Sensor calibration verification

Abstract

There is provided a method for verifying the calibration 40 of a sensor 4, represented by a calibrated relationship between measurements of an optical property of a sensing material 9 by the sensor 4 and the concentration of an analyte in a sample, the optical property of the sensing material 9 having a spectrum that varies with the concentration of the analyte in the sample, the spectrum having an isosbestic wavelength λ at which the optical property does not vary with the concentration of the analyte, the method comprising the step S10 of making measurements of the optical property at three or more wavelengths of light while the sensing material 9 is exposed to the sample; the step S16 of determining whether the measurements of the optical property are inconsistent with the calibrated relationship; and the step S18 of outputting a warning signal in response to the measurements of the optical property being inconsistent with the calibrated relationship.

Description

Technical Field

[0001] The present invention relates to a method for verifying the calibration of a sensor. In particular, the present invention relates to verifying the calibration of a sensor that measures the optical properties of a sensing substance.

Background Art

[0002] It is desirable in many fields to be able to determine the concentration of a specific analyte that may include a mixture of several different substances, both gaseous and non-gaseous, in the environment. For example, in some clinical settings such as dialysis treatment or patient monitoring in intensive care, it is important to be able to accurately determine in real time the carbon dioxide concentration or the ion concentration such as potassium or sodium in the patient's blood, and supplying continuous real-time measurement data to the clinician in an emergency treatment setting is often extremely beneficial as a means of guiding treatment management. Another example is the monitoring of a controlled environment in the food industry, where the presence of oxygen or contaminant gases can cause the risk of food spoilage and may be undesirable.

[0003] Some known types of sensors use luminescent compounds, such as fluorescent organic dyes having luminescence with properties that depend on the concentration of the target analyte. By exciting the luminescent compound while it is exposed to a sample containing the analyte and measuring its luminescence, the concentration of the analyte in the sample can be determined. This type of sensor can be operated continuously, and thus has the advantage that there is no need to periodically collect samples, such as blood samples or samples of the atmosphere in which food is stored, for analysis or other similarly cumbersome procedures. Other systems measure the transmission of light through a sensing substance and can use this to determine the concentration of the analyte in the sample.

[0004] However, the analyte concentration values ​​reported by sensors are prone to errors. The use of spectrophotometers and fluorophotometers has been promoted over the past 50 years, reducing many of the variables that previously caused analytical errors. These errors were generally caused by instrument drift from the light source, detector, and electronic noise within the detection system. Double-beam spectrophotometers and fluorophotometers, which automatically reference any absorption or fluorescence interference from the analyte in a blank sample without a detected chemical reaction, have also reduced errors in many measurements. However, some of these types of systems can be cumbersome and costly to maintain and operate.

[0005] The emergence of inexpensive fiber optic devices has once again highlighted the need for signal stability, as many of these devices, particularly those intended for invasive or semi-invasive continuous measurement of analytes in critically ill patients, are used over long periods of time. In these cases, long-term, low-level drift can result in highly inaccurate measurements. Drift in these fiber optic systems is often due to drift in the intensity or wavelength of light from a light source such as an LED or laser diode, drift in the sensitivity of the detection system, and photobleaching of the detected chemical reaction. Although several techniques have been developed to reduce drift, the values ​​reported by sensors can still drift over long periods of time.

[0006] In particular, it is currently difficult to determine whether or not the concentration values ​​reported by sensors are drifting, and by how much. If sensors provide inaccurate data without the operator's knowledge, making treatment decisions based on this data could be dangerous for the patient. To avoid this, sensor devices may require periodic recalibration of the sensors at time intervals based on estimated values ​​of drift. These time intervals tend to be chosen conservatively, placing an excessive burden on operators who need to frequently recalibrate their sensors. Furthermore, there is no guarantee that the values ​​will not drift faster than expected between calibrations. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Ge et al., “High-stability non-invasive autoclavable naked optical CO2 sensor”, Biosensors and Bioelectronics, 2003:18:857-865 [Non-Patent Document 2] Ge et al., “Study on low-cost calibration-free pH sensing with disposable optical sensors”, Analytica Chimica Acta, 2012:734:79-87 [Non-Patent Document 3] Rovati et al, “Plastic Optical Fiber pH Sensor Using a Sol-Gel Sensing Matrix”, MOH. YASIN Sulaiman W. Harun and Hamzah AROF, eds. Fiber Optic Sensors [Overview of the project] [Problems that the invention aims to solve]

[0008] Therefore, a method is needed to verify the values ​​reported by the sensor in order to provide clinicians and other workers with assurance that the reported values ​​are reliable, and to indicate when recalibration is necessary. [Means for solving the problem]

[0009] According to a first aspect of the present invention, a method is provided for verifying the calibration of a sensor, which is represented by a calibrated relationship between the concentration of an analyte in a sample and the measurement of the optical properties of a detection substance by the sensor, wherein the optical properties of the detection substance have a spectrum that changes with the concentration of the analyte in the sample, and the spectrum has isoabsorbent wavelengths in which the optical properties do not change with the concentration of the analyte, and the method is provided to measure the optical properties at three or more wavelengths of light while the detection substance is exposed to the sample, to determine whether the measurement of the optical properties is inconsistent with the calibrated relationship, and to output a warning signal in response to the measurement of the optical properties being inconsistent with the calibrated relationship.

[0010] By performing three or more measurements of spectra with isoabsorptive wavelengths, it is possible to determine whether the measurements match a previously calibrated relationship. The calibrated relationship can be thought of as defining a surface in three-dimensional space, where the dimensions are three measurements at various wavelengths. Points on the surface represent combinations of three values ​​that match the calibrated relationship. Therefore, whether the measurement matches the calibrated relationship can be determined by determining whether the optical property measurement defines a point in this space that is (or sufficiently close to) a point on the surface defined by the calibrated relationship. Having three measurements allows the method to detect drift or error that affects all wavelengths equally, as well as drift that affects various wavelengths in different ways. Outputting a warning signal when the calibration is determined to be invalid reduces the burden on the operator, because recalibration is only performed when there is a risk that the measurement is actually invalid. Furthermore, the operator is given assurance that the measurement is valid when no warning is issued.

[0011] In one embodiment, the method further includes deriving a concentration measurement of the analyte in the sample from the measurement according to the calibrated relationship described above. Determining the concentration is likely to be required for reporting to the operator, and therefore calculating it as part of the verification process means that the concentration measurement can be required to verify the calibration and that the calibration does not need to be separately associated with the raw measurement for calibration purposes.

