FALSE-POSITIVE HIV-1 TEST-RESULTS IN A LOW-RISK SCREENING SETTING OF VOLUNTARY BLOOD DONATION

Context.-Persons at risk of human immunodeficiency virus 1 (HIV-1) inf ection have been classified incorrectly as HIV infected because of Wes tern blot results, but the frequency of false-positive Western blot re sults is unknown. Objectives.-To determine the frequency of false-posi tive HIV-1 Western blot results in US blood donors and to make project ions to other screened populations. Secondarily, to validate an algori thm for evaluating possible false-positive cases. Design.-A retrospect ive cohort study of HIV-1 enzyme immunoassay (EIA) and Western blot re sults from large blood donor screening programs in which donors with s uspected false-positive Western blot results underwent HIV-1 RNA polym erase chain reaction (PCR) testing and follow-up HIV-1 serology. Setti ng.-Five US blood centers participating in the Retrovirus Epidemiology Donor Study. Participants.-More than 5 million allogeneic and autolog ous blood donors who successfully donated blood at 1 of the 5 particip ating centers from 1991 through 1995. Main Outcome Measures.-Rate of f alse positivity by Western blot and true HIV-1 infection status as det ermined by HIV-I RNA PCR and by serologic followup of blood donors mor e than 5 weeks after donation. Results.-Of 421 donors who were positiv e for HIV-1 by Western blot, 39 (9.3%) met the criteria of possible fa lse positivity because they lacked reactivity to p31. Of these, 20 (51 .3%) were proven by PCR not to be infected with HIV-1. The false-posit ive prevalence was 4.8% of Western blot-positive donors and 0.0004% (1 in 251 000) of all donors (95% confidence interval, 1 in 173 000 to 1 in 379 000 donors). Conclusions.-A false diagnosis of HIV-1 infection can result from the combination of EIA and Western blot testing in bl ood donor and other HIV-1 screening programs. Individuals with a posit ive Western blot result lacking the p31 band should be counseled that, although they may be HIV infected, there is uncertainty about this co nclusion. These individuals should be further evaluated by RNA PCR tes ting (if feasible) and HIV serologic analysis on a follow-up sample.