Abstract: In an assay where a ligand or organism is detected by a detector binding protein ("DBP") capable of generating a signal in proportion to the amount of ligand or organism bound to the DBP, the presence of the ligand or organism is confirmed by the use, prior to or concurrently with the DBP, of a confirmatory binding protein ("CBP") which binds to a second site on the ligand or organism, and thereby prevents the DBP from binding to the first site. Thus, a reduction in signal of a predetermined amount indicates the true presence of the ligand or organism, while failure to obtain signal reduction of a predetermined amount indicates that the original signal was a false positive due to assay artifact or detection of related ligands or organisms. In a preferred embodiment, the CBP is a monoclonal antibody and the organism is derived from Chlamydia species.