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