Abstract: An immunoassay for assaying an antigenic substance (Ag) in a fluid. The immunoassay is the type which comprises contacting the fluid with at least one first entity selected from a group consisting of an antibody (Ab) to the Ag, a soluble, labeled antibody (L-Ab.sub.a) to the Ag, and an antibody (Ab.sub.b) to the Ag bound to a solid support (SC). The immunoassay is characterized in that the fluid is contacted with at least one additional entity selected from a group consisting of at least one different type of soluble, labeled antibody (L-Ab.sub.c) to the Ag, at least one different type of antibody (Ab.sub.d) bound to a solid carrier (SC.sub.1), and at least one different type of antibody (Ab.sub.e) to the Ag. The SC.sub.1 is selected from a group consisting of SC, at least one different solid carrier (SC.sub.2), and mixtures thereof (SC and SC.sub.2). Each type of L-Ab.sub.c, Ab.sub.d -SC.sub.1, and Ab.sub.e has a lower average affinity constant (K) for Ag than each respective K of L-Ab.sub.a, Ab.sub.

Abstract: A direct particle agglutination immunoassay for assaying an antigenic substance (Ag) in a fluid. The immunoassay is the type which comprises:(a) contacting the fluid with an antibody (Ab) coated particle (P) to agglutinate the Ab coated particles; and(b) detecting the presence of agglutination.The immunoassay is characterized in that the fluid is contacted with at least one additional entity selected from a group consisting of at least one different type of antibody (Ab.sub.a) coated particle (P.sub.1) and at least one different type of antibody (Ab.sub.b). The P.sub.1 is selected from a group consisting of P, at least one different particle (P.sub.2), and mixtures thereof (P and P.sub.2). Each type of Ab.sub.a -P.sub.1 and Ab.sub.b has a lower average affinity constant (K) for Ag than the K of Ab-P and the additional entity is present in an amount sufficient to avoid a high antigen false negative effect.Also, a reagent of the type comprising Ab-P.