Abstract Body

Ensuring individuals are HIV-uninfected prior to pre-exposure prophylaxis (PrEP) initiation and continuation is critical to minimize the risk of developing resistance, but the optimal HIV testing algorithm for PrEP is not yet known. We evaluated the performance of rapid blood EIA (rEIA), lab-based p24-antigen/antibody (4th generation), and RNA tests in detecting HIV infection at PrEP screening (n=635), and initiation and follow-up (n=557) in the MSM and transgender women (TGW) enrolled in the US PrEP Demonstration Project, an open label study of FTC/TDF PrEP in San Francisco, Miami, and Washington DC.

MSM and TGW interested in PrEP underwent screening, followed by enrollment within 45 days, when PrEP was initiated. All participants (ppts) were HIV tested at screening, enrollment, and follow-up (weeks 4, 12, 24, 36, 48) with rEIA and 4th generation tests. Pooled RNA was performed at all visits in San Francisco, with quantitative or qualitative RNA only at enrollment in DC and Miami, respectively. Any positive test was confirmed using local testing algorithms.

At screening there were 16 (2.5%) ppts with a reactive rEIA and/or 4th generation test, 15 of whom were confirmed to be HIV infected. At enrollment 3 ppts had HIV infection: all were rEIA (-), 4th generation (-) and RNA (+), with viral loads of 120, 3343, and 51 copies/mL (all pre-treatment); 2 started ART as soon as the (+) RNA was known, and all viral loads were positive on repeat testing. Of 2 infections during follow-up, both were rEIA (+)/4th generation (+) at the visit with first evidence of infection; both had low or undetectable drug levels at seroconversion and no evidence of resistance on standard and ultrasensitive genotyping assays. During follow-up, of 2680 rapid EIAs there were 6 false positives in 2 ppts, for a specificity of 99.78% and a positive predictive value (PPV) of 25%; of 2673 4th gen tests there were 3 false positives in 2 ppts, for a specificity of 99.89% (PPV 50%). There were no false positive RNA tests at screening, enrollment or follow-up.

Rapid EIA and Lab-based 4th generation testing detected most HIV infection before PrEP initiation. Acute HIV infection should be ruled out with an RNA test before starting PrEP, if available, particularly in clients with recent exposure; low viral loads should not be assumed to be false positives. In this cohort, with high PrEP adherence, rapid EIA and lab based 4th generation tests were adequate to detect HIV infection during follow-up.