WHOSE BLOOD IS SAFER - THE EFFECT OF THE STAGE OF THE EPIDEMIC ON SCREENING FOR HIV

Background. With improvements in HIV antibody test (ELISA) performance , the window of time between infection and seroconversion becomes a ma jor source of error in HIV screening. The authors examined its impact on the false-reassurance rate (FRR). Methods. Test sensitivity was mod eled as the product of two factors: the inherent sensitivity (sensitiv ity when antibody is present) and the probability that antibody is pre sent in infected blood. A model of HIV and AIDS incidence was used to derive an estimate of the probability of remaining in the seronegative window (p(w)) among those who are infected. With plausible assumption s, this probability approaches 0.03. The FRR was then estimated as a f unction of the probability of remaining in the seronegative window, th e prevalence of HIV, and the inherent sensitivity of the ELISA test we re estimated. Results. The FRRs for two blood donor groups, one with a n HIV prevalence of 0.004 and a typical probability of remaining in th e seronegative window (p(w) = 0.03) and the other with a higher preval ence of 0.017 but fewer donors in the window (p(w) = 0.003), are equal (140 per million donors) if the blood is negative on a single ELISA t est. After two negative tests or a single test that can detect antibod y more reliably, however, the FRR is much higher in the group with the higher p(w) (= 120 per million compared with 50 per million), because the greater numbers of donors in the window more than offsets the low er prevalence. Conclusions. With improvements in inherent sensitivity of ELISA by virtue of technical progress or retesting, the prevalence of HIV infection may no longer play the critical role in degrading the results of blood screening. As inherent test performance improves, te sts are increasingly likely to miss infected blood because of the sero negative-window error rather than because of measurement error. Window error plays a proportionally greater role during the early stages of HIV dissemination in a population where the incidence of new HIV infec tion is high relative to the incidence of AIDS. These findings may exp lain, in part, the recent observation that cases of transfusion of con taminated blood often take place in areas where AIDS epidemics have st arted recently. They also suggest that the traditional strategy of sol iciting blood donors from low-prevalence populations may not always be optimal, unless such populations are truly low-risk.