A method of determining active concentration of an analyte in a sample comprises the steps of:
(a) contacting a laminar flow of the sample with a plurality of solid phase surfaces or surface area supporting a ligand capable of specifically binding the analyte, each surface or surface area having a different ligand density,
(b) determining the initial binding rate (dR/dt) of analyte to the ligand at each ligand-supporting surface or surface area,
(c) from the determined initial binding rates determining the initial binding rate corresponding to transport-limited interaction at the surfaces or surface areas, and
(d) from the initial binding rate determined in step (c) determining the active analyte concentration.