence of SEQ ID NO: 9 and the light chain of the capture antibody comprises an amino acid sequence of SEQ ID NO: 10. In some embodiments, the capture antibody comprises the heavy chain constant region and the light chain constant region amino acid sequences of Table 4. In some embodiments, the amino acid sequence of the heavy chain constant region of the capture antibody has 80-100% sequence identity to SEQ ID NO: 11. In some embodiments, the amino acid sequence of the heavy chain constant region of the capture antibody has 80%, 81%, 82%, 83%, 84%, 85%, 86%, Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO: 11. In some embodiments, the amino acid sequence of the light chain constant region of the capture antibody has 80-100% sequence identity to SEQ ID NO: 12. In some embodiments, the amino acid sequence of the light chain constant region of the capture antibody has 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO: 12. In some embodiments, the heavy chain constant region of the capture antibody comprises an amino acid sequence of SEQ ID NO: 11 and the light chain constant region of the capture antibody comprises an amino acid sequence of SEQ ID NO: 12. Immobilization of Capture Antibody In various embodiments, the capture antibody is immobilized on a surface. The term “immobilized” indicates that the capture antibody may be bound (e.g. covalently) to the surface, which may be chemically modified to bind the capture antibody. As used herein, the term “surface” may refer to a structure upon which an antibody (e.g., a capture antibody) is immobilized, either covalently or non-covalently. In some embodiments, the surface is a plate. In some embodiments, the surface is a particle. In some embodiments, the surface is a bead. In some embodiments, the surface is a magnetic microparticle (MP) bead that can be isolated from a solution by a magnet. In some embodiments, the surface is chemically modified to bind and immobilize the capture antibody. In some embodiments, the surface is Protein A or Protein G modified to bind monoclonal or polyclonal antibodies. In some embodiments, the capture antibody is immobilized to a surface through the interaction of streptavidin and biotin. In some embodiments, the surface is modified to covalently bind primary amine and / or sulfhydryl Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 residues. In some embodiments, the surface is chemically modified with tosyl groups to make it “tosylactivated”, such as with p-Toluene-sulfonyl groups. In some embodiments, tosyl groups on a surface bind primary amino and sulfhydryl groups to conjugate proteins to the surface of the magnetic particle. In some embodiments, when the capture antibody is immobilized on a surface, immobilization may occur before, at the same time as, or after the capture antibody is incubated with the analyte-containing sample. In some embodiments, the capture antibody is immobilized on a surface before the capture antibody is incubated with a biological sample. In some embodiments, the capture antibody is conjugated to the surface. In some embodiments, the capture antibody is immobilized on the surface of a plate. In some embodiments, the plate comprises for example 12-wells, 24-wells, or 96-wells. In some embodiments, the plate is clear plastic, such as polystyrene. In some embodiments, the plate is opaque or black. In some embodiments, when the capture antibody is immobilized to the surface of a particle, the particle may be magnetic. In some embodiments, the particle is a magnetic bead. In some embodiments, the magnetic bead diameter is about 1 µm to about 5 µm. In some embodiments the magnetic bead is a silica-coated iron oxide magnetic bead grafted with functional groups to facilitate the immobilization of the capture antibody. In some embodiments, the particle is a magnetic microparticle grafted with functional groups to facilitate the immobilization of the capture antibody. In some embodiments, the capture antibody is immobilized to the magnetic particle by a streptavidin-biotin linker. In some embodiments, the capture antibody is immobilized via a streptavidin-biotin linker to the surface using the Single Molecule Counting (SMC™) Capture Labeling Kit from Merck. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, the surface is modified to covalently bind primary amine and / or sulfhydryl residues, for example, by modification with p-Toluene-sulfonyl groups, or “tosylactivated”. Tosyl-activated surfaces provide reactive sulfonyl esters that covalently link antibodies or other ligands that contain primary amine or sulfhydryl groups to the surface. In some embodiments, tosyl groups bind primary amino and sulfhydryl groups to conjugate proteins to the surface of a magnetic particle. In some embodiments, the capture antibody is immobilized to a tosylactivated particle. In some embodiments, the capture antibody is immobilized to the particle by a covalent bond formed between the capture antibody and the tosylactivated particle. In some embodiments, the tosylactivated beads are Dynabeads™ MyOne™ (Invitrogen). In some embodiments, the capture antibody may be immobilized on a tosylactivated surface according to principles known in the art for the conjugation of molecules to surfaces (see, e.g., Otieno BA et al., Chapter Seven – Bioconjugation of Antibodies and Enzyme Labels onto Magnetic Beads. Methods in Enzymology Volume 571, Pages 135-150, 2016). In some embodiments, the capture antibody is conjugated to the surface at a concentration of 10- 40 µg of the capture antibody per mg of bead. In some embodiments, the tosylactivated beads are washed in sodium borate buffer. In some embodiments, the capture antibody is immobilized to the tosylactivated beads by incubating for 24 hours at 37°C with continuous rotation of the reaction tube. In some embodiments, the capture antibody is conjugated to the surface at a concentration of 10, 15, 20, 25, 30, 35 or 45 µg of antibody per mg of bead. In some embodiments, the capture antibody is conjugated to the surface at a concentration of 10, 11,12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37,38, 39, 40 µg of antibody per mg of bead. In some embodiments, the capture antibody is Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 conjugated to surface at a preferred concentration of 20 µg of antibody per mg of bead. In some preferred embodiments, the capture antibody is conjugated to the surface at a concentration of 20 µg of capture antibody per mg of a bead such as a magnetic bead, e.g., a tosylactivated magnetic bead. In some embodiments, the supernatant (e.g., containing unbound capture antibody) is discarded and the capture antibody immobilized to the surface is retained for the assay. In some embodiments, the capture antibody immobilized to the surface is blocked with blocking buffer (e.g., PBS with 0.5% BSA and 0.05% Tween20). In some embodiments, the immobilized capture antibody was blocked for 1 hour to 24 hours at 20°C to 40°C. In some embodiments, blocking of the capture antibody immobilized to the beads is performed with continuous agitation (e.g., continuous rotation). In some embodiments, the capture antibody immobilized to the surface is blocked in PBS with 0.5% BSA and 0.05% Tween20 overnight (e.g., 18 to 24 hours) at 37°C. In some embodiments, the capture antibody immobilized to the beads is stored (e.g., at 4°C) until use. Detection Antibody In various embodiments of the assays disclosed herein, a detection antibody is used that is selective for the target, and which contains an epitope different from the capture antibody or is otherwise capable of binding to the target when bound by the capture antibody. In some embodiments, the detection antibody is conjugated to a detectable tag, e.g., to provide a readout of the presence and / or amount of the detection antibody present in a sample by an instrument capable of detecting the signal from the tag. In some embodiments, the detection antibody is an anti-Aβ antibody that selectively binds Aβ at an epitope distinct from that bound by the capture antibody. In some embodiments, Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 the detection antibody binds to an epitope (e.g., in the N-terminal regions) of an Aβ protofibril with a greater affinity and lower dissociation constant (KD) than to other regions of Aβ or to other proteins. In some embodiments, the detection antibody binds to an epitope of Aβ protofibrils with a greater affinity and lower dissociation constant (KD) than to other forms of Aβ (e.g., oligomers, monomers). In some embodiments, the detection antibody binds to an epitope that becomes accessible for antibody binding when Aβ is organized into protofibrils. Table 16 contains exemplary anti-Aβ antibodies with predicted epitopes within the Aβ protein sequence. In some embodiments, the detection antibody binds the N-terminus of Aβ. In some embodiments, the detection antibody binds the first five amino acid residues of Aβ, Aβ1-5, of SEQ ID NO.: 28 or to an epitope comprising those first five amino acids. In some embodiments, the detection antibody binds the N-terminus of protofibrils, e.g., Aβ1-5(Table 9; SEQ ID NO.: 28). Exemplary detection antibodies with affinity to the N-terminus of Aβ include 3D6 (Table 6; SEQ ID NOs.: 19 and 20), 82E1 (Table 13; SEQ ID NOs.: 30 and 31) and 6E10 (Table 14; SEQ ID NOs.: 32 and 33) (Table 16). In some embodiments, 3D6 (Table 6; SEQ ID NOs.: 19 and 20) is an N-terminal binding antibody that may be used. In some embodiments, 3D6 may recognize an accessible epitope on Aβ protofibrils distinct from that bound by the capture antibody. In some embodiments, the detection antibody is 3D6 or comprises the CDRs and / or variable regions from 3D6 (Table 5, SEQ ID NOs.: 13, 14, 15, 16, 17, and 18; Table 6, SEQ ID NOs.: 19, 20). In some embodiments, antibodies with affinity to the C-terminus of Aβ are not used as detection antibodies, for example 21F12 (Table 15; SEQ ID NOs.: 34 and 35) and H31L21 (Table 16). In some embodiments, antibodies binding the C-terminus are used as detection antibodies. In some embodiments, lecanemab is used as the detection antibody, or an antibody Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 comprising the CDRs and / or variable regions from lecanemab (Table 1, SEQ ID NOs.: 1, 2, 3, 4, 5, and 6; Table 2, SEQ ID NOs.: 7 and 8). In some embodiments, the detection antibody comprises the CDRs and / or variable regions from 82E1 (Table 13, SEQ ID NOs.: 30 and 31). In some embodiments, the detection antibody is 82E1. In some embodiments, the detection antibody binds amino acid residues 3-8 of Aβ, Aβ3-8. In some embodiments, the detection antibody comprises the CDRs and / or variable regions from 6E10 (Table 14, SEQ ID NOs.: 32 and 33). In some embodiments, the detection antibody is 6E10. In some embodiments, the detection antibody competes for binding or binds the same epitope as any one or more of antibodies 3D6 (Table 6, SEQ ID NOs.: 19 and 20), 82E1 (Table 13, SEQ ID NOs.: 30 and 31) , and 6E10 (Table 14, SEQ ID NOs.: 32 and 33). In some embodiments, the detection antibody binds the same epitope or competes for binding with BAN2401. In some embodiments, the detection antibody comprises the CDRs and / or variable regions from BAN2401 (Table 1, SEQ ID NOs.: 1 (HCDR1), 2 (HCDR2), 3 (HCDR3), 4 (LCDR1), 5 (LCDR2), 6 (LCDR3); Table 2, SEQ ID NOs: 7 and 8). In some embodiments, the detection antibody is BAN2401. In some embodiments, the detection antibody comprises a set of six complementarity determining regions from the sets in Table 5. In some embodiments, the detection antibody comprises heavy chain complementarity determining regions (CDRs)(i.e., HCDR1, HCDR2, and HCDR3) comprising amino acid sequences of SEQ ID NOs.: 13 (HCDR1), 14 (HCDR2), and 15 (HCDR3) and light chain CDRs (i.e., LCDR1, LCDR2 and LCDR3) comprising amino acid sequences of SEQ ID NOs.: 16 (LCDR1), 17 (LCDR2), and 18 (LCDR3). In some embodiments, the detection antibody may have HCDR1, HDCR2, HCDR3 and LCDR1, LCDR2, LCDR3 amino acid sequences that have 80% to 100% sequence identity to the sequences of Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 5. In some embodiments, the detection antibody may have HCDR1, HDCR2, HCDR3, LCDR1, LCDR2, and LCDR3 amino acid sequences that have 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to the sequences of Table 5. In some embodiments, the detection antibody comprises the heavy chain variable region and light chain variable region of Table 6. In some embodiments, the heavy chain variable region has 80-100% sequence identity to SEQ ID NO.: 19. In some embodiments, the heavy chain variable region has 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO.: 19. In some embodiments, the light chain variable region has 80-100% sequence identity to SEQ ID NO.: 20. In some embodiments, the light chain variable region has 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%.98%, 99% or 100% sequence identity to SEQ ID NO.: 20. In some embodiments, the heavy chain variable region of the detection antibody comprises an amino acid sequence of SEQ ID NO.: 19 and the light chain variable region of the detection antibody comprises an amino acid sequence of SEQ ID NO.: 20 Detectable Labels In some embodiments, the detection antibody (e.g., 3D6) is conjugated to a detectable label, e.g., a tag e.g., a fluorophore. In some embodiments, the detection antibody is tagged using an antibody labeling kit, such as the SMC™ Detection Antibody Labeling kit (EMD Millipore).In some embodiments, the concentration of the detection antibody may be determined by measuring the level of the signal from the tag, e.g., by measuring the absorbance of a fluorescently-tagged detection antibody, e.g., at 280 nm In some embodiments, the detection Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 antibody conjugated to a fluorophore is diluted in a buffer, for example, Discovery Assay Buffer™ (EMD Millipore). In some embodiments, the detection antibody is diluted, e.g., to a concentration of 100 ng / mL, e.g., in Discovery Assay Buffer and filtered, e.g., through a 0.22 µm syringe filter (EMD Millipore). In some embodiments, a solution of detection antibody is stored at 4°C. As used herein, the term “tag” refers to a discrete unit that functions to detectably label a protein, i.e., is detectable and / or quantifiable by one or more means. In some embodiments, the tag is a detectable protein attached to an antibody disclosed herein. In some embodiments, the tag is a protein that may be incorporated into the 3’ or 5’ end of a DNA sequence of a protein coding gene in need of labeling (e.g., a detection antibody) to generate a fusion protein. In some embodiments, the tag is conjugated via chemical bonds, for example covalent chemical bonds. In some embodiments, the tag is conjugated to the labeled protein through the interaction of biotin and streptavidin. In some embodiments, the tag produces a fluorescent signal. In some embodiments, the tag is a quantum dot. In some embodiments, the tag is a fluorophore that absorbs photons of light and emits photons at a different wavelength. In some embodiments, the tag is a fluorescent protein, for example a protein that has inherent fluorescence, such as enhanced green fluorescent protein (EGFP) or yellow fluorescent protein (YFP). Generally, the tag is added to end of the protein in need of labeling. In some embodiments, the tag is conjugated to the detection antibody before, at the same time as, or after the detection antibody binds the analyte (e.g. forms the second immune complex). In some embodiments, the tag is conjugated to the detection antibody before binding the analyte and / or forming the second immune complex. In some embodiments, the tag is conjugated to the Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 detection antibody through the interaction of biotin and streptavidin. In some embodiments, the tag is conjugated to streptavidin and labels biotinylated detection antibody. In some embodiments, the tag is conjugated to biotin and labels streptavidin conjugated detection antibody. In some embodiments, the tag is a quantum dot, with optical and electronic properties that can be detected. In some embodiments, the tag is modified to bind primary amine groups of proteins, as a method of conjugating the tag and the detection antibodies. In some embodiments, the tag is a SULFO-TAG, for example a SULFO-TAG NHS-Ester. In some embodiments, the tag is a SULFO-TAG comprising N-hydroxysuccinimide esters that can bind to primary amine groups of proteins, including detection antibodies. In some embodiments, the SULFO-TAG comprises a fluorescent tag. In some embodiments, the tag (e.g., the fluorescent tag), e.g., the quantum dot, is attached to 3D6 as the detection antibody. Biological Sample In various embodiments, protofibril levels are determined in one or more biological samples obtained from a subject, e.g., samples obtained over the course of treatment. In one embodiment, the biological sample is obtained from a subject treated with lecanemab. In some embodiments, a biological sample obtained from a human subject may comprise body fluid or tissue. In some embodiments, human subject is alive. In some embodiments, the subject is deceased. In some embodiments, the biological sample is tissue (e.g., brain tissue) obtained during a biopsy. In some embodiments, the biological sample is brain tissue obtained from a Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 deceased individual. In some embodiments, the brain tissue is made into a brain tissue homogenate and / or tissue lysate. In some embodiments, the biological sample is a blood sample, including plasma and / or serum. In some embodiments, the biological sample is cerebrospinal fluid (CSF). In some embodiments, the biological sample contains no anti- Aβ antibody. In some embodiments, the biological sample contains an anti- Aβ antibody. In some embodiments, the biological sample contains lecanemab. In some embodiments, an Aβ protofibril level is measured in an undiluted biological sample, which may also be referred to as a “neat” sample. In some embodiments, the biological sample is diluted to a specific total protein concentration. In some embodiments, the biological sample is prepared at a concentration of 0.1 mg to 10 mg protein per ml of sample. In some embodiments, the biological sample is diluted at least 10-fold prior to the assay. In some embodiments, the sample is diluted 1, 2, 3, 4, 5, 6, 7, 8, 9, 10-fold. In some embodiments, the sample is diluted 1:1 with sample buffer. In some embodiments, the sample is diluted 1-fold to 4- fold. In some embodiments, the sample is diluted 1-fold, 2-fold, 3-fold, or 4-fold. In some embodiments the sample is diluted 1.5-fold. In some embodiments, the sample is diluted 2-fold. In some embodiments, the sample is diluted 3-fold. In some embodiments, the sample is diluted at least 100-fold prior to the assay. In some embodiments, the sample is diluted at least 1,000- fold prior to the assay. In some embodiments, the biological sample is treated to maintain the level of Aβ protofibrils. In some embodiments, the biological sample is diluted in a sample buffer to maintain the Aβ protofibril level. In some embodiments, the sample is diluted about 1.5-fold in a sample buffer, e.g., any of the sample buffers disclosed herein. In some embodiments, the CSF sample is diluted Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 about 1.5-fold. In some embodiments, the CSF sample is diluted 1.5-fold. In some embodiments, the sample is diluted about 2-fold in a sample buffer, e.g., any of the sample buffers disclosed herein. In some embodiments, the CSF sample is diluted about 2-fold. In some embodiments, the CSF sample is diluted 2-fold. In some embodiments, the sample is diluted about 3-fold in a sample buffer, e.g., any of the sample buffers disclosed herein. In some embodiments, the CSF sample is diluted about 3-fold. In some embodiments, the CSF sample is diluted 3-fold. In some embodiments, the biological sample is prepared in low protein binding microcentrifuge tubes. In some embodiments, the low protein binding tubes reduce sample-to- surface binding and sample loss. In some embodiments, the low protein binding microcentrifuge tubes are made from plastic, such as polypropylene, and may be treated to reduce protein binding. In some embodiments, the low protein binding tubes are Protein LoBind® Tubes (Eppendorf). In some embodiments, the biological sample is stored frozen, e.g., at around -80°C. In some embodiments, the biological sample is thawed at room temperature. In some embodiments, the biological sample is thawed on wet ice. In some embodiments, the sample is gently mixed by pipetting or vortexing to avoid air entering the sample and bubble formation. In some embodiments, the biological samples are not freeze thawed. In some embodiments, the biological samples are not freeze-thawed more than once. In some embodiments, the biological samples are not freeze-thawed more than 2 times. In some embodiments, the biological samples are not freeze-thawed more than 3 times. In some embodiments, the biological samples are freeze-thawed less than 3 times. In some embodiments, the biological sample may be obtained from a fasted subject. In some embodiments, additional biological samples may be obtained from subjects at the same time of day (e.g., morning), e.g., over the course of treatment with lecanemab. In some Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 embodiments, the biological sample may be serially sampled from the same subject for longitudinal analysis. In some embodiments, the biological sample may be obtained from the subject prior to treatment (e.g., with lecanemab) to establish a baseline level of protofibril. In some embodiments, one or more additional biological samples may be obtained following the initiation of a treatment. Assay Conditions In some embodiments, prior to incubating the immobilized capture antibody with a sample comprising the analyte (e.g., Aβ protofibrils), the antibody is washed to remove capture antibody that is not immobilized on a surface. In some embodiments a sample comprising analyte is contacted with capture antibody to form a first immune complex. In some embodiments, the first immune complex comprises the immobilized capture antibody bound to the analyte (e.g., Aβ protofibrils). In some embodiments, the first immune complex is washed to remove unbound material in the biological sample. In some embodiments, the sample and / or first immune complex is contacted with the detection antibody to form a second immune complex. In some embodiments, the second immune complex comprises the capture antibody (e.g., immobilized capture antibody), the analyte, and the detection antibody. In some embodiments, the second immune complex is washed to remove excess, unbound detection antibody. In some embodiments, the capture antibody is diluted, e.g., in Discovery Assay Buffer (EMD Millipore). In some embodiments, the first immune complex is formed by incubating the biological sample with the capture antibody at room temperature. In some embodiments, the first immune complex is formed by incubating the biological sample with the capture antibody at Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 27°C. In some embodiments, the first immune complex is formed by incubating the biological sample with the capture antibody at 4°C. In some embodiments, the first immune complex is formed in a still solution, or solution only agitated intermittently (e.g., tapping periodically, e.g., every 20-30 minutes). In some embodiments, the first immune complex is formed in a continuously agitated solution (e.g., on a plate shaker, tube rotator). In some embodiments, the first immune complex formed in a solution incubated on a plate shaker set to 600-1000 rpm. In some embodiments, the first immune complex formed in a solution incubated on a plate shaker set to 700-900 rpm. In some preferred embodiments, the first immune complex formed in a solution incubated on a plate shaker set to 800 rpm. In some embodiments, the first immune complex is formed by incubating the biological sample with the capture antibody for 0.5-5 hours. In some embodiments, the first immune complex is formed by incubating the biological sample with the capture antibody for 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0 hours. In some embodiments, the first immune complex is formed by incubating the biological sample with the capture antibody for 1-24 hours. In some embodiments, the first immune complex is formed by incubating the biological sample with the immobilized capture antibody for 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 hours. In some embodiments, the first immune complex is formed by incubating the biological sample with the immobilized capture antibody for 0.5-2 days. In some embodiments, the first immune complex is formed by incubating the biological sample with the immobilized capture antibody for 0.5, 1, 1.5, or 2 days. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some preferred embodiments, the first immune complex is formed by incubating the biological sample with the immobilized capture antibody for 2 hours with continuous agitation by a plate shaker set to 800 rpm at 27°C. In some embodiments, after an incubation period to form the first immune complex between the capture antibody and analyte, the first immune complex is washed, e.g., with a buffer, e.g., 10-fold diluted System Wash Buffer with Proclin™ (EMD Millipore) to remove unbound materials. In some embodiments, after the incubation period the first immune complex is washed using the SMC™ (EMD Millipore) washer. In some embodiments, a second immune complex is formed by incubating the sample containing the first immune complex with the detection antibody in a buffer, e.g., a dilution buffer. In some embodiments, the detection antibody is diluted with a buffer, such as Discovery Assay Buffer™ (EMD Millipore). In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody at room temperature. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody at 27°C. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody at 4°C. In some embodiments, the second immune complex is formed in a still solution, or solution only agitated intermittently (e.g., tapping periodically, e.g., every 5-30 minutes). In some embodiments, the second immune complex is formed in a continuously agitated solution (e.g., on a plate shaker or tube rotator). In some embodiments, the second immune complex is formed in a solution incubated on a plate shaker set to 600-1000 rpm. In some embodiments, the second immune complex formed in a solution incubated on a plate shaker set to 700-900 rpm. In Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 some preferred embodiments, the second immune complex is formed in a solution incubated on a plate shaker set to 800 rpm. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody for 0.5-5 hours. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody for 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, or 5.0 hours. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody for 1-24 hours. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody for 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 hours. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody for 0.5-2 days. In some embodiments, the second immune complex is formed by incubating the first immune complex with the detection antibody for 0.5, 1, 1.5, or 2 days. In some embodiments, the second immune complex is formed by incubating the sample containing the first immune complex with the detection antibody for 1.5 hours with continuous agitation by a plate shaker set to 800 rpm at 27°C. In some embodiments, the second immune complex is washed with a wash buffer comprising Triton-X. In some embodiments, the buffer comprises 1-5% Triton-X-100, boric acid 0.1-1%, disodium tetraborate decahydrate 0.1-1% and 2-methyl-4-isothiazolin-3-one and ProClin (e.g., ProClin 300). In some embodiments, after the incubation period the second immune complex is washed with a buffer, such as 10-fold diluted SMC™ System Wash Buffer with ProClin™ (EMD Millipore). In some embodiments, after the incubation period the first immune complex is washed using the SMC (EMD Millipore) washer apparatus. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, the Aβ protofibril level may be determined using an instrument capable of measuring a signal from the tag conjugated to the detection antibody. In some embodiments, the signal is measured using a plate reader. In some embodiments, the signal is measured using a fluorescence microscope. In some embodiments, the signal is measured using a digital ELISA. In some embodiments, the signal is measured using a digital single molecule counting instrument (e.g., SMCxPRO). In some embodiments, the instrument is a SMCxPRO. In some embodiments, the detection antibody in the second immune complex and / or the tag on the second antibody is dissociated to measure the signal from the tag of the detection antibody. In some preferred embodiments, the detection antibody comprises an antibody conjugated to a fluorescent tag. In some embodiments, the second immune complex is dissociated in an acidic elution buffer (e.g., Elution Buffer B™, EMD Millipore). In some embodiments, the second immune complex is incubated in the elution buffer for 1-20 minutes. In some embodiments, the second immune complex is incubated in the elution buffer for 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 minutes. In some embodiments, the second immune complex is incubated in the elution buffer at room temperature. In some embodiments, the second immune complex is incubated in the elution buffer at 27°C. In some embodiments, the second immune complex is incubated in the elution buffer at 4°C. In some embodiments, the second immune complex is incubated in the elution buffer without agitation and / or with intermittent mixing (e.g., at 1-2 minute intervals). In some embodiments, the second immune complex is incubated in the elution buffer with continuous agitation (e.g., plate shaker or tube rotator). In some highly preferred embodiments, the second immune is dissociated by incubation in the elution buffer for 10 minutes on a plate shaker at 800 rpm at 27°C. