Abstract: An assay for determining the presence of a threshold concentration of an analyte in a sample comprises first reacting the sample with an amount of anti-analyte selected to reduce the free analyte to a marginally detectable concentration. The sample is then contacted with anti-analyte immobilized on a test region or indicator zone on a solid phase, whereby the residual free analyte may be bound. The binding of free analyte to immobilized anti-analyte is detected by a variety of techniques to indicate the presence of the threshold concentration. By employing limited amounts of anti-analyte on the solid phase, the change between maximum binding of label and no binding of label will be responsive to very small changes in the analyte concentration originally present in the sample.

Abstract: Method and apparatus are provided for performing immunoassays employing a device comprising a relatively small test zone referred to as an immunosorbing zone, and a relatively large liquid absorbing zone in liquid receiving relationship with said immunosorbing zone. The immunosorbing zone includes a member of an immunological pair ("mip")--ligand and antiligand--bound to a support.A signal producing system is employed in conjunction with said device having as one component a signal label bound to a mip. The signal producing system provides for production of a detectible signal in the immunosorbing zone in relation to the amount of analyte in a sample.