Predictive value of past and current screening tests for syphilis in blooddonors: changing from a rapid plasma reagin test to an automated specific treponemal test for screening

BACKGROUND: This study evaluated the change from a rapid plasma reagin (RPR ) test to an automated specific treponemal test (PK-TP) in screening for sy philis in blood donors. STUDY DESIGN AND METHODS: A cross-sectional seroprevalence analysis was per formed on 4,878,215 allogeneic blood donations from 19 American Red Cross B lood Services regions from May 1993 through September 1995. Positive predic tive values relative to the confirmatory fluorescent treponemal antibody ab sorption test (FTA-ABS) were calculated. Differences in seroprevalence were compared in RPR and PK-TP tests for 1) unconfirmed and confirmed tests, 2) first-time and repeat donors, and 3) "recent" versus "past" infections. Do nation data from three additional Red Cross regions were evaluated for repe at donation patterns of blood donors who had a donation that was positive i n a serologic screening test for syphilis. The value of RPR and PK-TP tests as surrogate markers for HIV infection was compared. RESULTS: Reactive rates were lower but the positive predictive values was h igher for the PK-TP test than for the RPR test. Initially, donors screened by PK-TP were more likely to be confirmed as positive than were donors scre ened by RPR, but these rates became comparable. It is estimated that a sing le HIV window-period donation was removed by serologic testing for syphilis each year of this study period. CONCLUSIONS: The change to the PK-TP test resulted in a lower repeatedly re active rate, better prediction that a confirmed-positive test for syphilis would occur in testing in the FTA-ABS, fewer donations lost, and comparable deferral rates. Because of the high rate of reactivity to serologic testin g for syphilis among donors previously confirmed positive for syphilis, ind efinite deferral after a confirmed-positive index donation may be warranted . Serologic testing for syphilis is ineffective as a marker of HIV-infectio us window-period donations.