[0012] In one embodiment, the calibrated relationship includes a relationship between the concentration and the ratio between measurements of the optical properties at a pair of wavelengths, and the step of deriving the concentration measurement includes calculating the ratio between measurements of the optical properties at two wavelengths and deriving the concentration measurement from the ratio according to the calibrated relationship. Using the ratio between two measurements is advantageous because a multiplicative factor can compensate for a specific common cause of error that affects the measurements at all wavelengths.

[0013] In one embodiment, multiple concentration measurements include measurements calculated using the ratio between two measurements of the optical properties not performed at the isoabsorptural wavelengths. Since measurements at isoabsorptural wavelengths do not change with concentration, using the ratio of measurements taken away from isoabsorptural wavelengths can enhance the contrast between the ratios calculated at various concentrations of the analyte.

[0014] In one embodiment, the method further includes determining a deviation measurement of the optical properties from the calibrated relationship, and determining that the measurement of the optical properties is inconsistent with the calibrated relationship if the deviation measurement exceeds a predetermined threshold. The deviation measurement can be used to indicate the level of drift from the calibrated relationship and to set a well-defined appropriate level at which the method should issue a warning.

[0015] In one embodiment, the method further includes deriving multiple concentration measurements of the analyte in the sample from the measurements at various wavelengths according to the calibrated relationship, wherein the deviation measurement is a measurement of variation between the multiple concentration measurements. Using the three measurements, the multiple concentration measurements can be obtained using various measurements or combinations thereof. If these concentration measurements do not coincide, this indicates that the calibrated relationship is no longer valid.

[0016] In one embodiment, the calibrated relationship includes a relationship between the concentration and a plurality of ratios between the measurements of the optical properties at each pair of wavelengths, and the step of deriving a plurality of concentration measurements of the analyte in the sample includes calculating the ratios between the measurements of the optical properties at each pair of wavelengths and deriving the plurality of concentration measurements from the ratios according to the calibrated relationship. As described above, the ratios take into account the exclusion of several types of common sensor errors. If the ratios are inconsistent, this allows the method to detect that other types of errors are large enough to invalidate the measurement.

[0017] In one embodiment, the above variation measurement value is the coefficient of variation. This is a convenient technique that provides a variation measurement value for a set of values equivalent to the variation of other sets of values even when the absolute luminosities are substantially different.

[0018] In one embodiment, one of the above three or more wavelengths is the above equal absorption wavelength. By directly measuring at the equal absorption wavelength, it becomes possible to detect a specific type of error earlier because the value measured at the equal absorption wavelength should not change with the concentration, and thus the change indicates an error.

[0019] <00-00082>In one embodiment, one of the above three or more wavelengths is the wavelength at which the spectrum of the above optical property of the above detection substance has a maximum or minimum. This provides the greatest contrast between measurements at various concentrations, thereby enhancing the sensitivity and reliability of the measurement.

[0020] In one embodiment, the above optical property is one of absorption and emission, and the above spectrum is one of an absorption spectrum and an emission spectrum, respectively. The absorption spectrum and the emission spectrum are convenient choices for measuring the optical property because they can be measured without requiring angular resolution measurement or other complex measurement techniques.

[0021] In one embodiment, the above optical property is emission, and performing the measurement of the above optical property at three or more different wavelengths includes exciting the above detection substance with light of a first wavelength for each of the above three or more wavelengths, and measuring the intensity of the light emitted by the above detection substance at each of the above three or more different wavelengths, and the above first wavelength is the same for each of the above three or more wavelengths. This measurement method only requires that single-wavelength light is generated by the device and transmitted through the detection substance, thereby simplifying the configuration of the light emitter required in the device. This measurement method is most suitable when the detection substance has a single excitation band but emits light at multiple wavelengths.

[0022] In one embodiment, the optical property is absorption, and performing a plurality of measurements of the optical property at three or more different wavelengths includes, for each wavelength of the three or more wavelengths, exciting the detection substance with light at each of the three or more wavelengths, and measuring the intensity of the light emitted by the detection substance at a second wavelength, where the second wavelength is the same for each of the three or more wavelengths. This measurement method only requires a single detector because light is emitted at the same wavelength for various excitation wavelengths. This measurement method is most suitable when the detection substance can be excited in multiple different excitation bands but emits light at a single wavelength.

[0023] In one embodiment, the optical property is absorption, and performing a plurality of measurements of the optical property at three or more different wavelengths includes irradiating the detection substance with light at each of the three or more wavelengths, and measuring the intensity of the light transmitted by the detection substance at each of the three or more wavelengths. This measurement method also only requires a single detector and is advantageous in physical configurations where it is difficult to capture light emitted from the detection substance.

[0024] In one embodiment, the detection substance includes two species in an equilibrium state, and the equilibrium between the two species depends on the concentration of the analyte in the sample. The two species in the equilibrium state can result in an isosbestic point in the spectrum of the detection substance, and due to the different optical properties of the two species, it becomes possible to detect changes in the concentration of the analyte.

[0025] In one embodiment, the analyte is one of carbon dioxide and hydrogen ions. These are particularly desirable analytes to measure in biological or clinical implementations, and having confirmation of the effectiveness of the measurement can be crucial for patient treatment.

[0026] In one embodiment, the sample includes blood. Blood is a medium commonly collected in a clinical setting and indicates the levels of many important substances in the body.

[0027] In one embodiment, the detection substance includes 8-hydroxypyrene-1,3,6-trisulfonic acid. This is a readily available pH-sensitive dye for detecting carbon dioxide concentration in the blood.

[0028] In one embodiment, performing the above measurement of the optical properties includes taking raw measurements at multiple points in time and averaging the raw measurements over time. Time averaging can reduce the method's sensitivity to transient changes in measurements that do not exhibit the long-term drift that the method is initially intended to detect. This reduces the occurrence of false alarms.

[0029] In one embodiment, the process further includes an initial calibration step to determine the calibrated relationship between the concentration of the analyte and the measurement of its optical properties. The initial calibration provides a baseline to compare with later measurements and confirmation of the initial effectiveness of the sensor device calibration.