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, the eluate resulting from incubation with the elution buffer and comprising the detection antibody and / or tag dissociated from the second immune complex is diluted with buffer that may neutralize the pH of the elution buffer (e.g. Buffer D™, EMD Millipore). In some embodiments, the signal from the dissociated detection antibody is measured by digital ELISA. In some embodiments, the signal from the dissociated detection antibody is measured by single molecule counting. In some embodiments, the signal from the dissociated detection antibody is not measured using a SIMOA® instrument. In some embodiments, the signal from the dissociated detection antibody is measured using a single molecule counting instrument. In some embodiments, the signal from the dissociated detection antibody is measured using a SMCxPRO® instrument (EMD Millipore). In some embodiments, the signal from the detection antibody is measured using a Single Molecule Array (SIMOA®) instrument (Quanterix). In some embodiments, the Aβ protofibril level of a biological sample may be determined relative to a control sample from a healthy subject. In some embodiments, the Aβ protofibril level of a biological sample may be determined relative to the mean Aβ protofibril level of two or more healthy subjects. In some embodiments, Aβ protofibril levels may be reported in a subject as a fold change relative to a baseline level of Aβ protofibril level in the same subject prior to treatment, e.g., with lecanemab. In some embodiments, a control sample is obtained from an age and / or gender matched subject. In some embodiments, a control sample is obtained from a subject with co-morbidities that are not associated with AD. In some embodiments, a control sample is obtained from a subject with mild cognitive impairment and / or early AD. In some embodiments, a control sample is obtained from a subject who does not have AD. In some embodiments, a control sample is obtained from the same subject as the Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 test sample, but obtained prior to the start of treatment, e.g., to establish a baseline level of protofibril before treatment, e.g., treatment with lecanemab. In some embodiments, the Aβ protofibril level is a concentration determined using an Aβ1-42protofibril standard curve. In some embodiments, the concentration of protofibrils in a biological sample may be determined by linear regression from a Aβ protofibril standard curve. In some embodiments, the Aβ protofibril standard is generated from known amounts of oligomerized Aβ monomers, for example Aβ1-42 monomers. In some embodiments the Aβ protofibril standard is generated by size exclusion chromatography (SEC). In some embodiments, the Aβ protofibril standard is generated by incubating Aβ1-42 monomers dissolved in 10 mM NaOH to form 100 µM and then diluted to 50 µM with 2X PBS. In some embodiments, the Aβ protofibril standard is generated by incubating 50 µM of dissolved Aβ1-42 monomers at 37°C for 90 min to form the protofibrils. In some embodiments, the Aβ protofibril standard is isolated by size exclusion chromatography with Superdex 7510 / 600 GL (GE). In some embodiments, the concentration of the Aβ1-42 protofibril standard is determined with V- PLEX Aβ1-42 Peptide (4G8) Kit (Meso Scale Diagnostics). In some embodiments, the Aβ protofibril standard is diluted to a stock concentration of 100 nM. In some embodiments, the Aβ protofibril standard is diluted to a stock concentration with PBS +0.6% Tween 20. In some embodiments, the protofibril standard is stored at -80°C. In some embodiments, the Aβ protofibril standard is diluted from 2 pM to 0.003 pM. In some embodiments, the Aβ protofibril standard is diluted in a diluent, for example Discovery Standard Diluent™ (EMD Millipore). In some embodiments, the Aβ protofibril standard is generated by a serial dilution (e.g. a 2-fold serial dilution). In some embodiments, the standard curve is generated from at least three and up to 10 concentrations to generate a range of points for the standard curve. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, alternate assays may be used that are capable of detecting a signal from the anti-protofibril capture antibody, e.g., using alternate detection devices and / or labels such as horse radish peroxidase (HRP). In some embodiments, these assays may use lecanemab as the capture antibody and 3D6 as the detection antibody. In some embodiments, these assays may comprise plate-based sandwich ELISAs and / or the detection of a signal from the labeled detection antibody. In some embodiments, these protofibril assays may comprise detection by a Quanterix SIMOA® instrument. In some embodiments, these protofibril assays may comprise single molecule counting assays, e.g., with detection by a SMCxPRO® instrument. In some embodiments, these assays may have a lower limit of quantification greater than 1 pM and detect concentrations of PF greater than 1 pM in samples. In some embodiments, these assays may be used to for the further identification of capture antibody and the detection antibody pairs. In some embodiments, these assays may quantify PF generated from recombinant Aβ peptides. In some embodiments, these assays quantify PF generated from recombinant Aβ peptides and diluted in body fluid (e.g., cerebrospinal fluid). Uses of the Aβ Protofibril Assay The PF assay methods discussed herein may be used to quantify Aβ protofibril in a biological sample (e.g., a blood sample, a CSF sample, etc.). The disclosure and methods discussed herein may be used for determining an Aβ protofibril level in a biological sample (e.g., a blood sample, a CSF sample), e.g., by comparison to a control level. The biological sample may contain an anti-Aβ antibody (e.g., lecanemab). Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, a subject’s Aβ protofibril level may be used for diagnostic purposes. In some embodiments, a higher level of Aβ protofibril (as measured using a method disclosed herein) relative to a level in a control sample indicates an increased likelihood of having AD or risk of developing AD in the future. In some embodiments, a particular threshold level of Aβ protofibril indicates an increased likelihood of having AD or risk of developing AD in the future. In some embodiments, an Aβ protofibril level may be used for clinical applications such as clinical staging of AD disease (e.g., Alzheimer’s disease, Pre-AD, or early Alzheimer’s disease). In some embodiments, an Aβ protofibril level may be used alone or in conjunction with one or more additional criteria, such as one or more biomarkers and / or cognitive tests, to diagnose and / or provide clinical staging of a patient having or suspected of having AD. In some embodiments, an AD diagnosis and / or AD clinical staging may be determined based upon an Aβ protofibril level alone or in combination with other AD biomarkers, such as: intermediate or elevated levels of amyloid plaque in the brain as measured, e.g., by amyloid PET (e.g., a centiloid measure of about 20-40, e.g., a measure of about 20-32), fluorodeoxyglucose (FDG) PET, tau PET, cerebrospinal fluid level of Aβ1-42 and / or Aβ1-42 / 1-40 ratio, cerebrospinal fluid level of microtubule binding region (MBTR)-tau, cerebrospinal fluid level of total tau, cerebrospinal fluid level of neurogranin, cerebrospinal fluid level of neurofilament light peptide (NfL), and / or blood biomarkers as measured in the serum or plasma (e.g. levels of Aβ1-42, the ratio of two forms of amyloid-β peptide (Aβ1-42 / 1-40 ratio, e.g., a ratio of between about 0.092- 0.094 or below about 0.092), plasma levels of plasma total tau (T-tau), levels of phosphorylated tau (P-tau) isoforms (including tau phosphorylated at 181 (P-tau181), 217 (P-tau217), and 231 (P-tau231)), MTBR-tau243, glial fibrillary acidic protein (GFAP) and neurofilament light (NfL). Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, an Aβ protofibril level may be used in conjunction with one or more cognitive tests (e.g., MMSE, ADAS-Cog, CDR-SB, ADCOMS, and ADCS MCI-ADL) to diagnose and / or provide clinical staging of a patient having or suspected of having AD. Further guidance for methods of using one or more biomarkers for AD clinical applications can be found in WO2023283650A1, PCT / US2023 / 081441, and PCT / US2022 / 079571 (incorporated herein by reference in their entirety). In some embodiments, the methods disclosed herein may be used to measure changes in the disease burden and / or progression, e.g. there is a relationship between the level of Aβ protofibril and AD disease which can be used for diagnostic or prognostic purposes. For example, elevated levels of Aβ protofibril may distinguish subjects with MCI from cognitively normal subjects or indicate a patient at a more severe stage of the disease. In some embodiments, the Aβ protofibril level in a subject is compared to the Aβ protofibril level in a control. In some embodiments, the Aβ protofibril level in a subject is compared to the Aβ protofibril level a control subject who has been diagnosed with preclinical AD. In some embodiments, the Aβ protofibril level in a subject is compared to the Aβ protofibril level in a control subject who has been diagnosed with early AD. In some embodiments, the Aβ protofibril level in a subject is compared to the Aβ protofibril level in a control subject who has been diagnosed with mild AD dementia. In some embodiments, the Aβ protofibril level in a subject is compared to the Aβ protofibril level in a control subject who has been diagnosed with MCI due to AD – intermediate likelihood. In some embodiments, the Aβ protofibril level in a subject is compared to the Aβ protofibril level in a control subject who has been diagnosed with AD with moderate dementia. In some embodiments, the Aβ protofibril level in a subject is compared to the Aβ protofibril level in a control subject who has been diagnosed with AD with severe dementia. In some Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 embodiments, a change in an Aβ protofibril level over time in a subject indicates AD disease progression (e.g. an increasing Aβ protofibril level over a period of time may indicate that the disease is progressing). In some embodiments, the change over time of the Aβ protofibril level in a subject indicates a change in disease stage. For example, an increase in Aβ protofibril level over a period of time may indicate that a subject with pre-clinical AD has transitioned to mild AD. In some embodiments, a slowing in the rate of increase in Aβ protofibril levels in a subject indicates a slowing of disease progression. In some embodiments, an Aβ protofibril level that does not change over time may indicate stable disease. In some embodiments, the methods disclosed herein comprise measuring an Aβ protofibril level to monitor treatment efficacy. In some embodiments, an Aβ protofibril level that does not change (e.g., reduce) over time after initiating a treatment may continue on therapy. In some embodiments, a patient who exhibits a change, e.g., an increase in Aβ protofibril levels or a change, e.g., an increase in the rate of accumulation of Aβ protofibril levels over time after treatment commences may continue on therapy and / or move to a maintenance regimen. In some embodiments, the measured increase in Aβ protofibril levels reflects lecanemab-bound Aβ protofibril level in a subject’s CSF sample. These changes may be measured relative to a control patient (e.g., one receiving placebo) or relative to samples taken at earlier time points for the patient who is being evaluated for the change in Aβ protofibril levels. In some embodiments, the control patient is a patient who has received treatment with an anti-Aβ protofibril antibody. In some embodiments, the control patient has received treatment with an anti-Aβ protofibril antibody and did not respond to said treatment (e.g., the subject is amyloid positive after treatment with the anti-Aβ protofibril antibody). In some embodiments, the control patient has Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 received treatment with an anti-Aβ protofibril antibody and did respond to said treatment (e.g., the subject is amyloid negative after treatment with the anti-Aβ protofibril antibody). In some embodiments, the methods comprise measuring an Aβ protofibril level from a subject having or suspected of having AD before treatment and / or again in another sample during treatment (although it is to be understood that additional doses may be administered in between the sampling time points). In some embodiments, treatment may be titrated on the basis of a change in an Aβ protofibril level. In some embodiments, a change in the Aβ protofibril level and / or in the rate of accumulation of Aβ protofibril after receiving one or more doses of an anti-Aβ protofibril antibody, such as lecanemab, indicates treatment efficacy. In some embodiments, an increase in the Aβ protofibril level and / or in the rate of accumulation of Aβ protofibril after receiving one or more doses of an anti-Aβ protofibril antibody, such as lecanemab, indicates treatment efficacy. In some embodiments, the increase is relative to the level seen in a patient not treated with the anti-Aβ protofibril antibody, e.g., as compared to a patient receiving a placebo treatment. In some embodiments, the increase is relative to a patient who has received treatment with an anti-Aβ protofibril antibody. In some embodiments, the increase is relative to a patient who has received treatment with an anti-Aβ protofibril antibody and did not respond to said treatment (e.g., the subject is amyloid positive after treatment with the anti-Aβ protofibril antibody). In some embodiments, the increase is relative to a patient who has received treatment with an anti-Aβ protofibril antibody and did respond to said treatment (e.g., the subject is amyloid negative after treatment with the anti-Aβ protofibril antibody). In some embodiments, a subject demonstrating treatment efficacy is given a further dose of lecanemab. In some embodiments, a subject demonstrating treatment efficacy is moved to a Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 maintenance regimen, and / or if a change in an Aβ protofibril level is observed after 18 months of treatment. In some embodiments, a subsequent dose of treatment is given after the second sampling if a change, e.g., an increase in the Aβ protofibril level or in the rate of accumulation in the Aβ protofibril is detected. In some embodiments, an increase in the Aβ protofibril level and / or the rate of increase after receiving one or more doses of an anti Aβ protofibril antibody indicates disease progression. In some embodiments, this increase associated with disease progression is measured after a period of time when the patient has received treatment. In some embodiments, earlier changes in the Aβ protofibril level and / or the rate of increase may be used to measure treatment response, while later measurements may indicate disease progression. In some embodiments, the Aβ protofibril level of a subject treated with an anti-Aβ protofibril antibody, such as lecanemab, will change over the course of treatment with the anti- Aβ protofibril antibody. In some embodiments, the Aβ protofibril level of a subject treated with an anti-Aβ protofibril antibody, such as lecanemab, will increase at one point in time after administration of the anti-Aβ protofibril antibody, then decrease at a second point in time after administration of the anti-Aβ protofibril antibody. In some embodiments, the Aβ protofibril level of a subject treated with the anti-Aβ protofibril antibody will initially increase during treatment with the anti-Aβ protofibril antibody, then later change over time after subsequent treatment with the anti-Aβ protofibril antibody. Without being bound by theory, administration of an anti-Aβ protofibril antibody, such as lecanemab, may initially cause the migration of Aβ protofibrils (e.g., amyloid plaques) from the brain into the subject’s CSF, resulting in an initial increase in the level of soluble Aβ protofibril in the subject’s CSF after administration of anti-Aβ protofibril antibody. Anti-Aβ protofibril antibody treatment may bind and stabilize soluble Aβ protofibrils, Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 resulting in a longer half-life of the bound Aβ protofibrils that further leads to the increase in the level of soluble Aβ protofibril in the subject’s CSF during treatment with the anti-Aβ protofibril antibody. In some embodiments, an elevated Aβ protofibril level may be used to select a patient suitable for, and / or more likely to respond to, treatment with an anti-protofibril antibody such as lecanemab. In some embodiments, a subject with elevated Aβ protofibril level, e.g., as determined by the methods disclosed herein, is administered lecanemab. In some embodiments, an Aβ protofibril level may be used for selecting a patient for treatment (e.g., with an anti- protofibril antibody such as lecanemab). In some embodiments, an Aβ protofibril level may be used for determining if a patient is suitable for treatment with an anti-protofibril antibody such as lecanemab. In some further embodiments, the methods disclosed herein may be used to measure an Aβ protofibril level to monitor the effects of a treatment (e.g. treatment with an anti- protofibril antibody), and / or to alter a treatment regimen based on the changes in the Aβ protofibril level. In some embodiments, the methods disclosed herein may be used to monitor lecanemab efficacy and / or to alter a lecanemab treatment regimen based on the changes in the Aβ protofibril level. In some embodiments, the anti-Aβ protofibril antibody, e.g., lecanemab, is administered, e.g., intravenously, at a dose of 5 mg / kg to 25 mg / kg. In some embodiments, the anti-Aβ protofibril antibody is administered at a dose of 5 mg / kg to 20 mg / kg. In some embodiments, the anti-Aβ protofibril antibody is administered at a dose of 5 mg / kg to 15 mg / kg. In some embodiments, the anti-Aβ protofibril antibody is administered at a dose of 5 mg / kg to 10 mg / kg. In some embodiments, the anti-Aβ protofibril antibody is administered at a dose of 5 mg / kg, 10 mg / kg, 15 mg / kg, 20 mg / kg, 25 mg / kg, or 30 mg / kg. In some embodiments, the anti-Aβ protofibril antibody is administered at a dose of 10 mg / kg. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 In some embodiments, the anti-Aβ protofibril antibody, e.g., lecanemab, is administered, e.g., subcutaneously, at a dose of 300-1000 mg. In some embodiments, the dose is 300 mg to 800 mg, 300 mg to 400 mg, 400 mg to 500 mg, 400 mg to 450 mg, 450 mg to 500 mg, 500 mg to 600 mg, 500 mg to 550 mg, 550 mg to 600 mg, 600 mg to 700 mg, 600 mg to 650 mg, 650 mg to 700 mg, 700 mg to 800 mg, 700 mg to 750 mg, or 750 mg to 800 mg. In some embodiments, the dose is 300 mg, 310 mg, 320 mg, 330 mg, 340 mg, 350 mg, 360 mg, 370 mg, 380 mg, or 390 mg. In some embodiments, the dose is 400 mg, 410 mg, 420 mg, 430 mg, 440 mg, 450 mg, 460 mg, 470 mg, 480 mg, or 490 mg. In some embodiments, the dose is 500 mg, 510 mg, 520 mg, 530 mg, 540 mg, 550 mg, 560 mg, 570 mg, 580 mg, or 590 mg. In some embodiments, the dose is 600 mg, 610 mg, 620 mg, 630 mg, 640 mg, 650 mg, 660 mg, 670 mg, 680 mg, or 690 mg. In some embodiments, the dose is 700 mg, 710 mg, 720 mg, 730 mg, 740 mg, 750 mg, 760 mg, 770 mg, 780 mg, or 790 mg. In some embodiments, the dose is 800 mg, 820 mg, 840 mg, 860 mg, 880 mg, 900 mg, 920 mg, 940 mg, 960 mg, 980 mg, or 1000 mg. In some embodiments, the subcutaneous dose is 360 mg administered subcutaneously weekly. In some embodiments, the dose is 500 mg administered subcutaneously weekly. In some embodiments, the dose is 720 mg administered subcutaneously weekly. In some embodiments, one or more maintenance doses are administered after a treatment dosing period. In some embodiments, the maintenance dose is commenced after a period of time on the treatment dose, e.g., after 12 or 18 or 24 months. In some embodiments, the maintenance dose is commenced after a period of time on the treatment dose as determined by a change in the level of Aβ protofibrils in samples from the subject, e.g., as compared to samples taken before the start of the treatment period. In some embodiments, one or more maintenance doses are administered half as frequently (e.g., if a treatment dose of 10 mg / kg Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 biweekly is administered, the maintenance dose may be 10 mg / kg every four weeks or monthly). In some embodiments, the maintenance dose is half of the treatment dose. In some embodiments, the maintenance dose is 250-500 mg, e.g., administered subcutaneously. In some embodiments, the maintenance dose is 250 mg, 260 mg, 270 mg, 280 mg, 290 mg, 300 mg, 310 mg, 320 mg, 330 mg, 340 mg, 350 mg, 360 mg, 370 mg, 380 mg, or 390 mg. In some embodiments, the maintenance dose is 400 mg, 410 mg, 420 mg, 430 mg, 440 mg, 450 mg, 460 mg, 470 mg, 480 mg, 490 mg, or 500 mg. In some embodiments, the initial treatment dose is 720 mg administered subcutaneously weekly, followed by a maintenance dose of 360 mg administered subcutaneously weekly. In some embodiments, the initial treatment dose is 500 mg administered subcutaneously weekly, followed by a maintenance dose of 250 mg administered subcutaneously weekly. In some embodiments, a subject may start on an intravenously administered therapy, e.g., lecanemab administered at 10 mg / kg biweekly, and then switch to a subcutaneous maintenance dose, e.g., lecanemab administered at 250 mg. Kits In various embodiments, kits are disclosed herein, for detecting Aβ protofibrils in biological samples, comprising one or more of the capture antibodies or antigen binding fragments described herein and one or more of the detection antibodies or antigen binding fragments described herein. In certain embodiments, the kit comprises a capture antibody or antigen binding fragment comprising a heavy chain variable region and a light chain variable region, wherein the heavy chain variable region comprises three heavy chain complementarity determining regions (HCDR1, HCDR2, and HCDR3), and the light chain variable region comprises three light chain complementarity determining regions (LCDR1, LCDR2, and Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 LCDR3), wherein HCDR1 comprises the amino acid sequence of SEQ ID NO: 1, HCDR2 comprises the amino acid sequence of SEQ ID NO: 2, and HCDR3 comprises the amino acid sequence of SEQ ID NO: 3; and wherein LCDR1 comprises the amino acid sequence of SEQ ID NO: 4, LCDR2 comprises the amino acid sequence of SEQ ID NO: 5, and LCDR3 comprises the amino acid sequence of SEQ ID NO: 6. In certain embodiments, the capture antibody or antigen binding fragment comprises a heavy chain variable region and a light chain variable region, wherein the heavy chain variable region comprises an amino acid sequence of SEQ ID NO: 7 and the light chain variable region comprises an amino acid sequence of SEQ ID NO: 8. In some embodiments, the capture antibody comprises antibody BAN2401 or an antigen binding fragment thereof. In some embodiments, the kit comprises a capture antibody or antigen binding fragment immobilized to a surface, preferably a magnetic particle. In certain embodiments, the detection antibody or antigen binding fragment in the kit comprises a heavy chain variable region and a light chain variable region, wherein the heavy chain variable region comprises three heavy chain complementarity determining regions (HCDR1, HCDR2, and HCDR3), and the light chain variable region comprises three light chain complementarity determining regions (LCDR1, LCDR2, and LCDR3), wherein HCDR1 comprises the amino acid sequence of SEQ ID NO: 13, HCDR2 comprises the amino acid sequence of SEQ ID NO: 14, and HCDR3 comprises the amino acid sequence of SEQ ID NO: 15; and wherein LCDR1 comprises the amino acid sequence of SEQ ID NO: 16, LCDR2 comprises the amino acid sequence of SEQ ID NO: 17, and LCDR3 comprises the amino acid sequence of SEQ ID NO: 18. In certain embodiments, the detection antibody or antigen binding fragment comprises a heavy chain variable region and a light chain variable region, wherein the heavy chain variable region comprises an amino acid sequence of SEQ ID NO: 19 and the light Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 chain variable region comprises an amino acid sequence of SEQ ID NO: 20. In some embodiments, the detection antibody comprises antibody 3D6 or an antigen binding fragment thereof. In some embodiments, the kit comprises a detection antibody or antigen binding fragment conjugated to a tag, preferably a fluorescent tag, such as a fluorescent tag detectable by a single molecule counting instrument (e.g. SMCxPRO). In various embodiments, the kit comprises: an immobilized capture antibody selective for Aβ protofibrils, a fluorescently tagged detection antibody capable of binding Aβ protofibrils already bound to the capture antibody, sample dilution buffer, washing buffer(s), and dissociation buffers. In another embodiment, any of the preceding kits may further comprise tubes treated to reduce protein binding (e.g. Protein LoBind® Eppendorf for example 1.5 ml tubes). In some embodiments, the kit does not provide reagents for a SIMOA assay. In some embodiments, the kit includes a dissociation buffer. In some embodiments, the kit provides reagents for a SMCxPRO assay. In another embodiment, any of the preceding kits may further comprise instructions for using the one or more antibodies or antigen binding fragments to detect Aβ protofibrils in a sample from a subject according to the methods disclosed herein. The instructions may call for the sample to be cerebrospinal fluid or blood. The kits may further comprise additional components for use in sample collection and / or with a classic ELISA or a digital ELISA. EXAMPLES The present disclosure is further illustrated by the following examples that should not be construed as limiting. The contents of all references, patents, and published patent applications Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 cited throughout this application, as well as the figures, are incorporated herein by reference in their entirety for all purposes. Example 1. Aβ and biological fluid preparation Aβ1-42Protofibril Aβ1-42Protofibril (PF) was produced by the method below. Aβ1-42 Protofibril was made from Aβ1-42 aggregates by Size Exclusion Chromatography (SEC) purification. In brief, Aβ1-42peptide (AnaSpec, USA, Cat#64129) was dissolved in 10 mM NaOH to form 100 µM and then diluted to 50 µM with 2X PBS.50 µM solution was incubated at 37°C for 90 min to form the protofibrils. After the incubation, 10% Tween20 was added to form 0.6% Tween20 in solution and the protofibril solution was centrifuged at 16,000 g for 5 min at room temperature. The supernatant was collected for further purification by size exclusion chromatography to separate PF from small oligomer and monomer. Superdex 7510 / 600 GL (GE, Cat#1004-90) was used for the purification. Monitoring 214 nm, first peak which was eluted in void volume was collected as Aβ1-42 PF.6.5 µM Aβ1-42 PF solution was aliquoted in Protein LoBind tube 1.5 mL (Eppendorf, Germany) and stored at -80°C before use. Concentration of Aβ1-42 PF was determined using V-PLEX Aβ42 Peptide (4G8) Kit (Meso Scale Diagnostics, USA, Cat#K150-SLE) by the measurement of PF solution treated with 50% formic acid followed by neutralization of pH. 6.5 µM PF solution was further diluted to 100 nM with PBS+0.6% Tween20 and aliquoted as the working stock. Working stock was stored at -80°C until use. Aβ1-40, Aβ1-16peptide Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Lyophilized Aβ1-40peptide (Cat#4307-v) and Aβ1-16peptide (Cat#4359-v) were purchased from Peptide Institute, Inc. (Osaka, Japan). They were dissolved in 10 mM NaOH to make 100 µM solution. They were used for the evaluation of assay selectivity after appropriately diluted with Discovery Standard Diluent (EMD Millipore, USA, Cat#02-0560-00). Human brain homogenate Human brain tissues were purchased from TRANS-HIT BIOMARKERS (Canada).0.5 g of brain tissue was frozen with liquid nitrogen and crushed by MULTI-BEADS SHOCKER MB601(S) (Yasui Kikai, Japan). To the crushed tissue, buffer solution (1mM NaHCO3, 1mM MgCl2, 0.5mM CaCl2, 10% Sucrose, protease inhibitor cocktail (Nakalai Tesque, Japan, Cat#03969-21) and phosphatase inhibitor cocktail (Nakalai Tesque, Japan, Cat#07575-51) was added to make 200 mg tissue weight / mL homogenate. According to the protein concentration determined using Pierce 660 nm Protein Assay Kit (Thermofisher, USA, Cat#), each homogenate was diluted to 3 mg / mL protein concentration. The homogenate diluted to 3 mg / mL protein concentration was aliquoted and stored at - 80°C before use.

