Issues in Human Immunodeficiency Virus (HIV) Screening Programs

Unlike test sensitivity and specificity, the false positive and negative predictive values (probabilities of mislabeling an individual being tested) depend heavily on the prevalence of the infection of the human immunodeficiency virus (HIV) as well as the quality of the kit. A consequence of this dependence is that the false positive predictive value can reach a high magnitude such as 0.9; that is, 90 % of the positive tests are false. This raises many important issues pertaining to the current practice of HIV screening such as to how to control these misclassification errors, how to interpret test results, and how to estimate prevalence using test results. These issues are examined in detail here by considering the factors that dictate the quality of a screening program. Some real data examples are used to illustrate the importance of this consideration in designing programs to achieve the desired goals. The rationale behind the common two-step sequential protocol in HIV screening is examined to point out its limitations under practical situations. Finally, the use of entropy in evaluating the informativeness of a screening program is discussed. Am J Epidemiol 1992;136:244-55. bias (epidemiology); HIV; information theory; predictive value of tests; sensitivity and specificity (epidemiology) The spread of the human immunodefi-ciency virus (HIV) has been devastating; it is estimated that already over a million per-sons in the United States have been infected with the virus (1,2). While progress has been made in treating the deadly disease, a vac-cine does not seem to be forthcoming in the near future. Serious steps have been taken Received for publication July 15, 1991, and in final for