[0030] Herein, embodiments of the present invention will be described as non-limiting examples with reference to the accompanying drawings. [Brief explanation of the drawing]

[0031] [Figure 1] This is a schematic diagram of a sensor device in which the present invention can be implemented. [Figure 2] This is a schematic diagram of possible sensor-probe configurations for in vitro measurement of analyte concentrations in blood. [Figure 3] This is a schematic diagram of possible sensor-probe configurations for intravascular measurement of analyte concentrations in blood. [Figure 4] This is a schematic diagram of possible sensor-probe configurations for subcutaneous measurement of analyte concentrations in blood. [Figure 5] This is a graph of the absorption spectrum of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) over a range of pH values. [Figure 6]This graph shows both the absorption and emission spectra of HPTS at different pH values. [Figure 7] This graph shows the absorbance spectra of phenol red, an alternative detection substance, at various pH values. [Figure 8] This is a graph of the emission spectrum of seminaphtharhodafluor (SNARF®), a further alternative detection substance at various pH values. [Figure 9] This is a flowchart for finding the calibrated relationship. [Figure 10a] This graph shows the calibrated relationship between the concentration of the analyte in the sample and the ratio of optical property measurements performed at various wavelengths. [Figure 10b] This graph shows the calibrated relationship between the concentration of the analyte in the sample and the ratio of optical property measurements performed at various wavelengths. [Figure 10c] This graph shows the calibrated relationship between the concentration of the analyte in the sample and the ratio of optical property measurements performed at various wavelengths. [Figure 10d] This graph shows the calibrated relationship between the concentration of the analyte in the sample and the ratio of optical property measurements performed at various wavelengths. [Figure 11] This is a flowchart showing how to verify sensor calibration. [Figure 12] This graph shows the effect of changes in analyte concentration on the measurement of optical properties. [Figure 13] This graph shows the effect of a certain amount of drift on the measurement of optical properties. [Figure 14] This graph shows optical property measurements that do not match the calibrated relationship. [Modes for carrying out the invention]

[0032] This disclosure provides a method for verifying the calibration of a sensor. Figure 1 shows a type of sensor device in which the method disclosed herein can be used. An example of such a sensor 4 may be a pH sensor for detecting carbon dioxide concentration. The sensor device includes a sensor 4, which includes a detection substance 9 and an analysis system 30.

[0033] The sensor 4 includes a light source 10 configured to emit light used to measure the optical properties of the detected substance 9. For example, if the detected substance 9 is a luminescent compound, the light source 10 can be any light source capable of emitting light at the wavelength and intensity required to excite the luminescent compound. For example, the light source 10 may include a laser diode or an LED. The light source 10 may be a continuous light source, a light source using vibration intensity, or a pulsed light source.

[0034] Sensor 4 further includes a detector 14 configured to detect light received from the substance to be detected. The detector 14 can be any device capable of generating a signal in response to receiving light at a wavelength emitted by the substance to be detected 9. For example, the detector 14 may include a charge-coupled device, an energy element sensor, a photodiode, or a photoresistor. The signal output by the detector 14 can represent the intensity of the light received from the substance to be detected 9.

[0035] Sensor 4 includes an optical fiber 16 arranged to guide light to the sensing substance 9 and light from the luminescent compound. The optical fiber uses total internal reflection to prevent light loss from the fiber. This means that light can be efficiently transmitted to and from the sensing substance, improving the signal and preparing for higher quality and more reliable measurements. They can also be made small and flexible, making them particularly suitable for sensors that must be inserted into a patient's body. For example, the optical fiber 16 may include a PMMA fiber optic component. The optical fiber 16 functions as an optical waveguide, and where appropriate, any other suitable optical waveguide can be used instead of the optical fiber 16.

[0036] Sensor 4 includes a sensor probe 8 on which a detection substance 9 is provided. The sensor probe 8 may be the component of sensor 4 that is directly exposed to the sample. In one embodiment, the sample includes blood. Sensor 4 further includes a connector 21 configured to connect the sensor probe 8 to a light source 10 and a detector 14. Part or all of sensor 4 may be disposable. This is convenient in clinical situations where sensor 4 is used to measure analyte concentrations in a patient's body. In such cases, the portion of sensor 4 inserted into the patient must be sterile and cannot be reused between patients. For example, only the sensor probe 8 containing the detection substance 9 may be disposable, while the detector 14 or light source 10 may not be disposable.

[0037] The analysis system 30 is configured to perform the method by controlling the sensor 4 and processing the signals received from the detector 14. The analysis system 30 may also be configured to calibrate the sensor and / or derive concentration measurements of the analyte based on measurements by the sensor 4. The analysis system 30 can be connected to the sensor 4 via a wired connection, such as a serial or Ethernet® connection, or another type of interface specifically designed for the sensor device. Alternatively, a wireless connection such as Bluetooth® or Wi-Fi may be used. The analysis system can also receive signals output by the detector 14 and transmit those signals to the sensor 4 to control, for example, the light source 10.

[0038] Figures 2 to 4 show specific examples of the sensor 4 used in clinical settings and when the sensor 4 includes the sensor probe 8.

[0039] Figure 2 shows an embodiment in which sensor 4 is a bypass sensor. Such a sensor can be used in an external blood pump to monitor the concentration of an analyte in the blood being pumped. Measurements of analyte concentrations, particularly oxygen or carbon dioxide, can be used as part of controlling the blood pumping rate by the external blood pump, for example, to maintain an appropriate oxygen supply level to the blood. In this case, a disposable sensor probe 8 is attached to the bypass loop 61 so that the sensing substance 9 is exposed to the blood passing through the bypass loop. A connector 21 connects the disposable sensor probe 8 to the rest of sensor 4. A thermistor or another suitable temperature sensor 20 is mounted inside the sensor probe 8 to measure the temperature of the blood.

[0040] Figure 3 shows an embodiment in which sensor 4 is an intravascular sensor. The sensing material 9 is located at the tip of an optical fiber 16 inserted into the patient via a catheter. The sensor probe 8 includes the fiber optic component 16 together with a temperature sensor 20. The sensor probe 8 is connected to the rest of sensor 4 via a connector 21.

[0041] Figure 4 shows an embodiment in which sensor 4 is an interstitial fluid sensor. In this case, sensor 4 includes a sensor probe 8 and an outer component 54. The sensor probe 8 punctures the skin 52 and measures the concentration of analytes in the interstitial fluid. A retractable needle can be used to puncture the skin 52, and sensor 4 is wirelessly connected to an analysis system 30. Alternatively, the analysis system 30 may be located within the outer component 54. A temperature sensor is provided to measure skin temperature. This temperature sensor can be located within the sensor probe 8 that penetrates the skin 52, or it can be located near the skin 52 within the outer component 54.

[0042] The detection substance 9 can be any suitable substance having optical properties that have a spectrum that changes with the concentration of the analyte in the sample, and this spectrum has isoabsorbent wavelengths whose optical properties do not change with the concentration of the analyte. The detection substance 9 can be provided in a sensor probe 8 fixed within a polymer layer. The optical property of the detection substance 9 can be absorption intensity, and its spectrum can be an absorption spectrum.

