Detection of early HIV infection and estimation of incidence using a sensitiveness-sensitive enzyme immunoassay testing strategy at anonymous counseling and testing sites in San Francisco

Timely estimates of HIV incidence are needed to monitor the epidemic and ta rget primary prevention but have been difficult to obtain. We applied a sen sitive/ less-sensitive (S/LS) enzyme immunoassay (EIA) testing strategy to stored HIV-positive sera (N = 452) to identify early infections, estimate i ncidence, and characterize correlates of recent seroconversion among person s seeking anonymous HIV testing in San Francisco from 1996 to 1998 (N = 21, 292). Sera positive on a sensitive EIA but negative on a less-sensitive EIA were classified as early HIV infections; sera positive on both EIA were cl assified as long standing. Seventy-nine sera were from people with early HI V infection. Estimated HIV incidence was 1.1% per year (95% confidence inte rval [CT], 0.68%-1.6%) overall and 1.9% per year (95% CI, 1.2%-3.0%) among men who have sex with men (MSM). Early HIV infection among MSM was associat ed with injection drug use, unprotected receptive anal sex, and multiple se x partners in the previous year. No temporal trend in HIV incidence was not ed over the study period. The S/LS strategy provides a practical public hea lth tool to identify early HIV infection and estimate HIV incidence in a va riety of study designs and settings.