Background:  Undetected acute HIV infections pose a significant transmission risk because high viral loads increase the potential biological transmissibility.  Persons unaware of their infection status may engage in risky behaviors, further contributing to HIV transmission risk.  To address this key aspect of HIV care and prevention, acute HIV infection cases were identified among sexually transmitted diseases (STD) clinic patients in San Francisco.

Methods:  A consecutive sample of all persons presenting for voluntary confidential HIV testing at the San Francisco municipal STD clinic were evaluated for HIV infection by standard enzyme-linked immunoassays (EIAs).  To identify acute HIV infections, EIA-negative specimens were screened using nucleic acid amplification testing (NAAT) that detects the presence of HIV-RNA.  HIV-negative testers were used as the comparison group for the analysis of acute HIV infections because these cases otherwise would have been considered HIV-negative by standard HIV antibody testing prior to initiation of the NAAT protocol.  Associations between acute HIV infections and demographic characteristics and risk behaviors were assessed using the Chi-square statistical test.

Results:  A total of 3,789 persons were evaluated for HIV infection from January through December 2004.  Eleven acute HIV infections were detected among the 136 newly diagnosed HIV infections.  All 11 acute HIV infections were detected in men who have sex with men, of which 4 were Caucasian, 3 were Latino, 2 were African-American, and 2 were Asian/Pacific Islander.  Persons with acute HIV infections were more likely to have a known HIV-positive partner (p=0.0461) and a history of hepatitis B (p=0.0016), syphilis (p<0.0001), and Chlamydia (p=0.0027), compared to persons testing HIV-negative.

Conclusions:  Acute HIV infection cases contributed to an 8.8% increase in the rate of HIV case identification.  Incorporation of the NAAT protocol to detect acute HIV infections represents a substantial improvement over current HIV screening algorithms that solely use standard antibody testing.  Enhanced case detection may be especially significant if undiagnosed acute HIV infection results in risk of secondary transmission or missed opportunity for early care.