[0043] In some embodiments, the detection substance 9 is a luminescent compound. The optical property of the luminescent compound can be its luminescence emission intensity (fluorescence emission intensity or phosphorescence emission intensity, etc.), and its spectrum can be an emission spectrum. Another optical property of the luminescent compound can be its luminescence lifetime (fluorescence lifetime or phosphorescence lifetime, etc.).

[0044] In applications where the amount of detection substance 9 is small, it may be difficult to detect absorption by the detection substance 9 relative to the background of the excitation light. Therefore, the detection substance is preferably a luminescent compound having a luminescence emission spectrum. In particular, it is preferable that the luminescent compound emits light over a wavelength range different from the wavelength range in which the luminescent compound is excited, because this makes it easy to distinguish between the excitation light and the light emitted from the detection substance.

[0045] If the detection substance 9 is a luminescent compound, the luminescence emission spectrum can be either a fluorescence emission spectrum or a phosphorescence emission spectrum. However, phosphorescence emission spectra are generally weaker than fluorescence emission spectra because they involve spin-forbidden transitions. Therefore, in order to give the detection substance 9 a strong photoresponse to excitation light, it is preferable that the luminescent compound has a fluorescence emission spectrum that changes when the luminescent compound interacts with the analyte.

[0046] Therefore, the detection substance 9 is preferably a luminescent compound containing a phosphor. The phosphor is a portion that can absorb and re-emit light through fluorescence emission. Typically, phosphors absorb light in the visible region of the electromagnetic spectrum. Phosphors also typically emit light in the visible region of the electromagnetic spectrum. The "visible region of the electromagnetic spectrum" refers to electromagnetic radiation having wavelengths of approximately 400 nm to approximately 700 nm. Phosphors can also absorb and / or emit radiation outside the visible region of the electromagnetic spectrum. Therefore, in preferred embodiments, the detection substance 9 is a luminescent compound containing a phosphor, and the fluorescence emission spectrum of the phosphor changes in the presence of the analyte.

[0047] Changes in the optical properties of luminescent compounds (such as the emission spectrum of luminescent compounds) are caused by interactions with the analyte. Possible forms of interaction between the analyte and the luminescent compound include: Protonation of luminescent compounds, Deprotonation of luminescent compounds, Collision quenching of the excited state of luminescent compounds, Bonding of lone pairs of electrons in luminescent compounds, Any other non-covalent interactions to facilitate bonding, Includes.

[0048] Other forms of interaction are possible. These interactions alter one or more optical properties of the luminescent compound, and these properties can be detected optically.

[0049] In some cases, the analyte does not bind to the luminescent compound, such as when the interaction between the analyte and the luminescent compound is involved in collision quenching of the luminescent compound. However, in other cases, chemical bonds, such as ionic or covalent bonds, can be formed between the analyte and the luminescent compound. In such cases, the luminescent compound may contain a receptor moiety. The receptor moiety is a portion that can bind to the analyte. It is preferable for the luminescent compound to contain a receptor moiety because the receptor moiety generally conveniently binds to the analyte and not to other chemical species. Therefore, luminescent compounds containing a receptor moiety generally produce an optical signal that is particularly associated with the analyte, and this optical signal has low sensitivity to interference from other species. Many examples of luminescent compounds that can be used as detection substances for various different analytes are listed below for illustrative purposes only.

[0050] In one example, a luminescent compound is given by formula (I), that is [ka] The portion may include pyranine or a derivative thereof. See, for example, Non-Patent Document 1. This portion can be bonded to the polymer layer at any point other than the hydroxyl group. The compound of formula (I) does not contain a separate acceptor and phosphor, and the phosphor itself acts as the acceptor. The portion of formula (I) can be used to detect acid or CO2 because CO2 forms an acid (carbonic acid) in the presence of water. In the presence of an acid (such as carbonic acid formed by CO2), the hydroxyl group of the portion of formula (I) is protonated. However, as the concentration of the acid or CO2 decreases, the hydroxyl portion is deprotonated, leaving a delocalized negative charge throughout the phosphor and altering the fluorescence emission and fluorescence absorption spectra of the compound. This change is particularly enhanced when the luminescent compound containing the portion of formula (I) is immobilized in a polymer matrix with a phase transfer agent. An example of a phase transfer agent is hexadecyltrimethylammonium hydroxide.

[0051] A suitable pyranine derivative that can be used is of formula (II), namely, [ka] This is the relevant part.

[0052] For example, see Non-Patent Document 2.

[0053] In another example, a luminescent compound is given by formula (III), namely, [ka] This may include the portion or derivative thereof. This portion can be bonded to the polymer of the polymer layer at any point. The compound of formula (III) behaves similarly to the portions of formula (I) and (II), i.e., it does not contain a separate acceptor and phosphor, and the phosphor itself acts as the acceptor. In the presence of an acid (such as carbonic acid formed by CO2), the hydroxyl group of the portion of formula (III) is protonated. However, as the concentration of the acid or CO2 decreases, the hydroxyl portion is deprotonated, leaving a delocalized negative charge throughout the phosphor and altering the fluorescence emission and fluorescence absorption spectra of the compound. The absorbance spectrum of the compound of formula (III) is shown in Figure 8.

[0054] Other luminescent compounds are known and often commercially available, and these compounds can also be used as luminescent compounds. In some examples, the detection substance includes 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS).

[0055] Further examples of luminescent compounds that can be used to detect acids or CO2 are listed below, namely: [ka] That is the case.

[0056] The emission and excitation spectra of this luminescent compound are shown in Figure 7. See, for example, Non-Patent Document 1.

[0057] Further examples of luminescent compounds that can be used to detect acids or CO2 are, namely, [ka] That is the case.

[0058] The emission and excitation spectra of this luminescent compound are shown in Figure 7. See, for example, Non-Patent Document 3.

[0059] From the above, it should be clear that sensor 4 can be used for the optical detection of a wide variety of analytes. The analytes may be, for example, ions, gases, inorganic compounds, or organic compounds. The analyte may be present as a gas in the sample, or alternatively, dissolved or suspended in another substance, such as interstitial fluid or blood. If the analyte is an organic compound, it is generally a small organic compound, for example, an organic compound containing fewer than 20 carbon atoms. Specific examples of small organic compounds include sugars, sugar alcohols, and metabolites such as urea or ketones. Particularly preferred examples of analytes are CO2 and acids (H + It is a pH sensor.

