Clinicians can now base clinical decisions on the results of rigorous studies of the performance of diagnostic tests. In selecting the tests, clinicians should take into account their sensitivity (the proportion of patients with the disease who have a positive test result) and specificity (the proportion of patients without disease who have a negative test results). Sensitivity and specificity are affected by the "spectrum" of patients studied i.e. the severity of disease in those with disease and the clinical characteristics of those without disease. Test results are interpreted by predictive values--the proportion of patients with a positive (negative) test result who have (do not have) the disease. Predictive values depend on both test sensitivity and specificity and disease prevalence. The information content of a test is further increased by taking into account where the cut-off point between normal and abnormal is placed, the degree of abnormality of the result, and the results of the preceding tests (done either in parallel or in series). Describing test performance in terms of likelihood ratios facilitates this process. Readers should be able to critique published studies of diagnostic tests which are still far from perfect.