Biomarker validation of recent unprotected sexual intercourse in a prospective study of young women engaged in sex work in Phnom Penh, Cambodia.

BACKGROUND Accurate measurement of unprotected sex is essential in HIV prevention research. Since 2001, the 100% Condom Use Program targeting female sex workers (FSWs) has been a central element of the Cambodian National HIV/AIDS Strategy. We sought to assess the validity of self-reported condom use using the rapid prostate-specific antigen (PSA) test among Cambodian FSWs. METHODS From 2009 to 2010, we enrolled 183 FSWs in Phnom Penh in a prospective study of HIV risk behavior. Prostate-specific antigen test results from the OneStep ABAcard were compared with self-reported condom use in the past 48 hours at quarterly follow-up visits. RESULTS Among women positive for seminal fluid at the first follow-up visit, 42% reported only protected sex or no sex in the detection period. Discordant results were more likely among brothel and street-based FSW versus entertainment (56% vs. 17%), recent (last 3 months) amphetamine-type stimulant (ATS) users (53% vs. 20%), and those with 5 or more partners in the past month (58% vs. 13%). In multivariable regression models, positive PSA results were associated with recent ATS use (adjusted risk ratio [ARR], 1.5; 95% confidence interval [CI], 1.1-2.2), having a nonpaying last sex partner (ARR, 1.7; CI, 1.2-2.5), and sex work venue (ARR, 3.0; CI, 1.4-6.5). Correspondingly, women with a nonpaying last sex partner were more likely to report unprotected sex (ARR, 1.5; CI, 1.1-2.2), but no associations were found with sex work venue or ATS use. CONCLUSIONS Results confirm the questionable validity of self-reported condom use among FSW. The PSA biomarker assay is an important monitoring tool in HIV/sexually transmitted infection research including prevention trials.