Abstract
Background
Emtricitabine triphosphate (FTC-TP), the phosphorylated anabolite of emtricitabine, can be quantified in dried blood spots (DBS). We evaluated FTC-TP in DBS as a predictor of viral suppression and evaluated self-reported adherence as a predictor of FTC-TP.
Methods
Persons living with HIV (PLWH) on an FTC-containing regimen were prospectively recruited. A DBS and HIV viral load were obtained during routine clinical visits. Self-reported adherence for 3 days, 30 days and 3 months was captured. Generalized estimating equations were used to estimate the adjusted odds ratio (aOR) of viral suppression for quantifiable FTC-TP versus below the limit of quantification (BLQ). The utility of self-reported adherence to predict quantifiable FTC-TP was assessed by calculating the area under receiver operating characteristic (ROC) curve.
Results
One thousand one hundred and fifty-four person-visits from 514 participants who had DBS assayed for FTC-TP were included in the analysis. After adjusting for age, gender, race, BMI, ART class, ART duration, estimated glomerular filtration rate and CD4+ T cell count, the aOR (95% CI) for viral suppression for quantifiable FTC-TP versus BLQ was 7.2 (4.3–12.0; P < 0.0001). After further adjusting for tenofovir diphosphate, the aOR was 2.1 (1.2–4.0; P < 0.015). The area under the ROC curve for 3 day self-reported adherence was 0.82 (95% CI 0.75–0.88) compared with 0.70 (95% CI 0.62–0.77, P = 0.004) and 0.79 (95% CI 0.71–0.86, P = 0.32) for 3 month and 30 day self-reported adherence, respectively.
Conclusions
In PLWH, FTC-TP from DBS is a strong predictor of viral suppression, even after adjusting for tenofovir diphosphate, and was best predicted by 3 day self-reported adherence.
Introduction
Adherence to ART is key for persons living with HIV (PLWH) to achieve and maintain viral suppression, to avoid progression to AIDS and to prevent HIV transmission.1 Many methods have been described to quantify adherence among individuals on ART,2–4 but they face several limitations, including social desirability bias of self-reporting and inaccuracies of pill counts.4 Thus, new methods that can objectively quantify ART adherence are required.
Recently, new methods that reflect longer periods of adherence and exposure have been developed. Amongst them is the quantification of tenofovir diphosphate, the phosphorylated anabolite of tenofovir, in RBCs using dried blood spots (DBS).5 Tenofovir diphosphate in the DBS is a measure of cumulative tenofovir exposure5 owing to its long intracellular half-life of 17 days.5 This adherence biomarker has been widely applied to quantify adherence and predict efficacy of tenofovir disoproxil fumarate/emtricitabine in HIV pre-exposure prophylaxis.6,7 In HIV treatment, tenofovir diphosphate was recently shown to be a strong predictor of viral suppression8 and to correlate with other measures of adherence, such as pharmacy refills9 and real-time adherence monitoring.10 However, owing to its long half-life in both RBCs and DBS, tenofovir diphosphate is best used as a marker of adherence over the preceding 6–8 weeks9 and cannot quantify changes in recent adherence. Comparatively, emtricitabine triphosphate (FTC-TP), the phosphorylated anabolite of emtricitabine, can be simultaneously quantified in the same DBS sample along with tenofovir diphosphate, and reflects recent dosing owing to its shorter half-life of 35 h in both RBCs and DBS, thereby being more sensitive to recent changes in adherence.11 To date, the association of FTC-TP in DBS with viral suppression and with other methods used to quantify adherence in HIV infection (such as self-reporting) has not been evaluated, which was the goal of this study.
