The value of clinical laboratory tests as aids in diagnosis and prognosis depends on their sensitivity, specificity, and accuracy. Few tests fulfill the desirable criteria for use as screening tests in the asymptomatic, normal population. In the symptomatic patient a test of high sensitivity for detecting disease has great clinical value since a negative result tends to exclude the index diagnosis for consideration at that point in time. A test of high specificity for a single disease invariably has decreased sensitivity but has diagnostic value when positive. Data on the indirect immunofluorescent antinuclear antibody test (FANA) provide criteria to justify its wide application as a clinical laboratory test. A negative FANA result essentially excludes active systemic lupus erythematosus (SLE). Quantitation by serum titration of the positive FANA and attention to the nuclear fluorescence patterns provide indices of specificity for diagnosis of the various FANA-positive autoimmune diseases. Similarly, negative results of indirect fluorescent antibody tests for antithyroid antibodies essentially exclude Hashimoto's thyroiditis, and a negative test for antimitochondrial antibodies in biliary cirrhosis tends to exclude the surgically amenable form due to biliary stones.