[0060] The method disclosed herein relates to the verification of the calibration of a sensor 4, which is represented by a calibrated relationship between the concentration of an analyte in a sample and the measurement of the optical properties of a detected substance 9 by the sensor 4 (e.g., sensor 4 in Figure 1). Individual sensors 4, such as those described above, have variations in their manufacture and behavior, and these variations need to be calibrated before use in order that the values ​​reported by the sensor device are accurate. The calibrated relationship can be determined using a method such as the one shown in Figure 9.

[0061] Figure 9 shows a method for determining a calibrated relationship between the concentration of an analyte and the measurement of its optical properties. In step S20, the optical properties of the detection substance 9 are measured while the detection substance 9 is exposed to a sample containing the analyte, and in step S22, the concentration of the analyte in the sample is measured by an independent method. For example, the sample can be a specially prepared calibration sample, and the concentration of the analyte in the sample can be known. Alternatively, the sample can then be analyzed using another device with known behavior to independently determine the concentration of the analyte. Steps S20 and S22 are repeated for many different samples with analytes of various concentrations. The number of repetitions required varies depending on the calibration method and the required precision. For example, if the change in the optical properties of the detection substance 9 with respect to the concentration of the analyte is known to follow a specific model, only a few repetitions may be needed to determine the parameters of the model, and the exact number depends on the model. Alternatively, if no such model exists, many repetitions may be required to enable the behavior at other concentrations to ensure reliable interpolation from the calibration measurement.

[0062] In step S24, the analysis system 30 determines the calibration of sensor 4 using the measured values ​​of the optical properties and concentrations. This may involve fitting the model to the measurements performed in steps S20 and S22 to determine the model parameters, or it may involve interpolation between calibration measurements. The output of step S24 is calibration data 40, which includes calibrated relationships that can be used to determine the concentration of the analyte in a new sample from the measurement of the optical properties of the detection substance 9 while the detection substance 9 is exposed to a new sample.

[0063] An example of calibration data 40 is shown in Figure 10. Figure 10(a) shows the absorption spectrum obtained from a fluorescent carbon dioxide sensor exposed to increasing concentrations of carbon dioxide. When the luminescent compound 9 was excited at three different excitation wavelengths, namely 405 nm (intensity increases with carbon dioxide concentration), 417 nm (isoabstract point), and 469 nm (intensity decreases with carbon dioxide concentration), the fluorescence output of the compound was measured at 540 nm. Using these sets of spectra, a calibrated relationship in the form of a graph, such as the graphs shown in Figures 10(b) to 10(d), can be defined for measurements at various wavelengths. Figures 10(b) to 10(d) show the concentration of the analyte as a function of the ratio of the optical property measurements performed at 405 nm, 417 nm, and 469 nm in Figure 10(a), according to the calibrated relationship obtained in step S24 of the method in Figure 9. As will be explained later, using the ratio of two measurements at various wavelengths has advantages in some embodiments.

[0064] The calibrated relationship defines the spectrum of the optical properties, such as the spectrum shown in Figure 10(a), corresponding to various concentrations of the analyte in the sample. A continuous series of such spectra, corresponding to other possible values ​​of concentration, exist between, above, and below the spectrum shown in Figure 10(a), and all of these spectra are defined by the calibrated relationship. Thus, the calibrated relationship relates to the concentration of the analyte and the measurement of the optical properties. As will be described in more detail below, the method for verifying the calibration disclosed herein involves performing three or more measurements of the optical properties. Once three measurements of the optical properties are performed, the calibrated relationship defines a surface in three-dimensional space, where the dimensions of this space are the possible values ​​of each of the three measurements, and the points on the surface represent combinations of the three values ​​that coincide with the calibrated relationship. Each point on the surface has a corresponding value of the concentration of the analyte. This can be used to determine whether the measurements coincide with the calibrated relationship.

[0065] During continuous operation of sensor 4, many factors can cause drift in the measurement of optical properties. In some embodiments, light source 10 includes multiple separate light sources, one for each wavelength measured, and any of these light sources may drift in wavelength or intensity after calibration, potentially inducing a change in the measured intensity of light received by the detector. Furthermore, if there is interference (optical or chemical) in either of the two absorption peaks after calibration, this also causes a noticeable change in intensity. External materials present in the sample that emit fluorescence can also cause errors. During continuous measurement, it is difficult to distinguish between variations in optical property measurement due to errors and "true" variations in measurement due to changes in analyte concentration. Errors may mask changes in analyte concentration or lead to a false indication that the analyte concentration has changed. This is extremely important when the data is presented to clinicians who follow the data to administer treatment to patients.

[0066] This type of error can be detected because the detection substance 9 has a spectrum with an isosbestic point. At isosbestic wavelengths, the optical properties do not change with the concentration of the analyte. In one embodiment, the detection substance 9 contains two species in equilibrium, and the equilibrium between the two species depends on the concentration of the analyte in the sample. This equilibrium gives rise to an isosbestic point. Figure 5 shows isosbestic point A at 418 nm. i This is shown in Figures 12-14 as λ i This is schematically illustrated by [figure]. The isosbestic point is the wavelength at which the absorption of light by the detection substance 9, which contains two species, remains constant as the equilibrium between those species changes. In the example in Figure 5, the absorption of light remains constant even when the pH changes. This can be explained as follows:

[0067] The absorbance of the detected substance is determined by Lambert-Beer's law, that is A = εlc equation 1 Represented by, During the ceremony, A is absorption at a specific wavelength, ε is the absorbance or molar extinction coefficient of the detected substance at a specific wavelength. l is the optical path length through the detected substance. c is the concentration of the detected substance.

[0068] Absorbance is given by equation 2, that is,

number

[0069] In the example in Figure 5, the analytical concentration of the detected substance 9 is: c1 + c2 = c equation 3 It remains constant in the overall equilibrium state, In the formula, c1 and c2 are the concentrations of two species in equilibrium contained in the detected substance 9.

[0070] Assuming the optical path lengths are the same for both species, the total absorbance of the detected substance at a given wavelength is: A = l(ε1c1 + ε2c2) equation 4 That is the case.

[0071] However, at the isosbestic point, the absorbances of the two species are the same, that is, ε1=ε2=ε equation 5 That is the case.

[0072] Therefore, the absorbance at the isosbestic point is, A i =l(ε1c1+ε2c2)=lε(c1+c2)=lεc equation 6 That is the case.