Methods
Study design and participants
PLWH were prospectively recruited from the University of Colorado Hospital Infectious Diseases Group Practice at the time of their clinical visits. Inclusion criteria required that participants were ≥18 years of age, had HIV infection being treated with a tenofovir-based regimen, and were scheduled for a blood draw for routine laboratory analysis [which included HIV viral load (VL)]. The only exclusion criterion was pregnancy, owing to the unknown pharmacokinetics of FTC-TP in DBS in pregnant women. Permission was obtained from the participant’s primary care provider prior to study personnel having any contact with the participants. If participants were interested in the study, they met with the study personnel and underwent the process of informed consent prior to sample collection. After written consent was obtained, 4–6 mL of whole blood was collected in an EDTA tube via the same venipuncture used during the participant’s routine laboratory draw. Additional samples were obtained at the participant’s next clinic visit, whenever blood for an HIV VL was drawn by the clinical provider, for a total of up to three visits in a 48 week period (with a minimum time of 14 days between visits to allow for one half-life of tenofovir diphosphate5).
Ethics
This study was approved by the Colorado Multiple Institutional Review Board (COMIRB#13-2104) and registered at clinical trials.gov (NCT02012621). Study enrolment was initiated in June 2014, and follow-up for the last enrolled participant was concluded in July 2017. All participants completed informed consent prior to participation in the study.
Quantification of FTC-TP and tenofovir diphosphate in DBS
After collection, 25 µL of whole blood from the EDTA tube was spotted onto each circle of a Whatman 903 Protein Saver card and allowed to dry for at least 3 h (up to overnight). Once dry, DBS cards were stored at −80°C in plastic bags with desiccant and humidity indicators until analysis. FTC-TP and tenofovir diphosphate, for which stability in stored frozen DBS samples at −80°C has been demonstrated, were quantified from a 3 mm punch using LC–MS/MS, as previously reported.5,12
Self-reported adherence
At each study visit, the participants were asked about their 3 month, 30 day and 3 day adherence to their current ART using a visual analogue scale that ranged from 0% to 100% adherence, with intervals at 10% increments, as previously described.8,13,14 Participants were asked to draw a mark on the scale indicating their estimated percentage of ART taken over the preceding period of time. If a mark was placed between two intervals (i.e. between 60% and 70%), the participants were asked to estimate a precise percentage of ART taken over that time frame.
HIV viral load
HIV-1 VL was quantified at the University of Colorado Hospital clinical laboratory using the Roche COBAS 6800 HIV test (range of quantification 20–107 copies/mL). This laboratory is certified by the Clinical Laboratory Improvement Amendment of 1988 (CLIA). An undetectable HIV VL was therefore defined as <20 copies/mL.
Statistical analysis
FTC-TP in DBS at each person-visit was categorized as quantifiable or below the limit of quantification (BLQ, 0.1 pmol/sample), as previously described.11 Tenofovir diphosphate in DBS at each person-visit was categorized based on previous data in HIV-negative persons and PLWH.8,15 Self-reported adherence was categorized as <50%, 50%–84%, 85%–99% and 100%, based on previous clinically significant thresholds.16–20
The primary outcome for the study was HIV VL, which was evaluated in all study visits as part of routine patient care. As mentioned, DBS samples were simultaneously collected along with every HIV VL assessment. From study initiation (June 2014) to August 2016, all DBS samples were consecutively analysed independently of the HIV VL result (as originally planned). However, our capacity to analyse all collected DBS within the entire cohort was limited by practical and logistical constraints. Thus, after August 2016 the assay strategy was modified, focusing only on assaying all available DBS samples obtained for participants with a detectable HIV VL at one or more study visits, and discontinuing DBS assays for participants who were virologically suppressed at all study visits. Similar to a case (i.e. viraemic)–control (i.e. suppressed) study, extended within a longitudinal setting, this outcome-dependent sampling method ensured that data on the most informative patients (i.e. patients who were either viraemic at all visits or who switched VL status between visits) were included in the analysis, while maintaining statistical power by analysing all available samples from participants with at least one detectable HIV VL at any person-visit.8,21
Differences in the proportion of quantifiable FTC-TP were compared in the viraemic and suppressed groups according to participant characteristics using Fisher’s exact test or the χ2 test, as appropriate. At baseline, χ2 tests for trend were used to analyse the association between FTC-TP quantification and self-reported adherence. Generalized estimating equations, with a logit link, were utilized to estimate the OR of viral suppression comparing a quantifiable FTC-TP versus BLQ, while accounting for repeated measures (i.e. repeated HIV VL and DBS). The resulting OR was adjusted (aOR) for age, gender, race, BMI, estimated glomerular filtration rate (eGFR, calculated using the modification of diet in renal disease equation), CD4+ T cell count, ART class and duration of ART.8 An additional aOR including these variables plus tenofovir diphosphate was calculated to account for tenofovir exposure. The utility of self-reported adherence to predict a quantifiable FTC-TP in DBS was assessed by calculating the area under the receiver operating characteristic (ROC) curve using logistic regression for each of the three measures obtained (i.e. 3 days, 30 days and 3 months), and compared using the 3 day adherence measure as a reference. Statistical analyses were performed using SAS v9.4. A P value <0.05 was considered statistically significant.
