Diagnostic or screening tests are used to help determine whether or not a patient has a certain condition or disease. The ability of a diagnostic test to correctly classify subjects is expressed by the four test characteristics-sensitivity, specificity, predictive value positive, and predictive value negative. This paper describes these characteristics and discusses methods for choosing optimal tests or cutoff points to maximize expected value considering the consequences of incorrect diagnoses. Data drawn from ongoing studies of facial pain are used to illustrate some of these concepts.