Background: The efficacy of HIV pre-exposure prophylaxis (PrEP) depends upon adherence.
Daily PrEP adherence was assessed over 12 months using two biological measures and two measures of self-report (SR). Drug concentrations in plasma and peripheral blood mononuclear cells (PBMC), and patient SR to clinicians and via online surveys were evaluated. Concordance between SR and biological measures was assessed.
Methods: PRELUDE is an open-label PrEP demonstration project of 327 high-HIV risk, predominantly (96%) gay/bisexual men in New South Wales (NSW), Australia. Participants self-reported the number of tablets taken in the previous week to clinicians, and the last three months via online surveys. SR adherence required daily dosing, as prescribed. Blood was collected from 108 consecutively presenting participants 1, 6 and 12 months post-PrEP initiation. Liquid chromatography-mass spectrometry was used for drug quantification. Biological adherence was defined as plasma tenofovir (TFV) ≥40.0 ng/mL and PBMC tenofovir-diphosphate (TFV-DP) ≥16.8 fmol/106 cells, indicative of daily dosing in the three days and one week prior to blood collection, respectively. Results were analysed using descriptive statistics and ANOVAs with Bonferroni''s post-hoc correction. Kappa tests were used to determine measurement agreement.
Results: Amongst participants with blood samples from all three time points (n=87), mean adherence was 92% (95% confidence interval 88 - 95%) across all measures (Table 1). Median (interquartile range) PBMC TFV-DP concentrations significantly decreased between months 1 (100 (64 - 176) fmol/106 cells) and 6 (61 (40 - 94) fmol/106 cells; p< 0.001), and months 6 and 12 (51 (29 - 74) fmol/106 cells); p=0.01), but not in plasma or SR data (p>0.05). Concordance between PBMC TFV-DP and plasma TFV, survey SR, and clinician SR was 87%, 92%, and 87% respectively.
Conclusions: Very high levels of daily adherence to PrEP amongst early adopters were confirmed using four different measures, although there was a decrease in PBMC TFV-DP over time. Strong concordance between blood drug concentrations and SR data suggests that SR may be a suitable measure of adherence in clinical practice. To ensure effective ongoing PrEP use, longer-term observation is required.

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