Immunoassay of free substances in biological fluids

Abstract

A method of determining free analyte level in a sample containing the analyte in free and bound portions is accomplished in sequential steps as follows. The sample is combined with a known first liquid volume containing soluble anti-analyte receptor in excess of the free analyte in the sample. The resultant mixture is incubated for a period of time sufficient to allow the binding reaction of the free analyte with the anti-analyte receptor to reach substantial completion to form a first incubation mixture. A known second liquid volume is added thereafter which contains soluble conjugate sufficient to reduce the sample proportion in the resultant mixture to no greater than 10% v/v, with the conjugate comprising a labeled analyte or labeled analog thereof, to allow the conjugate to bind with the anti-analyte receptor not bound in the first incubation mixture to form a second incubation mixture. Thereafter, separation means is introduced to the second incubation mixture which comprises a solid phase coated with immobilized binder for the anti-analyte receptor, which binder is in excess of the anti-analyte receptor to bind substantially with all of the anti-analyte receptor in the incubation mixture. The solid phase is then separated and washed from the mixture. The amount of conjugate that is bound to the solid phase is measured, and the free analyte concentration in the sample is determined.