[0073] Therefore, the absorbance at the isosbestic point does not change with the analyte concentration, but is dependent on the optical path length l, the analyte concentration c of the detected substance, and the absorbance of the detected substance 9 at the isosbestic wavelength. This is also useful when the emission of the detected substance is measured. Emission by each substance is proportional to the absorbance by each substance, and therefore the emission spectrum exhibits isosbestic points for the same reasons that the absorption spectrum exhibits isosbestic points. Due to the presence of isosbestic points, changes in analyte concentration cause inverse changes in optical properties at wavelengths above and below the isosbestic point, because a decrease in the concentration of one substance causes an increase in the concentration of the other, and vice versa. This means that it is possible to distinguish between changes caused by changes in analyte concentration and changes caused by errors, as will be further explained below.

[0074] Figure 11 shows a method for verifying the calibration of a sensor that utilizes this insight. The calibration is represented by a calibrated relationship, such as that obtained using the method in Figure 9. In one embodiment, the method includes an initial calibration step of determining a calibrated relationship between the concentration of an analyte and the measurement of its optical properties, for example, using the method in Figure 9. Sensor 4 is a sensor 4 as shown in Figure 1, and includes a detector 14 and a light source 10. Sensor probe 8 contains a detecting substance 9, such as the detecting substance described above, and sensor 4 measures the optical properties of the detecting substance 9.

[0075] In step S10, the method includes measuring the optical properties of the detection substance at three or more wavelengths of light while the detection substance is exposed to the sample. The measurements performed in step S10 can be obtained in various ways, depending to some extent on the special selection of the detection substance 9. If the optical property is absorption, step S10 includes irradiating the detection substance 9 with light at each of the three or more wavelengths and measuring the intensity of the light transmitted through the detection substance 9 at each of the three or more wavelengths.

[0076] The sensing material 9 is preferably a luminescent compound, and more preferably a fluorescent compound. The sensor 4, including the fiber optic component 16, is deployed with a fluorescent compound having a single absorption peak that, when excited with single-wavelength light, exhibits two overlapping emission peaks with isosbestic points. In this case, the optical property is emission, and step S10 includes exciting the sensing material 9 with light of a first wavelength for each of three or more wavelengths, and measuring the intensity of the light emitted by the sensing material 9 at each of the three or more different wavelengths, where the first wavelength is the same for each of the three or more wavelengths. This implementation may be preferred in some situations because only single-wavelength light is required to produce two overlapping peaks in the emission spectrum. This reduces the complexity of the light source 10, and furthermore, any change in the output from the light source 10 equally affects both emission peaks. This makes it possible to effectively remove the change as an error by the ratiometric method, which will be further described below. However, this type of implementation requires that the detector 14 be able to distinguish between light at various wavelengths.

[0077] For other sensing materials such as HPTS, excitation at various wavelengths results in a single emission peak. For example, as illustrated in Figure 7, the signals obtained by exciting HPTS at 405 nm, 470 nm, and 418 nm result in a single fluorescence emission at 525 nm. If light at three wavelengths is provided in time-separated pulses, the fluorescence signals from excitation at three different wavelengths can be analyzed in time, and the absorption due to excitation at each wavelength can be determined. In this case, the optical property is absorption, and step S10 includes exciting the sensing material 9 with light at each of the three or more wavelengths, and measuring the intensity of the light emitted by the sensing material 9 at a second wavelength, where the second wavelength is the same for each of the three or more wavelengths. This would be preferable when the detector 14 can only detect the intensity of light and cannot detect the wavelength, but it is required that the light source 10 can emit light at multiple different wavelengths.

[0078] In one embodiment, one of the three or more wavelengths is an isosbestic wavelength. As discussed above, the absorbance at the isosbestic point is directly dependent on the optical path length l, the analytical concentration c of the detected substance, and the absorbance of the detected substance at the isosbestic point. When one of the three or more wavelengths is an isosbestic wavelength and the absorbance at the isosbestic point is determined at the calibration point, any changes in the optical path length and the analytical concentration of the detected substance (e.g., due to photobleaching or chemical interference) can be directly detected during subsequent use by monitoring the change in the isosbestic point.

[0079] In one embodiment, one of three or more wavelengths is the wavelength at which the spectrum of the optical properties of the detected substance has its maximum or minimum. The change in optical properties with respect to the analyte concentration is greatest at the point where the spectrum has its maximum or minimum, and therefore this embodiment further enhances contrast. The selection of wavelengths can be made by considering where the spectrum has its maximum or minimum at a particular analyte concentration, e.g., zero.

[0080] In one embodiment, step S10 includes taking raw measurements at multiple points in time and averaging these raw measurements over time. Averaging the raw measurements over time reduces the method's sensitivity to variations or transient changes in the optical properties, thereby reducing the likelihood of falsely generating a warning signal.

[0081] In step S12, the method includes deriving multiple concentration measurements of the analyte in the sample from measurements at various wavelengths according to a calibrated relationship. Calibration of sensor 4 provides a relationship between the concentration of the analyte and the measurement of its optical properties. Furthermore, in many systems, the concentration values ​​need to be calculated in advance for reporting to the user. Therefore, using concentration to determine whether the measurement is in agreement with the calibrated relationship means that no separate comparison using raw measurements is required. However, in some embodiments, the method may not include the step of calculating the concentration, and the determination of whether the measurement is in agreement with the calibrated relationship may be made directly based on the measurement of the optical properties.

[0082] In step S12, the calibrated relationship includes a relationship between concentration and multiple ratios between optical property measurements at each pair of wavelengths, and step S12 includes calculating the ratios between optical property measurements at each pair of wavelengths and deriving multiple concentration measurements from those ratios according to the calibrated relationship. This is advantageous because the multiple measurements compensate for a specific type of error or drift, and therefore the sensor 4 does not need to be recalibrated to compensate for these errors. For example, as shown in Figure 6, there are HPTS absorption peaks at 405 nm (A1) and 470 nm (A2). Therefore, using equation 1 for the ratio of the two peaks shown in Figure 5,

number

[0083] The two concentrations c1 and c2 correspond to the concentrations of the two species contained in the detected substance discussed above, assuming that the absorption at each of the two peaks is significant due to the species associated with the peaks. Since the optical path lengths are the same for light at both wavelengths (especially when both are transmitted along the same optical fiber 16), this condition is eliminated, and the ratio eliminates this possible variable, thereby reducing measurement drift that may be caused by changes in optical path length due to, for example, temperature-related expansion and contraction of the optical fiber 16. Furthermore, I=I010 -ATherefore, in embodiments where the measurement method uses the same light source 10 to perform measurements at both wavelengths, changes in the intensity I0 of the light source 10 do not affect the peak ratio, because any change in I0 results in a proportional change in I. Consequently, the frequency of recalibration is reduced, and less user time is consumed. The results using the ratio are shown in Figure 13. For each value of analyte concentration, there is a set of effective spectra of optical properties. The two solid lines show two effective spectra at analyte concentration C1, corresponding, for example, to various values ​​of optical path length or incident intensity of light from the light source 10. Similarly, the dotted line represents two such spectra for analyte concentration C2. With respect to spectra at various concentrations, intermediate spectra exist over the entire range of values ​​that the incident intensity can take, for example. However, even when multiple concentration measurements are derived using the measurement ratio, this does not mean that the calibration relationship persists between subsequent measurements. Other errors may affect the measurement at one of three or more wavelengths from the measurement at other wavelengths in different ways, and thus still cause drift. Therefore, even when ratios are used, the need to verify calibration remains.