[0002] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Human cerebrospinal fluid Human cerebrospinal fluid (CSF) was purchased from PrecisionMed, LLC. (USA) and stored at -80°C before use, and avoided freeze-thaw cycling. In the case of sample preparation, spike recovery and dilution test, Protein LoBind 1.5 mL tube (Eppendorf, Germany) was used. CSF was thawed in room temperature and mixed gently by vortex mixer on the day of use. BAN2401 BAN2401103 mg / mL solution was aliquoted and stored at -80°C until use. Example 2. Conjugation and labeling of antibodies Preparation of BAN2401 immobilized magnetic particles BAN2401 was immobilized to Dynabeads MyOne Tosylactivated (Invitrogen, USA, Cat#65501) following to instructions of the beads, and used for the assay on SMCxPRO (EMD Millipore, USA). Briefly, Tosylactivated beads were washed by 0.1M sodium borate buffer (pH 9.5) several times. After initial wash, BAN2401 and 0.1M sodium borate buffer (pH 9.5) and 3M ammonium sulphate were added to the beads and then reaction mixture was incubated for 24 hour at 37°C with continuous rotation of the reaction tube, 20 µg of BAN2401 was added per 1 mg of beads. After the generation of covalent link between BAN2401 and the beads, supernatant was removed and blocking buffer (PBS with 0.5% BSA and 0.05% Tween20) was added to the remaining beads, and then reaction tube was further incubated for overnight at 37°C with continuous rotation. After blocking, the beads were washed several times with wash / storage buffer (PBS with 0.1% BSA and 0.05% Tween20) and then beads solution was prepared for 20 Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 mg / mL.20 mg / mL BAN2401 immobilized beads solution was used as stock solution and stored at 4°C until use. Preparation of dye-labeled 3D6 Anti Aβ N-terminal antibody clone 3D6 was purchased from ADx Neurosciences NV (Gent, Belgium). Labeling with fluorescent dye was conducted using SMC Detection Antibody Labeling Kit (EMD Millipore, USA, Cat# 03-0076-02) following the kit instruction. Concentration of the labeled antibody was determined from the absorbance at 280 nm. The dye labeled 3D6 was stored at 4°C until use. Example 3. Detection of Aβ1-42PF BAN2401 was covalently immobilized to the magnetic particles (MP), Dynabeads MyOne Tosylactivated (Invitrogen, USA) according to the instruction manual of the beads. In 96 well 500μL V-bottom plate (Axygen, USA, Cat# P-96-450V-C), BAN2401-bound MP solution which was diluted to 100 µg / mL in Discovery Assay Buffer (EMD Millipore, USA, Cat#02- 0474-00) was added at 50 µL to 150 µl of Aβ1-42PF which was diluted from 2 to 0.003 pM in Discovery Standard Diluent (EMD Millipore, USA, Cat#02-0560-00) followed by incubation for 2 h at 27°C with 800 rpm shaking on the plate shaker (BIOSAN LTD., Latvia, Cat# PST-60HL- 4). After washing with 10-fold diluted System / Wash Buffer w / Proclin (EMD Millipore, USA, Cat#02-0111-03) by the washer for SMC (EMD Millipore, USA, Cat#95-0004-05), Fluorescent- labeled detection antibody 3D6 was diluted with Discovery Assay Buffer to a final concentration of 1000 ng / mL and filtered through a 0.22 µm syringe filter (EMD Millipore, USA, Cat#SLGPR33RS) and the diluted antibody was added at 20 µL / well. The 96-well was incubated Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 for 1.5 hour at 27°C with 800 rpm shaking. After washing step, buffer was aspirated from well and 11.5 µL / well of Elution Buffer B (EMD Millipore, USA, Cat#02-0297-00) was added, followed by 10 min incubation at 27°C with 800 rpm shaking. After the 10 min incubation, 10 µL of eluate was collected and transferred to 384 glass bottom plate (Aurora, USA, Cat#ABB2- 00160A) which was filled with 10 µl / well of Buffer D (EMD Millipore, USA, Cat#02-0368-00). The plate was sealed with Nunc sealing tape for multiwell plates (Thermofisher Scientific, USA, Cat#276014) and then it was read by the SMCxPRO instrument. Response (readout of SMCxPRO) was obtained depending on Aβ1-42PF concentration even at very low level as tens of fM order (Fig.1). LLOQ (Lower Limit Of Quantification) and ULOQ (Upper Limit Of Quantification) of this assay were 0.01 pM and 2 pM, respectively. Example 4. Selectivity to PF, compared to monomer Aβ Aβ1-42 PF, Aβ1-40, and Aβ1-16 were measured on SMCxPRO. Each Aβ species was diluted with Discovery Standard Diluent (EMD, USA, Cat#02-0560-00) in Protein LoBind 1.5 mL tube (Eppendorf, Germany). Selectivity was estimated by dividing the interpolated values by the theoretical values based on an Aβ1-42 PF standard curve at comparable protein concentrations. Aβ1-16was used to represent complete monomer which includes epitopes of both BAN2401 and 3D6 (Englund et al. “Sensitive ELISA detection of amyloid-β protofibrils in biological samples”, Journal of Neurochemistry, 2007, 103, 334–345). In the preparation of Aβ1-40solution, formation of oligomerized Aβ species were not avoidable even amount of them was very small. Aβ1-16 was therefore more precisely reflect cross- reactivity to monomer of this PF assay rather than Aβ1-40. Calculated selectivity to PF was 3.2 or Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 2 million-fold greater than that to Aβ1-16. This assay was over 1,000,000-fold selective to PF compared to Aβ monomer (Fig.2). Example 5. Dilution linearity, spike recovery 100 nM PF stock was diluted with Discovery Standard Diluent to make 10 pM and 5 pM PF solution. Dilution linearity: 6.2 µL of 10 pM Aβ1-42 PF was spiked to 613.8 µL CSF and 2-fold serially diluted. The samples were measured on SMCxPRO. Spike Recovery: 6.2 µL of 5 pM Aβ1-42 PF or Discovery Standard Diluent was spiked to 303.8 µL CSF (Spiked in CSF and neat CSF).6.2 µL of 5 pM Aβ1-42PF was spiked to 303.8 µL Discovery Standard Diluent and measured. Measured value of the sample was PF spiked (Spiked in buffer). From neat to 8-fold dilution, good dilution linearity was obtained (Fig.3). Spike recovery was also good (Table 10). It was confirmed that this assay was not interfered from sample matrix. Example 6. BAN2401 interference 103 mg / mL BAN2401 was diluted with Discovery Standard Diluent (EMD Millipore, USA, Cat#02-0560-00) to the concentration of 25 µg / mL.6.2 µL of 25 µg / mL BAN2401 or Discovery Standard Diluent was spiked to 303.8 µL of CSF to make samples containing 0 or 500 ng / mL BAN2401. Samples were prepared using Protein LoBind 1.5 mL tube (Eppendorf, Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Germany). They were used for the assay on SMCxPRO and their measured values were compared. Addition of BAN2401 to CSF did not interfere the assay result (105% compared to BAN24010 ng / mL) (Fig.4). It was confirmed that this assay was not affected by the presence of 500 ng / mL BAN2401 in CSF. This indicates that the assay can be used to quantify Aβ PF in a sample from a subject administered lecanemab. Example 7. Measurement of brain homogenate Brain homogenate (N=12, 6 AD and 6 Control) was measured on SMCxPRO. Before measurement, each homogenate was centrifuged at 13,000 g for 10min and then supernatant was 5000-fold diluted with Discovery standard diluent (EMD Millipore, USA, Cat#02-0560-00). The diluted supernatant was used for the assay on SMCxPRO. Calibration curve was generated using Aβ1-42 PF. It was not statistically significant but brain homogenate from AD showed higher trend compared to that from control (Fig.5). Example 8. CSF measurement AD and Control CSF specimens purchased from PrecisionMed were measured on SMCxPRO. Annotations (AD or Control) were assigned by the vender based on symptoms. CSF were stored at -80°C before use. The samples were thawed and mixed on the day of use. CSF were used as neat for the measurement. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 All CSF specimens could be detected in the measurement range (0.01 pM-2 pM). Significant difference was not observed between AD and Control groups (Fig.6(A) and Fig. 6(B)). Example 9. Evaluation of anti-Aβ detection antibodies by sandwich ELISA In combination with BAN2401 as a capture antibody, detection antibodies were evaluated for Aβ1-42 PF detection by sandwich ELISA. Three anti Aβ N-terminal antibodies and two anti-Aβ 1-42 C-terminal antibodies were evaluated (Table 16). The N-terminal antibodies are 3D6 (ADx Neurosciences, Belgium), 82E1 (IBL, Japan) and 6E10 (Biolegend, USA). The C- terminal antibodies are 21F12 (ADx Neurosciences, Belgium) and H31L21 (Thermofisher Scientific, USA, Cat#700254). The detection antibodies were biotinylated using SIMOA®Homebrew Assay Development Kit (Quanterix, USA, Cat#101354) according to the kit instructions. BAN2401 was adjusted with 50 mM Tris / HCl (pH 7.5) to a concentration of 2 µg / mL, and then injected at 100 µl / well in a Nunc Maxisorp 96 well plate (Thermofisher Scientific, USA). The plate was sealed and stood for overnight at 4 ̊C. After the antibody solution was removed by aspiration, SuperBlock Blocking Buffer (Thermofisher Scientific, USA, Cat#37515) was injected at 250 µl / well, followed by the incubation at room temperature for 30 min. The plate was washed three times with wash buffer (PBS with 0.05% Tween20) and then series of Aβ1-42 protofibril solution were added at 50 µL / well and incubated for 1.5 hour at room temperature. After washing for three times, 1 µg / mL of the biotinylated detection antibodies were added at 100 µL / well and incubated for 1.5 hour at room temperature. After washing and 30 min incubation with Streptavidin-HRP (R&Dsystems, USA), 3,3',5,5'- Tetramethylbenzidine (TMB) Liquid Substrate System for ELISA (KPL, SeraCare Life Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Sciences) was injected at 100 µl / well and reacted at room temperature for 20 minutes. Reaction was terminated by the addition of 100 µL / well 1M H2SO4 and Optical Density (450 to 650 nm) was then measured by a spectrophotometer, SpectraMAX190 (Molecular Devices, USA). Aβ1-42PF, the biotinylated detection antibodies and Streptavidin-HRP were diluted in PBS with 0.5% BSA and 0.05% Tween20. The N-terminal antibodies showed higher signal compared to the C-terminal antibodies (Fig.7). Among the N-terminal antibodies, 3D6 was selected because it showed the highest signal in combination with BAN2401 as a capture antibody. Example 10. Comparison of antibody pairs, BAN2401 / 3D6 and BAN2401 / BAN2401 (MSD- ECL) Comparison of antibody pairs, BAN2401 / 3D6 and BAN2401 / BAN2401 was conducted on Meso Scale Discovery Electrochemiluminescence (MSD-ECL) platform (Fig.8(A) and Fig.8(B)). Biotinylated capture antibody and SULFO-TAG labeled detection antibody were used for MSD-ECL platform. Biotinylated BAN2401 was prepared using EZ-Link™ NHS- PEG4-Biotin (Thermo Fisher Scientific, USA, Cat# A39259) and SULFO-TAG labeled 3D6 and BAN2401 were prepared using MSD GOLD SULFO-TAG NHS-Ester Conjugation Pack (Meso Scale Discovery, USA, Cat#R31AA). The biotinylated capture antibody, the SULFO-TAG labeled detection antibody and Aβ1-42 protofibril were diluted with PBS + 0.5% BSA + 0.05% Tween20 and used.150 µL of blocking solution (1% BSA in PBS) was added to wells of biotinylated microplate, MSD GOLD 96-well Small Spot Streptavidin SECTOR Plate (Meso Scale Discovery, USA, Cat#L45SA) and the plate was incubated for 1 hour at room temperature. After 1 time wash with PBS+0.05% Tween20, 25 µL of 1 µg / mL biotinylated BAN2401 solution Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 was added to the wells and the plate was incubated for 1 hour at room temperature. The plate was washed 3 times with PBS+0.05% Tween20 and 25 µL of series of diluted Aβ protofibril standard were added to appropriate wells. After 1 hour incubation at room temperature and 3 times wash with PBS+0.05% Tween20, 25 µL of 1 µg / mL SULFO-TAG labeled 3D6 or BAN2401 were added and incubated for 1 hour at room temperature. The plate was washed 3 times with PBS + 0.05% Tween20 and 150 µL of 1:1 mixture of MSD Read Buffer T (4x) (Meso Scale Discovery, USA, Cat#R92TC) and milli-Q water was added to the wells. Electrochemiluminescence signal was read by MESO SECTOR S 600 (Meso Scale Discovery, USA, Cat# IC0AA-0). BAN2401 / 3D6 showed higher ECL signal compared to BAN2401 / BAN2401. BAN2401 / 3D6 pair is more optimal for MSD-ECL platform. Example 11. Comparison of antibody pairs, BAN2401 / 3D6 and BAN2401 / BAN2401 (SMCxPRO) Comparison of antibody pairs, BAN2401 / 3D6 and BAN2401 / BAN2401 was conducted on SMCxPRO platform (Fig.9). Assay procedure was following to the method described in elsewhere (Example 3).1000 ng / mL of fluorescent-labeled 3D6 or BAN2401 were used as detection antibody. Aβ1-42 PF was diluted with Discovery Standard Diluent (EMD Millipore, USA, Cat#02-0560-00) to make 0.1 pM and 1 pM solution. BAN2401 / 3D6 effectively worked for the detection of Aβ1-42 PF, compared to BAN2401 / BAN2401. BAN2401 / 3D6 showed 30 times (1 pM) and 12 times (0.1 pM) larger S / N ratio than BAN2401 / BAN2401. Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Example 12. Comparison of platform, BAN2401 / 3D6 on SMCxPRO®and SIMOA®Comparison of platform, SMCxPRO® and SIMOA® was conducted using the same antibody pair, BAN2401 / 3D6. For SMCxPRO® assay, BAN2401 coated magnetic beads and fluorescent dye labeled 3D6 were prepared following the procedure described elsewhere (Example 2) and measurement was conducted following the method previously described (Example 3). For SIMOA® assay, BAN2401 coated beads and biotinylated 3D6 were prepared using SIMOA® Homebrew Assay Development Kit (101354, Quanterix, Billerica, MA, USA) according to the manufacturer’s instructions. BAN2401 coated beads was diluted with Beads Diluent Buffer (Quanterix, USA, Cat#101362), biotinylated 3D6 and Aβ1-42 protofibril were diluted with Homebrew Detector / Sample Diluent (Quanterix, USA, Cat#101359). The 25 µL of BAN2401 beads solution and 100 µL of diluted Aβ1-42 protofibril were added to the microplate, SIMOA® 96-well Assay Plate (Quanterix, USA, Cat#101457) and incubated for 30 min at 30°C with 800 rpm shaking. After incubation, the plate was washed with Wash Buffer A (Quanterix, USA, Cat#103078) twice on the plate washer (BioTek, USA, Cat#405TSRVS) following the washing procedure installed by Quanterix. After washing, 100 µL of 0.3µg / mL biotinylated 3D6 was added to wells and incubated for 10 min at 30°C with 800 rpm shaking. After second reaction, the plate was washed with Wash Buffer A twice on the plate washer. After second wash, 100 µL of 150 pM SBG solution (SBG concentrate was diluted with SBG Diluent) (101361; SIMOA Enzyme and Substrate Kit, Quanterix, Billerica, MA, USA) was added to the plate and incubated for 10 min at 30°C with 800 rpm shaking. After SBG incubation, the plate was washed with Wash Buffer A twice on the plate washer, and then the plate was washed with Wash Buffer B (Quanterix, USA, Cat#103079) twice on the plate washer. The plate after wash Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 was set on SIMOA SR-X analyzer (Quanterix, Billerica, MA, USA, Cat#102917) to read AEB (signal of SIMOA platform). Compared to SIMOA platform, SMCxPRO showed better S / N ratio for the measurement of Aβ1-42 protofibril (Fig.10(A) and Fig.10(B)). Example 13. Selection of magnetic beads for sensitive detection of Aβ1-42 PF on SMCxPRO Dynabeads MyOne Tosylactivated (Invitrogen, USA, Cat#65501) was compared to the beads supplied as default beads for SMCxPRO platform by the vendor. Coating of BAN2401 to the default beads for SMCxPRO (SMC_BAN2401) was conducted using SMC™ Capture Labeling Kit (EMD Millipore, USA, Cat# 03-0077-02). Coating of BAN2401 to Dynabeads MyOne Tosylactivated (TA_BAN2401) was conducted by following the procedure described in elsewhere (Example 2). Human IgG1 (Sigma, USA, Cat#I5154) was also used to immobilize on SMC default beads (SMC_IgG) and Dynabeads MyOne Tosylactivated (TA_IgG), conjugation method was the same for BAN2401 beads. BAN2401 beads and IgG1 beads were used for human CSF measurement on SMCxPRO. Compared to SMC default beads, Dynabeads MyOne Tosylactivated beads showed higher signal in human CSF measurement without increasement of non-specific signal from IgG1 beads (Fig.11 and Table 11). Example 14. Selecting antibody amount on beads coating procedure (SMCxPRO) To optimize antibody immobilization procedure, antibody amount during coating reaction was investigated. Coating procedures were same for 3 reaction conditions except Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 antibody / beads ratio in the immobilization reaction. Evaluated conditions were 40 µg BAN2401 / mg magnetic beads, 20 µg BAN2401 / mg magnetic beads and 10 µg BAN2401 / mg magnetic beads. Coating procedure was the same as described elsewhere (Example 2). Aβ1-42 protofibril and human CSF were measured with each BAN2401 coated beads according to the method described in elsewhere (Example 3). Discovery Standard Diluent was measured as buffer blank. 20 µg BAN2401 per 1 mg magnetic beads was selected for the preparation procedure judging from S / N ratio in PF standard and CSF measurement (Fig.12(A) and Fig.12(B) and Table 12). Example 15. Clinical uses of the protofibril assay Example 15a. Clinical diagnosis and staging of AD A biological sample will be obtained from a patient and the Aβ protofibril level will be measured and used for diagnostic purposes such as clinical staging of AD disease (e.g., Alzheimer’s disease, Pre-AD, or early Alzheimer’s disease). Specifically, the Aβ protofibril levels will be measured and compared to the levels Aβ protofibril measured in age matched control amyloid-negative subjects, cognitively impaired amyloid negative subjects, cognitively unimpaired amyloid positive subjects, mild cognitive impairment amyloid positive subjects, and young normal control amyloid negative subjects. The Aβ protofibril levels in a subject will be used alone or in conjunction with one or more additional criteria, such as one or more biomarkers and / or cognitive tests, to diagnose and / or provide clinical staging of a patient having or suspected of having AD. Example 15b. AD disease monitoring Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 The Aβ protofibril levels in a subject will be measured at different time points in the same patient over the course of weeks, months, and years and correlated with changes in the disease burden and / or progression over time in a patient having or suspected of having AD. The change in Aβ protofibril levels in a subject over time will be used to determine the rate of AD disease progression. The Aβ protofibril levels in a subject will be measured in conjunction with one or more additional criteria, such as one or more biomarkers and / or cognitive tests, to monitor clinical progression of a patient having or suspected of having AD. Example 16. Analysis of CSF Aβ-PF levels in a placebo-controlled clinical study of lecanemab. CSF samples collected from the 410 subjects (Placebo=207, Lecanemab=203) had a CSF sample at baseline and 253 subjects (Placebo=127, Lecanemab=126) had a CSF sample at baseline and at least one post-baseline sample. Table 17 provides patient demographic data for the study, including the overall study population (n=875 and n=859) and subjects who had a CSF sample at baseline (n=207 and n=203). Other biomarker assessments including amyloid-PET, and CSF biomarkers including neurogranin were described previously and used for correlation analyses. Baseline CSF Aβ-PF showed significant correlation with brain amyloid plaques, measured with amyloid-PET (Pearson correlation: r=-0.202, p=0.0005; Fig 13A). Baseline CSF Aβ-PF showed significant correlation with neurogranin, a biomarker of synaptic dysfunction (Pearson correlation: r=0.153, p=0.0127; Fig 13B). In the placebo arm, there was an increase from baseline in CSF Aβ-PF levels which significantly correlated with the increase from baseline observed in CSF neurogranin levels (Table18). In the lecanemab-treatment arm, the CSF total Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Aβ-PF levels (free and bound) increased relative to baseline at each of the post-baseline assessments (p=0.0126 at 12 months and numerically higher at 18 month; Fig.14A and Table 19). Additional analysis showed the significant correlation of the change in CSF PF with other AD biomarkers from baseline to 18 months in subjects treated with placebo, while lecanemab- treated subjects did not experience a significant correlation between CSF PF levels and other AD biomarkers (Table 18). In the subjects achieving amyloid-negativity defined as Centiloid<30 at 18 months, the lecanemab-treatment group demonstrated a larger percent change from baseline compared with subjects who remained amyloid-positive (Centiloid> / =30 at 18 months) (Fig.14B and Table 20).

