In a placebo-controlled PrEP efficacy trial of tenofovir disoproxil fumarate ( TDF ) and TDF with emtricitabine ( FTC/TDF ) among 4747 African women and men with an HIV-infected partner , we measured plasma tenofovir concentrations from participants in the active PrEP arms : 29 HIV seroconverters ( cases ) and 196 randomly selected controls who remained uninfected .