[0084] In one embodiment, multiple concentration measurements include measurements calculated using the ratio between two measurements of optical properties that are not performed at isoabsorptural wavelengths. Since measurements at isoabsorptural wavelengths do not change with analyte concentration, using the ratio between two measurements of optical properties that are not performed at isoabsorptural wavelengths increases the contrast of the ratio as the analyte concentration changes, thereby increasing the sensitivity of the concentration measurements. In particular, using the ratio between two measurements of optical properties, i.e., one of the two measurements performed at a wavelength greater than isoabsorptural wavelength and the other of the two measurements performed at a wavelength less than isoabsorptural wavelength, results in a greater improvement in contrast.

[0085] In step S14, the method includes determining a deviation measurement of the optical property from the calibrated relationship. In the method of Figure 11, the deviation measurement is a variation measurement between a plurality of concentration measurements calculated in step S12. However, in embodiments that do not include calculating concentration measurements, the deviation measurement can be calculated directly from the optical property measurements. Concentration measurements can be obtained individually from each measurement such that the measurement at any particular wavelength has a different value for each concentration of the analyte. Comparing the concentration measurements obtained using various measurements among the measurements provides a convenient way to determine whether the measurements are in agreement with the calibrated relationship. Alternatively, in embodiments where one of the three or more wavelengths is an isosbestimulus wavelength, the deviation measurement may be the deviation of the optical property measurement at an isosbestimulus wavelength from the optical property measurement at an isosbestimulus wavelength during calibration.

[0086] In one embodiment, the deviation measurement is the coefficient of variation. Depending on what is most appropriate, other variation measurements, such as a range of concentration measurements, standard deviation, or equivalents, may be used.

[0087] In step S16, the method includes determining whether the optical property measurement is inconsistent with the calibrated relationship. In the method shown in Figure 11, the optical property measurement is determined to be inconsistent with the calibrated relationship if the deviation measurement obtained in step S14 exceeds a predetermined threshold. If three or more measurements are in agreement with the calibrated relationship, the multiple concentration measurements obtained in step S14 should be the same. For example, in Figure 12, the concentration corresponding to the black spot on the solid line at λ1 is the same as the concentration corresponding to the black spot on the solid line at λ2, i.e., C1. However, the concentration corresponding to the white spot at λ2 is the concentration between C1 and C2 where the optical property spectrum intersects with the white spot. Therefore, the concentration measurements obtained from the white spot and the black spot are different, and the measurement as a set is inconsistent with the calibrated relationship, indicating that some drift has occurred since calibration. Comparing deviation measurements to a predetermined threshold is advantageous because this threshold provides a well-defined threshold above which measurements are considered inconsistent, and it offers a quantitative measure of how much the measurement has drifted over time. Therefore, the measurements reported by sensor 4 can continue to be used as long as it is known that the drift is not yet large enough to cause a significant error in the reported value, thereby reducing the frequency to which sensor 4 must be recalibrated.

[0088] As explained with respect to step S24 of the method in Figure 9, once three optical property measurements are performed, the calibrated relationship defines a surface in three-dimensional space, where the dimensions of this space are the possible values ​​of each of the three measurements, and the points on the surface represent combinations of the three values ​​that coincide with the calibrated relationship. Therefore, more comprehensively, step S16 includes determining whether the optical property measurements define a point in this space that is (or sufficiently close to) a point on the surface defined by the calibrated relationship. In one embodiment, the deviation measurement is the measurement of the shortest distance from the surface defined by the calibrated relationship to the point defined by the optical property measurements. For example, if the optical property measurements (represented by black dots) at three or more wavelengths fall on the solid line in Figure 12, the concentration of the analyte can be determined to be C1. If the optical property measurements fall on the dotted line, the concentration of the analyte can be determined to be C2. However, if the optical property measurements at three different wavelengths fall on λ1 and λ i If the measurements are indicated by black spots at λ and white spots at λ2, the three measurements do not correspond to any of the spectra defined by the calibration, and these measurements can be determined to be inconsistent with the calibrated relationship. In this manner, the determination of whether the measurements are inconsistent with the calibrated relationship can be made directly based on the optical property measurements in some embodiments, without calculating the concentration values.

[0089] In the method shown in Figure 11, the multiple concentration measurements derived in step S12 are calculated using the ratio between measurements at two different wavelengths. The results using the ratio are clearly shown in Figure 13. For each value of analyte concentration, there exists a set of effective spectra of optical properties. The two solid lines show two effective spectra at analyte concentration C1, corresponding, for example, to various values ​​of optical path length or incident intensity of light from light source 10. Similarly, the dotted line represents two such spectra for analyte concentration C2. With respect to spectra at various concentrations, intermediate spectra exist over the entire range of values ​​that the incident intensity can take, for example. As shown in Figure 14, if the three measurements are those indicated by black dots, the possible spectra do not match the measurements at all, and it can be confirmed that the measurements are inconsistent with the calibrated relationship, even when a ratiometric method is used. Thus, the determination of inconsistency can be made only for errors that cannot be compensated using the ratio, and a warning signal is output only when an uncompensable drift occurs. Similarly, the absorbance at the isosbestic point is as follows, i.e.,

number

[0090] The ratio obtained during calibration, and then measured and compared consecutively after calibration, i.e.,

number

number

[0091] In step S18, the method includes outputting a warning signal in response to a measurement of optical properties being inconsistent with a calibrated relationship. This warning signal alerts the user that the value reported by the sensor device is no longer reliable, thereby preventing, for example, a clinician from making a clinical decision based on a misunderstanding regarding the concentration of a particular substance in a patient's blood. The warning signal alerts the user that the sensor 4 needs to be recalibrated. Therefore, recalibration is performed only when the sensor 4 has actually drifted to the point where it no longer reports a reliable value. This reduces the wasted time of performing periodic calibrations that may not be necessary. The warning can be provided in any preferred manner. For example, the warning may be an audible warning, a visual warning, or a combination thereof.