[0003] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 TABLES Table 1. Amino acid sequences of monoclonal antibody (mAb) CDRs mAb IgG chain SEQ ID NO Amino acid sequence BAN2401 HCDR1 1 SFGMH HCDR2 2 YISSGSSTIYYGDTVKG HCDR3 3 EGGYYYGRSYYTMDY BAN2401 LCDR1 4 RSSQSIVHSNGNTYLE LCDR2 5 KVSNRFS LCDR3 6 FQGSHVPPT Table 2. Amino acid sequences of mAb variable regions (CDRs shown in bold) mAb IgG chain SEQ ID NO Amino acid sequence BAN2401 Heavy chain 7 EVQLVESGGGLVQPGGSLRLSCSASGFTFSSFGMH variable region WVRQAPGKGLEWVAYISSGSSTIYYGDTVKGRF TISRDNAKNSLFLQMSSLRAEDTAVYYCAREGGY YYGRSYYTMDYWGQGTTVTVSS BAN2401 Light chain 8 DVVMTQSPLSLPVTPGAPASISCRSSQSIVHSNGN variable region TYLEWYLQKPGQSPKLLIYKVSNRFSGVPDRFSG SGSGTDFTLRISRVEAEDVGIYYCFQGSHVPPTFG PGTKLEIK

