Methods for simultaneously detecting both members of a binding pair

A method for simultaneously measuring both members A and B of a binding pair complex in a biological sample, said method comprising:

• a) providing a solid phase reagent, said solid phase reagent comprising a particle, said particle comprising capture antibodies having specific binding affinities for said member A of said binding pair complex, and wherein said particle does not comprise a capture antibody having specific binding affinity for said member B of said binding pair complex;
• b) contacting said biological sample with said solid phase reagent under conditions in which said member A, if present, becomes bound to said particle;
• c) contacting said solid phase reagent obtained from step (b) with first antibodies having specific binding affinities for said member A, wherein said first antibodies are labeled with a first label, and with second antibodies having specific binding affinities for said member B of said binding pair complex, wherein said second antibodies are labeled with a second label, wherein said first and second labels are different;
• and
• d) measuring said first and second labels on said solid phase reagent obtained from step (c).