Predictive value of a screening test for antibodies to HTLV-III

A test for the detection of antibodies to HTLV-III is available and will be widely used to screen donated blood to prevent transfusion-associated acquired immunodeficiency syndrome (AIDS). Based upon the sensitivity and specificity, the authors calculated the expected predictive values for different groups of asymptomatic individuals using Bayes' theorem. The prevalence of HTLV-III infection has great impact upon the positive and negative predictive values of the test. For a member of the general population there is a less than 3% chance that a positive test represents a true positive. High-risk patients, such as hemophiliacs, with a positive test will have a greater than 95% chance of actually having antibodies to HTLV-III, but the negative predictive value of the test in this group is less than ideal. The authors recommend that all positive tests for HTLV-III be confirmed by more specific methods when obtained in low-risk people. Members of high-risk groups for AIDS should continue to refrain from donating blood, despite the availability of the screening test.