[0004] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 3. Amino acid sequences of mAb heavy and light chains mAb IgG chain SEQ ID NO Amino acid sequence BAN2401 Heavy chain 9 EVQLVESGGGLVQPGGSLRLSCSASGFTFSSFGMHWVRQAPGKGLEWVAYISSGSSTIYYGDTVKGRFTISRDNAKNSLFLQMSSLRAEDTA VYYCAREGGYYYGRSYYTMDYWGQGTTV TVSSASTKGPSVFPLAPSSKSTSGGTAALGCL VKDYFPEPVTVSWNSGALTSGVHTFPAVLQS SGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT KVDKRVEPKSCDKTHTCPPCPAPELLGGPSVFL FPPKPKDTLMISRTPEVTCVVVDVSHEDPEVK FNWYVDGVEVHNAKTKPREEQYNSTYRVV SVLTVLHQDWLNGKEYKCKVSNKALPAPIE KTISKAKGQPREPQVYTLPPSREEMTKNQVS LTCLVKGFYPSDIAVEWESNGQPENNYKTTP PVLDSDGSFFLYSKLTVDKSRWQQGNVFSC SVMHEALHNHYTQKSLSLSPGK BAN2401 Light chain 10DVVMTQSPLSLPVTPGAPASISCRSSQSIVHSNGNTYLEWYLQKPGQSPKLLIYKVSNRFSGVPDRFSGSGSGTDFTLRISRVEAEDVGIYYCF QGSHVPPTFGPGTKLEIKRTVAAPSVFIFPPSDE QLKSGTASVVCLLNNFYPREAKVQWKVDNAL QSGNSQESVTEQDSKDSTYSLS STLTLSKADYEKHKVYACEVTHQGLSSPVT KSFNRGEC

