Background:
Pharmacologic metrics of PrEP adherence predict effectiveness in clinical trials. Self-report is suboptimal but used in clinical and many research settings due to the expense and delay of laboratory-based tests (e.g. dried blood spots (DBS) or hair measures via liquid chromatography tandem mass spectrometry (LC-MS/MS)). A urine rapid point-of-care (POC) tenofovir (TFV) antibody-based test has compared well to LC-MS/MS of urine. To inform clinical and research utility, we calculated predictive values against self-reported recent PrEP use and previously validated measures of recent and longer-term adherence from DBS via LC-MS/MS.
Methods:
Participants who reported currently taking PrEP in the RADAR community-cohort study of young men and transgender women who have sex with men were invited to this sub-study. At 3 monthly visits, participants completed a survey of daily PrEP use in the prior 7 days and provided DBS and urine samples. DBS samples were tested for TFV-diphosphate (TFV-DP) to estimate average dosing over the prior month and emtricitabine-triphosphate (FTC-TP) to assess recent dosing (past 2-3 d). A urine POC TFV test qualitatively assessed recent adherence (past 4–7 d) was performed.
Results:
A subset of 73 PrEP-users contributed 126 observations. Self-reported adherence was over-reported (87% for 4+ doses in last 7 days), versus urine TFV (69%), DBS FTC-TP (68%), and DBS TFV-DP (67%). The two objective metrics of short-term adherence performed similarly well in predicting longer term adherence via TFV-DP (see Table for predictive values). Self-report overestimated longer term adherence, but was better for short term adherence metrics.. In multivariable logistic regression analyses, the urine assay was a significant predictor of DBS TFV-DP (OR = 30.2, p < .0001); self-report did not add significantly to prediction.
Conclusions:
The urine POC TFV assay had excellent predictive values and self-report did not add significantly to prediction of adherence. The POC assay provides results in several minutes to enable same-visit counseling/intervention, requires no specialized training, and is projected to be low-cost. It could also be used for research where objective short term adherence metrics are needed.
Positive and negative predictive values for PrEP adherence