DEFINING THE SMALLEST ANALYTE CONCENTRATION AN IMMUNOASSAY CAN MEASURE

An immunoassay's minimal detectable concentration ((MDC), the smallest analyte concentration the assay can reliably measure, is one of its m ost important properties. Bayes' theorem is used to unify the five cur rent mathematical MDC definitions. The unified definition has signific ant implications for defining positive results for screening and diagn ostic tests, setting criteria for immunoassay quality control and opti mal design, reliably measuring biological substances at low concentrat ions, and, in general, measuring small analyte concentrations with cal ibrated analytic methods, As an illustration, we apply the unified def inition to the microparticle capture enzyme immunoassay for prostate-s pecific antigen (PSA) developed for the Abbott IMxm(TM) automated immu noassay system. The MDC of this assay as estimated by our unifying app roach is shown to be 4.1-7.1 times greater than currently reported, As a consequence, the ability of the assay to measure reliably small con centrations of PSA to detect early recurrences of prostate cancer is p robably overstated.