[Sensitivity and specificity in AIDS screening system]

The interest in the optimal combination of the diagnostic tests is growing in terms of reasoning rationale and cost-effectiveness of the diagnostic system. As an example of the two-stage test system, we investigated the sensitivity, specificity, and posterior probabilities of the HIV screening system with incomplete information of the conditional independency between the enzyme immunoassay and the Western Blot. The theoretical analysis of the probabilistic reasoning of the HIV screening system could provide us with the upper and lower limits of the predictive positive value, given the prior probability of HIV infection. Consequently, we could show that the lower and the upper boundaries of the predictive positive value are, respectively, 4.07% and 100% for the low risk group (prevalence of HIV: 0.01%), while 99.86% and 100% for the high risk group (prevalence of HIV: 62%). These analyses imply that clinicians must be very careful in the interpretation of the positive test result in the HIV screening system, depending on how the risk of HIV infection of the patient is estimated in advance. In addition to the probabilistic analyses of the test parameters, the approach by use of the utility theory is favorable for the issue of optimizing the diagnostic test sequences. It remains for future investigation.