[0005] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 4. Amino acid sequences of mAb constant regions mAb IgG chain Class SEQ ID NO Amino acid sequence BAN2401 Heavy chain IgG1 11 ASTKGPSVFPLAPSSKSTSGGTAALGCLV KDYFPEPVTVSWNSGALTSGVHTFPAVL QSSGLYSLSSVVTVPSSSLGTQTYICNVN HKPSNTKVDKRVEPKSCDKTHTCPPCPA PELLGGPSVFLFPPKPKDTLMISRTPEVT CVVVDVSHEDPEVKFNWYVDGVEVHN AKTKPREEQYNSTYRVVSVLTVLHQDW LNGKEYKCKVSNKALPAPIEKTISKAKG QPREPQVYTLPPSREEMTKNQVSLTCLV KGFYPSDIAVEWESNGQPENNYKTTPPV LDSDGSFFLYSKLTVDKSRWQQGNVFSC SVMHEALHNHYTQKSLSLSPGK BAN2401 Light chain kappa 12 RTVAAPSVFIFPPSDEQLKSGTASVVCLL NNFYPREAKVQWKVDNALQSGNSQESV TEQDSKDSTYSLSSTLTLSKADYEKHKV YACEVTHQGLSSPVTKSFNRGEC

[0006] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 5. Amino acid sequences of mAb CDRs mAb IgGchainSEQ ID NO Amino acid sequence3D6 HCDR1 13 NYGMS HCDR2 14 SIRSGGGRTYYSDNVKG HCDR3 15 YDHYSGSSDY 3D6 LCDR1 16 KSSQSLLDSDGKTYLN LCDR2 17 LVSKLDS LCDR3 18 WQGTHFPRT Table 6. Amino acid sequences of mAb variable regions (CDRs shown in bold) mAb IgG chain SEQ ID NO Amino acid sequence 3D6 Heavy chain 19 E VKLVESGGGL VKPGASLKLS variable region CAASGFTFSNYGMSWVRQNSDKRLEWVA SIRSGGGRTYYSDNVKGRFTIS RENAKNTLYL QMSSLKSEDT ALYYCVRYDH YSGSSDYWGQ GTTVTVSS 3D6 Light chain 20 YVVMTQTPLTLSVTIGQPAS variable region ISCKSSQSLLDSDGKTYLNWLLQRPGQSPK RLIYLVSKLDSGVPDRFTGS GSGTDFTLKISRIEAEDLGLYYCWQGTHFP RTFGGGTKLE IK

