We randomly selected a subcohort of 150 participants randomized to FTC/TDF in 3 FEM-PrEP sites ( Bondo , Kenya ; Bloemfontein , South Africa ; and Pretoria , South Africa ) to examine adherence levels over time and to assess factors associated with adherence , based on plasma tenofovir and intracellular tenofovir diphosphate drug concentrations in specimens collected at 4-week visit intervals .