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 most important properties. Bayes' theorem is used to unify the five current mathematical MDC definitions. The unified definition has significant implications for defining positive results for screening and diagnostic tests, setting criteria for immunoassay quality control and optimal design, reliably measuring biological substances at low concentrations, and, in general, measuring small analyte concentrations with calibrated analytic methods. As an illustration, we apply the unified definition to the microparticle capture enzyme immunoassay for prostate-specific antigen (PSA) developed for the Abbott IMx automated immunoassay system. The MDC of this assay as estimated by our unifying approach 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 concentrations of PSA to detect early recurrences of prostate cancer is probably overstated.