Predictive value of serologic tests.

An analysis of serologic tests for treponemal disease performed in Australian aboriginal communities is used to illustrate factors influencing the predictive value positive of serologic tests. Simple extrapolation of predictive value estimates, from prevalence, sensitivity, and specificity data, is complicated by variation of specificity between strata of a population, and the weighting of true positive and false positive results produced by the criteria for selecting individuals for testing. The predictive value positive of a particular test is greatest when it is used for incident cases suspected of having a disease, but lower when the same test is used to screen a whole population unless longitudinal data are available to exclude individuals with past disease. Preferential testing of individuals without active disease may produce very low predictive values. Empirical estimation of the predictive value of tests provides objective guidelines for decision making and enables increased predictability by modification of testing criteria.