PREDICTIVE VALUES OF USUAL BIOLOGIC TESTS FOR DETECTION OF HIV - CONSEQUENCES FOR SCREENING

This paper tries to review what is scientifically known about the pred ictive values of biological tests of HIV infection. The epidemiologica l situation for that infection is characterized by two facts : the ver y high values of sensitivity and specificity which are close to unity; the prevalence of seropositivity which is on average - at least in we stern countries -, very low (except for some small specific groups). U nder those conditions, Negative Predictive Values are always very clos e to unity, and the percentage of false negative tests is extremely lo w. Things are quite different for Positive Predictive Value, which var ies very rapidly with very small shifts or uncertainties about specifi city and prevalence. In the case when prevalence is very low (general population screening) and at the same time specificity is not excellen t (that means < 0,99 or even < 0,995), Positive Predictive Value is ve ry poor and the proportion of false positive tests rather important. I ndeed the analysis of scientific literature, using the method of <<bes t synthesis evidence>>, reveals numerous discrepancies as to the value of specificity among different tests. Figures vary a lot from one stu dy to another. It is not obvious which screening strategies are concer ned by the results, which finally entail a strong statistical uncertai nty. Finally, the figures published in the literature are given by hig h standard laboratories. One may fear the tests realized in routine la boratories are less reliable. As a conclusion, let us say that despite their very good quality, the biological tests, when used separately, should not be trusted without strong previous criticism when applied t o samples of the general population. Any biological screening should b e preceded by a clinical examination, including a precise inquiry, in order to detect people at risk, that means with a high prior probabili ty. Clinical dialogue has moreover another great interest : it allows health consulting and education, and calls for personal responsibility for both seropositive and negative subjects. It is the best choice of method to reach a high preventive effectiveness.