Accuracy of PrEP adherence measures among adolescents men who have sex with men and transgender women in Brazil


BACKGROUND: Adherence is fundamental to PrEP effectiveness. Direct measures of adherence are expensive and not sustainable in most PrEP services. Therefore, an indirect measure is a feasible alternative. The aim of this analysis was to assess the concordance of indirect measures for PrEP adherence comparing them with a direct measure.
METHODS: PrEP1519 is a daily PrEP demonstration cohort study among adolescent men who have sex with men (aMSM) and transgender women (aTGW) aged 15-19 years. Indirect measures such as medication possession ratio (MPR), pill-count, and self-report were compared to tenofovir diphosphate (TFV-DP) concentrations in dried blood spot (DBS). Participants receiving pills of FTC/TDF, had DBS collected and stored at each study visit, were asked to return their PrEP bottles and to recall PrEP use in the last 30 days. A sample of DBS from aMSM, DBS from all seroconverters, and all aTGW were sent for quantification of TFV-DP concentrations. The accuracy of each indirect measure was assessed using Areas under (AUC) the receiver operating characteristics (ROC) curves for protective TFV-DP levels '¥800fmol/punch equivalent to 4 doses per week or above. Sensitivity (SE), specificity (SP), and predictive values positive (PPV) and negative (NPV) were assessed for the best cutoff points identified by Youden index.
RESULTS: DBS data from 188 participants were included. Coincidental data for indirect measures were: MPR (N=185), pill-count (N= 68) and self-report (N= 174). The AUC was 0.55 (95%CI: 0.47'0.53) for MPR, AUC = 0.67 (95%CI: 0.54'0.80) for pill count, and AUC = 0.72 (95%CI: 0.65'0.80) for self-report. The best cut-off point was 0.90 for MPR with 80.3% SE, 40.4% SP, 25.4% PPV and 51.6% NPV; for pill count was 92.7% with 72.7% SE, 62.9% SP, 29.0% PPV and 35.1% NPV and 83.3% for self-report with 91.8% SE, 46.5% SP, 11.3% PPV and 44.6% NPV.
CONCLUSIONS: Pill count and self-report were able to discriminate adolescents with protective levels of TFV-DP. In addition, self-report adherence showed the best performance, and it is feasible and sustainable to be implemented in PrEP services for adolescents.

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