[0007] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 9. Amino acid sequences of Amyloid β Amyloid β SEQ ID NO Amino acid sequence Amyloid β 1-42 (Aβ1-42) 25 DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGL MVGGVVIA Amyloid β 1-40 (Aβ1-40) 26 DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGL MVGGVV Amyloid β 1-16 (Aβ1-16) 27 DAEFRHDSGYEVHHQK Amyloid β 1-5 (Aβ1-5) 28 DAEFR Amyloid β Precursor29MLPGLALLLLAAWTARALEVPTDGNAGLLAEPQProtein (APP)IAMFCGRLNMHMNVQNGKWDSDPSGTKTCIDTKEGILQYCQEVYPELQITNVVEANQPVTIQNWCK RGRKQCKTHPHFVIPYRCLVGEFVSDALLVPDKC KFLHQERMDVCETHLHWHTVAKETCSEKSTNLH DYGMLLPCGIDKFRGVEFVCCPLAEESDNVDSA DAEEDDSDVWWGGADTDYADGSEDKVVEVAE EEEVAEVEEEEADDDEDDEDGDEVEEEAEEPYEE ATERTTSIATTTTTTTESVEEVVREVCSEQAETGP CRAMISRWYFDVTEGKCAPFFYGGCGGNRNNFD TEEYCMAVCGSAMSQSLLKTTQEPLARDPVKLP TTAASTPDAVDKYLETPGDENEHAHFQKAKERL EAKHRERMSQVMREWEEAERQAKNLPKADKKA VIQHFQEKVESLEQEAANERQQLVETHMARVEA MLNDRRRLALENYITALQAVPPRPRHVFNMLKK YVRAEQKDRQHTLKHFEHVRMVDPKKAAQIRS QVMTHLRVIYERMNQSLSLLYNVPAVAEEIQDE VDELLQKEQNYSDDVLANMISEPRISYGNDALM PSLTETKTTVELLPVNGEFSLDDLQPWHSFGADS VPANTENEVEPVDARPAADRGLTTRPGSGLTNIK TEEISEVKMDAEFRHDSGYEVHHQKLVFFAEDV GSNKGAIIGLMVGGVVIATVIVITLVMLKKKQYT SIHHGVVEVDAAVTPEERHLSKMQQNGYENPTY KFFEQMQN Table 10. Spike recovery Spiked in buffer Spiked in CSF neat CSF Recovery (measured) (measured) (measured) 0.0953 0.1156 0.0161 104.4% Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 11. S / N ratio Beads Human IgG1BAN2401 Tosylactivated 2.06 5.15 SMC 2.42 3.07 Table 12. S / N ratio for 3 different beads in PF standard and human CSF measurement Sample 40 mg 20 mg 10 mg 0.003 pM std 1.47 1.92 1.20 0.01 pM std 1.71 3.00 2.11 CSF 3.11 5.57 3.07

[0008] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 13. Amino acid sequences of mAb variable regions (CDRs shown in bold) mAb IgG chain SEQ ID NO Amino acid sequence 82E1 Heavy chain 30EVKLVESGGGSVKPGGSLKVSCAASGFIFSNYGMSWVRQTPEKSLEWVASISRGGSTFYSDRVKGRFTISRENGRNILYLQMNSLRSEDTA IYYCVRYDYDEGATDYWGQGTTLTVSS 82E1 Light chain 31DWMTQTPLNLSVTIGQPASISCKSSQSLLDRDGKTYLNWLFQRPGQSPKRLIYLVSKLDSGVPDRFTGSGSGTDFTLKISRVE AEDLGVYYCWQGTHFPRTFG GGTKLEIK Table 14. Amino acid sequences of mAb variable regions (CDRs in bold) mAb IgG chain SEQ ID NO Amino acid sequence 6E10 Heavy chain 32EVQLQQSGAE LVKPGASVKLCTASGFNIKD TYIHWVKQRPEQGLEWIGRF DPVNVNTRYD SRFRGKATIT SDASSNTAYLH LNSLTSEDTA VYYCSRSYYN GRRRFTYWGQ GTLVTVSA 6E10 Light chain 33DIVMTQSPSS LTVTAGEKVALTCKASQSLLS SGNQKNYLTWYQQKPGQPPK LLIYWASIRE SGVPDRFTGS GSGTFFTLTI SSVQAEDLAVY YCQNDYNYPF TFGSGTKLEI K Table 15. Amino acid sequences of mAb variable regions (CDRs shown in bold) mAb IgG chain SEQ ID NO Amino acid sequence 21F12 Heavy chain 34EVQLQQSGPELLKPGASVKISCKASGFTFTDYTMHWMKQSHGKSLEWIGGI NPNSGGTIYN EKFKDKATLT VDKSSRTAYM ELRSLTSEDS AVYFCTRGVY DGYFYWGQGT LVTVSA 21F12 Light chain 35D VVMTQTPLSL PVSLGDQASISCRSSQSLVY SNGNTFLHWYLQKPGQSPKL LIYKVSTRFS GVPDRFSGSG SGSDFTLKIS RVEAEDLGIY FCSQTTHAPF TFGSGTKLAI R Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 16. Epitopes of anti-Aβ antibodies Clone Vendor Epitope 3D6 ADx NeuroSciences NV Aβ1-5 82E1 Immuno-Biological Laboratories Co., Ltd. Aβ1-5 6E10 Biolegend, Inc Aβ3-8 21F12 ADx NeuroSciences NVC-term of Aβ42(37-42)H31L21 Thermo Fisher Scientific C-term of Aβ42 Table 17. Subject Demographics and Baseline Characteristics Table 18. Correlation of CSF PF and AD biomarker changes from baseline to 18 months in subjects treated with lecanemab or placebo (Pearson r, *: P<0.05) 18 months Placebo LEC10-BW CSF PF vs Change %Change Change %Change CSF total tau 0.138 0.286* 0.066 0.206 CSF p-tau181 0.145 0.304* -0.001 0.158 CSF neurogranin 0.158 0.262* 0.172 0.173 CSF MTBR-tau243 0.075 0.252* 0.184 0.123 Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Table 19. Percent change in CSF PF from baseline in lecanemab treated subjects Treatment Month Adjusted Mean Lower SE Upper SE (% Change from baseline) Placebo 0 0.000 0.000 0.000 Placebo 12 19.091 4.333 33.849 Placebo 18 29.421 15.650 43.191 10 mg / kg bi-weekly 0 0.000 0.000 0.0000 10 mg / kg bi-weekly 12 59.432 44.112 74.752 10 mg / kg bi-weekly 18 45.137 30.726 59.549 Table 20. Percent change in CSF PF from baseline in subjects reaching amyloid negativity. Treatment Month Adjusted Mean (% Change from baseline) Placebo 0 0.000 Placebo 12 3.968 Placebo 18 2.702 LEC+ 0 0.000 LEC+ 12 50.253 LEC+ 18 21.898 LEC- 0 0.000 LEC- 12 82.427 LEC- 18 59.516

[0009] Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 Abbreviations Aβ amyloid beta Aβ(1-42) amyloid beta monomer from amino acid 1 to 42 AD Alzheimer’s disease CDR Complementarity Determining Region LC-MS / MS liquid chromatography – tandem mass spectrometry

Claims

Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 CLAIMS We claim:

1. A method of treating a subject having or suspected of having or at risk for developing Alzheimer’s disease (AD) or mild cognitive impairment (MCI) with an anti-Aβ protofibril antibody, comprising: obtaining a first biological sample from the subject prior to receiving the anti-Aβ protofibril antibody (i.e., a baseline sample); obtaining subsequent samples after treatment with the anti-Aβ protofibril antibody; measuring concentrations of Aβ protofibrils in the first and the subsequent samples; and continuing to administer the anti-Aβ protofibril antibody (e.g., lecanemab) if a change, e.g., an increase in Aβ protofibrils is detected between the first and subsequent samples; wherein the concentrations of Aβ protofibrils in the samples are measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody;Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample.

2. The method of claim 1, wherein a change in Aβ protofibrils between the first and subsequent samples is used to determine the treatment dose of anti-Aβ protofibril antibody.

3. The method of claim 1, wherein the subject is administered the same or increased dose of the anti-Aβ protofibril antibody in a subsequent dose if an increase in Aβ protofibrils between the first and a subsequent sample is detected.

4. The method of claim 1, wherein the subject is administered a maintenance dose of the anti-Aβ protofibril antibody if an increase in Aβ protofibrils between the first and a subsequent sample is detected.

5. The method of any one of claims 1 to 4, wherein the subsequent samples are obtained after 12-months of treatment with the anti-Aβ protofibril antibody.

6. The method of any one of claims 1 to 4, wherein the subsequent samples are obtained after 18-months of treatment with the anti-Aβ protofibril antibody.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 7. The method of any one of claims 1 to 6, wherein the subsequent samples are obtained after 12-months and 18-months of treatment with the anti-Aβ protofibril antibody.

8. The method of claim 1, wherein the rate of change in Aβ protofibril levels from baseline to 12 months is measured in a subject.

9. The method of claim 1, wherein the rate of change in Aβ protofibril levels from baseline to 18 months is measured in a subject.

10. The method of any one of claims 1 to 9, wherein the method further comprises comparing the Aβ protofibril level relative to a control, wherein the anti-Aβ protofibril antibody is continued to be administered if the Aβ protofibril level or the rate of change in the Aβ protofibril level is increased relative to the control.

11. The method of claim 10, wherein the control is a subject with AD or MCI who has not been treated with an anti-Aβ protofibril antibody.

12. The method of claim 10, wherein the control is a subject with AD or MCI who has been treated with a placebo.

13. A method of treating a subject having or suspected of having or at risk for developing AD or MCI with an anti-Aβ protofibril antibody, comprising:Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 obtaining a biological sample from the subject prior to receiving the anti-Aβ protofibril antibody; measuring a concentration of Aβ protofibrils in the sample; and administering the anti-Aβ protofibril antibody if there is an elevated Aβ protofibril level relative to a control; wherein the concentration of Aβ protofibrils in the sample is measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample.

14. The method of claim 13, wherein the control comprises an Aβ protofibril measurement in a biological sample obtained from a subject without AD, an average Aβ protofibrilEisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 measurement in biological samples obtained from a group of age-matched subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of young healthy subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of amyloid negative subjects with mild cognitive impairment, or an average Aβ protofibril measurement in biological samples obtained from a group of amyloid positive subjects without MCI.

