To the Editor.
—The recent article by Dr Gann and colleagues1 assesses the utility of random screening using prostate-specific antigen (PSA) for the detection of prostate cancer. The authors conclude that the use of a single PSA measurement is "highly informative." They evaluated their data using sensitivities (defined as the percentage of prostate cancer cases testing positive for PSA) and specificities (defined as the percentage of prostate cancer—free controls testing negative for PSA). However, sensitivity and specificity values, by themselves, do not determine the usefulness of a diagnostic test. Rather, usefulness is better determined by evaluating positive predictive values (PPVs) and negative predictive values (NPVs).2The PPV of a test is the probability that a person with an elevated test value has the disorder in question. Unlike sensitivity and specificity values, PPVs vary depending on the prevalence of the disease in the population. This dependence becomes particularly important
Brunswick DJ. The Utility of Prostate-Specific Antigen for Detecting Prostate Cancer. JAMA. 1995;274(8):607. doi:10.1001/jama.1995.03530080023019