[0092] In some embodiments, the method may prompt the user to take a confirmation sample. The measured values ​​of the optical properties can then be compared with predicted values ​​from calibration constants to calculate a correction multiplier, which can then be applied to each individual measured value, thus allowing for error correction and continued monitoring.

[0093] The measurements performed during the verification process can be the same measurements used to generate the concentration measurements reported to the user. Thus, in one embodiment, the method further includes deriving concentration measurements of the analyte in the sample from the measurements according to a calibrated relationship. This allows the value to be reported to the user for monitoring or determination of any suitable action. In one embodiment, the concentration measurement reported to the user can be the average of three or more concentration measurements calculated using three or more measurements of optical properties.

[0094] In one embodiment, the calibrated relationship includes a relationship between concentration and the ratio between measurements of optical properties at a pair of wavelengths, and the step of deriving the concentration measurement includes calculating the ratio between measurements of optical properties at two wavelengths and deriving the concentration measurement from that ratio according to the calibrated relationship. As discussed above, obtaining the concentration measurement using the ratio between measurements has advantages in compensating for certain types of errors. Therefore, it is equally advantageous to use it when reporting the value to the user. In one embodiment, the concentration measurement includes a measurement calculated using the ratio between two measurements of optical properties that were not performed at isoabsorptural wavelengths. This increases the contrast of the ratio, thereby increasing the sensitivity of the concentration measurement. The concentration measurement reported to the user can be calculated directly using the ratio between two measurements of optical properties that were not performed at isoabsorptural wavelengths, or it can be the average of multiple ratios calculated using three or more measurements of different pairs of optical properties.

Claims

1. A method for verifying the calibration of a sensor, which is represented by a calibrated relationship between the concentration of an analyte in a sample and the measurement of the optical properties of a detected substance by the sensor, wherein the optical properties of the detected substance have a spectrum that changes with the concentration of the analyte in the sample, and the spectrum has equiabsorbent wavelengths at which the optical properties of the detected substance do not change with the concentration of the analyte, The aforementioned method, The steps include: measuring the optical properties at three or more wavelengths of light while the detection substance is exposed to the sample; The steps include: deriving multiple concentration measurements of the analyte in the sample from the measurements at three or more wavelengths according to the calibrated relationship; A step of determining the deviation measurement of the optical properties from the calibrated relationship, wherein the deviation measurement is a variation measurement between the plurality of concentration measurements. If the deviation measurement value exceeds a predetermined threshold, the step of determining that the measurement of the optical characteristics does not match the calibrated relationship, A step of outputting a warning signal in response to the measurement of the optical properties being inconsistent with the calibrated relationship. A method that includes this.

2. The method according to claim 1, further comprising the step of deriving a concentration measurement of the analyte in the sample from the measurement according to the calibrated relationship.

3. The calibrated relationship includes the relationship between the concentration and the ratio between the measurement of the optical properties at a pair of wavelengths, The method according to claim 2, wherein the step of deriving the concentration measurement value includes the steps of calculating the ratio between the measurements of the optical properties at two wavelengths, and deriving the concentration measurement value from the ratio according to the calibrated relationship.

4. The method according to claim 3, wherein the concentration measurement value includes a measurement value calculated using the ratio between two measurements of the optical properties that were not performed at the same absorption wavelength.

5. The calibrated relationship includes the relationship between the concentration and a plurality of ratios between the measurements of the optical properties at each pair of wavelengths, The method according to claim 1, 2, or 4, wherein the step of deriving a plurality of concentration measurements of the analyte in the sample includes the steps of calculating the ratio between the measurements of the optical properties at each pair of wavelengths, and deriving the plurality of concentration measurements from the ratio according to the calibrated relationship.

6. The method according to any one of claims 1 to 5, wherein the measured value of variation is the coefficient of variation.

7. The method according to any one of claims 1 to 6, wherein one of the three or more wavelengths is the equiabsorbent wavelength.

8. The method according to any one of claims 1 to 7, wherein one of the three or more wavelengths is the wavelength at which the spectrum of the optical properties of the detection material has the maximum or minimum.

9. The method according to any one of claims 1 to 8, wherein the optical property is one of absorption and emission, and the spectrum is one of an absorption spectrum and an emission spectrum, respectively.

10. The optical property is light emission, and the step of measuring the optical property at three or more wavelengths is performed for each of the three or more wavelengths: A step of exciting the detection substance using light of a first wavelength, A step of measuring the intensity of light emitted by the detection substance at each of the three or more wavelengths. The method according to claim 9, wherein the first wavelength is the same for each of the three or more wavelengths.

11. The optical property is absorption, and the step of performing multiple measurements of the optical property at three or more wavelengths is as follows: A step of exciting the detection substance using light at each of the three or more wavelengths, A step of measuring the intensity of light emitted by the detection substance at a second wavelength. The method according to claim 9, wherein the second wavelength is the same for each of the three or more wavelengths.

12. The optical property is absorption, and the step of performing multiple measurements of the optical property at three or more wavelengths is as follows: The steps include irradiating the detection substance with light at each of the three or more wavelengths, A step of measuring the intensity of light transmitted through the detection substance at each of the three or more wavelengths. The method according to claim 9, including the method described in claim 9.

13. The method according to any one of claims 1 to 12, wherein the detection substance comprises two species in equilibrium, and the equilibrium between the two species depends on the concentration of the analyte in the sample.

14. The method according to any one of claims 1 to 13, wherein the analyte is one of carbon dioxide and hydrogen ions.

15. The method according to any one of claims 1 to 14, wherein the sample comprises blood or interstitial fluid.

16. The method according to any one of claims 1 to 15, wherein the detection substance comprises a luminescent compound.

17. The method according to claim 16, wherein the luminescent compound is a fluorescent compound.

18. The method according to claim 16 or 17, wherein the detection substance comprises 8-hydroxypyrene-1,3,6-trisulfonic acid.

19. The method according to any one of claims 1 to 18, wherein the step of measuring the optical properties includes the steps of performing raw measurements at multiple points in time and averaging the raw measurements over time.

20. The method according to any one of claims 1 to 19, further comprising an initial calibration step of determining the calibrated relationship between the concentration of the analyte and the measurement of the optical properties.

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