15. The method of claim 13, wherein the control Aβ protofibril measurement is a baseline measurement in a baseline sample obtained from the subject 6-18 months prior to the biological sample.

16. A method monitoring a patient treated with an anti-Aβ protofibril antibody comprising: obtaining a first biological sample from the subject prior to receiving the anti-Aβ protofibril antibody and a second biological sample from the subject after receiving the anti-Aβ protofibril antibody; and measuring concentrations of Aβ protofibrils in the samples; wherein the concentrations of Aβ protofibrils in the sample are measured using an Aβ protofibril assay, comprising contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQEisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; and detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and wherein a change, e.g., an increase, in the Aβ protofibril level in the second sample relative to the first sample indicates treatment efficacy.

17. The method of claim 16, wherein the second sample is obtained after 12-months of treatment with the anti-Aβ protofibril antibody.

18. The method of claim 16, wherein the second sample is obtained after 18-months of treatment with the anti-Aβ protofibril antibody.

19. The method of claim 16, wherein the rate of change in Aβ protofibril levels from baseline to 12 months is measured in a subject.

20. The method of claim 16, wherein the rate of change in Aβ protofibril levels from baseline to 18 months is measured in a subject.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 21. The method of any one of claims 16 to 21, wherein the method further comprises comparing the Aβ protofibril level relative to a control, wherein the anti-Aβ protofibril antibody is continued to be administered if the Aβ protofibril level or the rate of change in the Aβ protofibril level is increased relative to the control.

22. The method of claim 21, wherein the control is a subject with AD or MCI who has not been treated with an anti-Aβ protofibril antibody.

23. The method of claim 21, wherein the control is a subject with AD or MCI who has been treated with a placebo.

24. A method of diagnosing a subject with AD, comprising: obtaining a biological sample from the subject; and measuring a concentration of Aβ protofibrils in the sample; wherein the concentration of Aβ protofibrils in the sample is measured using an Aβ protofibril assay, comprising contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex;Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; and detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and wherein an elevated Aβ protofibril level relative to a control indicates that the subject has AD.

25. The method of claim 24, wherein the control comprises an Aβ protofibril measurement in a biological sample obtained from a subject without AD, an average Aβ protofibril measurement in biological samples obtained from a group of age-matched subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of young healthy subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of amyloid negative subjects with mild cognitive impairment, or an average Aβ protofibril measurement in biological samples obtained from a group of amyloid positive subjects without MCI.

26. The method of claim 24, wherein the control Aβ protofibril measurement is a baseline measurement in a baseline sample obtained from the subject 6-18 months prior to the biological sample.

27. A method of determining an AD clinical stage of a subject, comprising: obtaining a biological sample from the subject; andEisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 measuring a concentration of Aβ protofibrils in the sample; wherein the concentration of Aβ protofibrils in the sample is measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; and detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and wherein the Aβ protofibril level indicates the AD clinical stage of the subject.

28. The method of claim 27, wherein the Aβ protofibril level is compared to an Aβ protofibril level in a biological sample from a control.

29. The method of claim 28, wherein the control comprises an Aβ protofibril measurement in a biological sample obtained from a subject without AD, an average Aβ protofibril measurement in biological samples obtained from a group of age-matched subjectsEisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 without AD, an average Aβ protofibril measurement in biological samples obtained from a group of young healthy subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of amyloid negative subjects with mild cognitive impairment, or an average Aβ protofibril measurement in biological samples obtained from a group of amyloid positive subjects without MCI.

30. The method of claim 28, wherein the control Aβ protofibril measurement is a baseline measurement in a baseline sample obtained from the subject 6-18 months prior to the biological sample.

31. A method of identifying a subject suitable for treatment with an anti-Aβ protofibril antibody, comprising: obtaining a biological sample from the subject; and measuring a concentration of Aβ protofibrils in the sample; wherein the concentration of Aβ protofibrils in the sample is measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex;Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and wherein an elevated Aβ protofibril level relative to a control indicates that the subject is suitable for treatment with an anti-Aβ protofibril antibody.

32. The method of claim 31, wherein the control comprises an Aβ protofibril measurement in a biological sample obtained from a subject without AD, an average Aβ protofibril measurement in biological samples obtained from a group of age-matched subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of young healthy subjects without AD, an average Aβ protofibril measurement in biological samples obtained from a group of amyloid negative subjects with mild cognitive impairment, or an average Aβ protofibril measurement in biological samples obtained from a group of amyloid positive subjects without MCI.

33. The method of claim 31, wherein the control Aβ protofibril measurement is a baseline measurement in a baseline sample obtained from the subject 6-18 months prior to the biological sample.

34. A method detecting progression of Alzheimer’s disease (AD) comprising:Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 obtaining a first biological sample from a subject and a second biological sample from the subject at a later period in time; and measuring concentrations of Aβ protofibrils in the samples; wherein the concentrations of Aβ protofibrils in the sample are measured using an Aβ protofibril assay, comprising contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; and detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and wherein a change, e.g., an increase, in the Aβ protofibril level in the second sample relative to the first sample indicates AD progression.

35. The method of claim 34, wherein no change in the Aβ protofibril level in the second sample relative to the first sample indicates stable AD disease.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 36. The method of any one of claims 1 to 35, wherein contacting the biological sample with the capture antibody forms a first immune complex and contacting the first immune complex with the detection antibody forms a second immune complex.

37. The method of any one of claims 1 to 36, wherein the anti-Aβ protofibril capture antibody comprises a heavy chain variable region comprising SEQ ID NO: 7 and a light chain variable region comprising SEQ ID NO:

8.

38. The method of any one of claims 1 to 37, wherein the anti-Aβ protofibril capture antibody comprises a heavy chain region comprising SEQ ID NO: 9 and a light chain region comprising SEQ ID NO:

10.

39. The method of any one of claims 1 to 38, wherein the anti-Aβ protofibril capture antibody comprises a heavy chain constant region comprising SEQ ID NO: 11 and a light chain constant region comprising SEQ ID NO:

12.

40. The method of any one of claims 1 to 39, wherein the anti-Aβ protofibril capture antibody comprises lecanemab.

41. The method of any one of claims 1 to 40, wherein the label on the anti-Aβ detection antibody comprises a tag, wherein the tag comprises a protein tag, a fluorescent tag, a quantum dot tag, an aptamer tag, an oligonucleotide tag, a SULFO-TAG, or a biotin tag.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 42. The method of any one of claims 1 to 41, wherein the anti-Aβ detection antibody binds a region of Aβ that does not overlap a region bound by lecanemab, and / or wherein the anti- Aβ detection antibody does not compete for binding to Aβ protofibrils with lecanemab.

43. The method of any one of claims 1 to 42, wherein the anti-Aβ detection antibody binds to an N-terminus of an Aβ, e.g. Aβ1-5.

44. The method of any one of claims 1 to 43, wherein the anti-Aβ detection antibody comprises heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 13 (HCDR1), SEQ ID NO: 14 (HCDR2), and SEQ ID NO: 15 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ ID NO:16 (LCDR1), SEQ ID NO:17 (LCDR2), and SEQ ID NO: 18 (LCDR3).

45. The method of any one of claims 1 to 44, wherein the anti-Aβ detection antibody comprises a heavy chain variable region comprising SEQ ID NO: 19 and a light chain variable region comprising SEQ ID NO:

20.

46. The method of any one of claims 1 to 45, wherein the anti-Aβ detection antibody comprises 3D6.

47. The method of any one of claims 1 to 41, wherein the anti-Aβ detection antibody comprises lecanemab.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 48. The method of any one of claims 1 to 47, wherein the biological sample comprises a body fluid or tissue.

49. The method of claim 48, wherein the biological sample comprises cerebrospinal fluid, whole blood, plasma, serum, brain tissue homogenate, or brain tissue lysate.

50. The method of any one of claims 1 to 49, wherein the biological sample is stored at -80°C prior to contacting the sample with the capture antibody.

51. The method of any one of claims 1 to 50, wherein the method does not comprise repetitive freeze-thaw cycles of the biological sample.

52. The method of any one of claims 1 to 51, further comprising thawing the frozen biological sample and gently mixing and / or vortexing.

53. The method of any one of claims 1 to 52, wherein the biological sample is prepared and stored in low protein-binding tubes.

54. The method of any one of claims 1 to 53, wherein the biological sample is a brain homogenate.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 55. The method of any one of claims 1 to 54, wherein the biological sample is at a concentration 0.1-10 mg of protein per mL of sample.

56. The method of claim 55, wherein the biological sample is at a concentration of 1 mg of protein per mL of sample.

57. The method of any one of claims 1 to 56, wherein the biological sample is diluted 2-fold, optionally in a sample buffer.

58. The method of any one of claims 1 to 56, wherein the biological sample is a CSF sample and optionally the CSF sample is not diluted.

59. The method of any one of claims 1 to 56, wherein the biological sample is a CSF sample diluted 2-fold, optionally in a sample buffer.

60. The method of any one of claims 1 to 56, wherein the biological sample is a blood sample and optionally the blood sample is not diluted.

61. The method of any one of claims 1 to 56, wherein the biological sample is a blood sample diluted 2-fold, optionally in a sample buffer.

62. The method of any one of claims 1 to 61, wherein the anti-Aβ protofibril capture antibody is immobilized on a surface.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 63. The method of claim 62, wherein the surface is a plate, a bead, or a particle, optionally a magnetic bead or particle.

64. The method of claim 63, wherein the particle is a tosylactivated magnetic bead.

65. The method of claim 63 or claim 64, wherein the capture antibody is conjugated to the bead at a concentration of 10-40 µg of antibody per mg of bead.

66. The method of claim 65, wherein the capture antibody is conjugated to the bead at a concentration of 20 µg of antibody per mg of bead.

67. The method of claim 65, wherein the capture antibody is conjugated to the bead at a concentration of 20 µg antibody per mg of tosylactivated magnetic bead.

68. The method of any one of claims 1 to 67, wherein the tag is a fluorescent tag, and wherein the signal from the fluorescent tag is measured by the single molecule counting instrument.

69. The method of any one of claims 1 to 68, wherein the single molecule counting instrument is an SMCxPro instrument.Eisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 70. The method of any one of claims 1 to 69, further comprising measuring one or more additional plasma or CSF biomarkers and / or one or more cognitive measurements of AD or MCI.

71. The method of claim 70, wherein the one or more additional plasma or CSF biomarkers comprises Aβ 42:40 ratio, pTau217, pTau181, MTBR-tau243, NfL, amyloid plaques, and / or neurogranin.

72. The method of claim 71, wherein the biomarker comprises neurogranin or amyloid plaques.

73. The method of claim 70, wherein one or more cognitive measurements of AD or MCI comprise MMSE, ADAS-Cog, CDR-SB, ADCOMS, or ADCS MCI-ADL.

74. An anti-Aβ protofibril antibody for use in the treatment of a subject having or suspected of having or at risk of developing AD or MCI, wherein the treatment comprises: obtaining a biological sample from the subject prior to receiving the anti-Aβ protofibril antibody; measuring a concentration of Aβ protofibrils in the sample, wherein the concentration of Aβ protofibrils in the sample is measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) andEisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 light chain complementarity determining region (LCDR) sequences of SEQ ID NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and administering the anti-Aβ protofibril antibody if there is an elevated Aβ protofibril level relative to a control.

75. Use of an anti-Aβ protofibril antibody in the manufacture of a medicament for the treatment of a subject having or suspected of having or at risk of developing AD or MCI, wherein the treatment comprises: obtaining a biological sample from the subject prior to receiving the anti-Aβ protofibril antibody; measuring a concentration of Aβ protofibrils in the sample, wherein the concentration of Aβ protofibrils in the sample is measured using an Aβ protofibril assay, comprising: contacting the biological sample with an anti-Aβ protofibril capture antibody comprising heavy chain complementarity determining region (HCDR) sequences of SEQ ID NO: 1 (HCDR1), SEQ ID NO: 2 (HCDR2), and SEQ ID NO: 3 (HCDR3) and light chain complementarity determining region (LCDR) sequences of SEQ IDEisai Reference: ABBA-007-U2PC Attorney Docket No.08061.0069-00304 NO: 4 (LCDR1), SEQ ID NO: 5 (LCDR2), and SEQ ID NO: 6 (LCDR3) to form an immune complex; contacting the immune complex with a labeled anti-Aβ detection antibody; dissociating the immune complex to release the labeled anti-Aβ detection antibody; detecting a signal from the labeled anti-Aβ detection antibody using a single molecule counting instrument; and quantifying Aβ protofibrils in the sample; and administering the anti-Aβ protofibril antibody if there is an elevated Aβ protofibril level relative to a control.

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