For the analysis of plasma TFV concentrations, we used a case–cohort design,13 in which a ran- dom subcohort was selected from the active study- product groups (cohort) and was enriched with all remaining participants who had undergone HIV-1 seroconversion (cases). For every participant who underwent seroconversion, we sampled approximately three participants who remained uninfected with HIV-1 throughout participation. Predictors of TFV detection were assessed with the use of generalized estimating equations with a logistic link function and exchangeable working correlation matrix. We conducted multivariate survival analyses to assess the association between the detection of the drug in plasma obtained at follow-up visits and time to HIV-1 infection.14 Generalized-estimating-equation models with a binomial link, exchangeable correlation structure, and robust standard errors were used to determine baseline predictors of plasma TFV detection. A Cox regression was used to determine baseline predictors of HIV acquisition and to analyze the association between TFV detection and HIV acquisition.