Table 2.

TFV and FTC Concentrations in Hair Samples^a

Hair Sample No.	Duration of PrEP (Weeks)	TFV Concentrationb (ng/mg Hair)	FTC Concentration (ng/mg Hair)
1	32	0.026	0.582
2	10	0.026	0.477
3	57	0.014	0.121
4	25	0.050	0.619
5	NA	BLQ	BLQ
6	142	0.027	0.265
7	11	0.037	0.705
8	28	0.040	1.167
9	11	0.009	0.213
10	11	0.031	0.761
11	59	0.028	0.537
12	156	0.005	0.040
13	11	0.027	0.092
14	47	0.030	0.093
15	48	0.020	0.194

Abbreviations: BLQ, below lower limit of quantification; FTC, emtricitabine; NA, not available; PrEP, pre-exposure prophylaxis; TFV, tenofovir.

^aHair samples were analyzed by high-performance liquid chromatography-tandem mass spectrometry.

^bOn average, a TFV value of 0.038 ng/mg hair is consistent with dosing 7 days per week, and 0.023 ng/mg hair is consistent with dosing 4 days per week [35]. In our small subset, 73% (11 of 15) had TFV values indicating at least 4 days per week adherence to PrEP.