We randomly selected a subcohort of @ participants randomized to FTC/TDF in @ 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 @-week visit intervals .