Outcome 2: Sustained uptake and adherence to oral PrEP among female sex workers Similar to all medications, sustained uptake and adherence to PrEP is fundamental for efficacy in preventing HIV acquisition. In the four clinical trials of oral PrEP that demonstrated efficacy, study participants with high adherence had the greatest protection from HIV (Baeten 2012, Choopanya 2013, Grant 2010, Thigpen 2012). Likewise, across PrEP trials, there was a clear relationship between higher efficacy and a greater proportion of participants with study drug detectable in their blood. Two randomized clinical trials of daily oral PrEP among heterosexual women in Africa found no efficacy for HIV prevention; poor adherence to PrEP most likely explains these results (Marrazzo 2013, Van Damme 2012). For example, drug level testing in the FEM-PrEP Study found that less than 40% of a random sample of HIV negative women in the PrEP arm of the study had detectable levels of PrEP in their blood (Van Damme 2012). Analyses of high-risk subgroups of study participants in the Partners PrEP Study found PrEP efficacy estimates ranged from 64 to 84%, showing consistently high efficacy of PrEP in preventing HIV across various higher risk groups, including women (Murnane 2013).