Abstract: Disclosed is an assay for determining the presence of at least one analyte in a sample, which involves the steps of: (a) mixing the sample and predetermined amounts of first test microparticles having disposed thereon a binding molecule which binds a first analyte, and inert reference microparticles to form a reaction mixture, and to allow for the first binding molecule to bind the first analyte; b) counting the numbers of the non-reacted first test microparticles and the reference microparticles; and (c) comparing the number of non-reacted first test microparticles to the reference microparticles to thereby establish a first test value so that the presence of the first analyte in the sample may be determined. In a preferred embodiment, the first test value is compared to a control value for a non-reactive sample, and a change value is calculated therefrom.