Results
Patient population
The total study population enrolled was 807 participants, with 119 completing at least one visit (one pair of HIV VL and DBS samples), 244 completing at least two visits (two paired HIV VL and DBS samples) and 444 completing all three visits (three paired HIV VL and DBS samples). In total, 1936 DBS samples were collected in the entire cohort. Of these, 1199 DBS, obtained from 532 participants, were assayed, and 45 DBS samples obtained from 18 participants who were not on an emtricitabine-containing regimen were excluded from the analysis, reducing our sample size to 514 participants for whom at least one DBS sample was assayed at any study visit.
Of the 514 participants in whom FTC-TP in DBS was quantified at any study visit, 503 had drug concentrations available at baseline. The demographic characteristics of these 503 participants who were on an emtricitabine-containing ART at the enrolment visit, according to viral suppression status, are shown in Table 1. Among these participants, 170 (34%) were viraemic and 333 (66%) were suppressed. Fifty-two (16%) out of the 333 participants who were suppressed at baseline became viraemic at one of the follow-up visits. In comparison, 88 (52%) out of the 170 participants who were viraemic at baseline became suppressed at one of the follow-up visits. Overall, the majority of participants were men, with similar percentages of black, Hispanic and white participants between viraemic and suppressed participants. A larger percentage of viraemic participants were on an integrase strand-transfer inhibitor (INSTI)-based regimen when compared with suppressed participants (45% versus 32%), whereas a larger percentage of suppressed participants were on an NNRTI-based regimen in comparison with participants who were viraemic (35% versus 13%). Similarly, a larger percentage of viraemic participants were on a pharmacological booster such as ritonavir or cobicistat in comparison with participants who were suppressed (65% versus 44%). Longer duration of ART (i.e. >6 months) was more frequent in suppressed versus viraemic participants (88% versus 68%).
Table 1.
Baseline characteristics of participants at the enrolment visit
| Characteristic | Participants on FTC at baseline with available DBS (n = 503)a |
|
|---|---|---|
| viraemic (n = 170) | suppressed (n = 333) | |
| Age | 43 (33–51) | 46 (38–52) |
| Sex | ||
| male | 153 (90) | 278 (83) |
| female | 17 (10) | 55 (17) |
| Race | ||
| black | 34 (20) | 63 (19) |
| Hispanic | 34 (20) | 61 (18) |
| other | 8 (5) | 16 (5) |
| white | 94 (55) | 193 (58) |
| BMI (kg/m2) | ||
| <18.5 | 7 (4) | 14 (4) |
| 18.5–25 | 78 (46) | 132 (40) |
| >25–30 | 53 (31) | 117 (35) |
| >30 | 32 (19) | 68 (20) |
| ART class | ||
| INSTI-based | 77 (45) | 106 (32) |
| multiclass | 26 (15) | 31 (9) |
| NNRTI-based | 22 (13) | 115 (35) |
| b/PI-based | 45 (26) | 81 (24) |
| Pharmacological booster | ||
| no | 60 (35) | 187 (56) |
| yes | 110 (65) | 146 (44) |
| Time on ART (months) | ||
| <1 | 17 (10) | 4 (1) |
| 1–3 | 22 (13) | 22 (7) |
| >3–6 | 15 (9) | 14 (4) |
| >6 | 116 (68) | 293 (88) |
| Self-reported adherence <100%b | ||
| 3 days | 36 (21) | 36 (11) |
| 30 days | 82 (50) | 132 (40) |
| 3 months | 98 (70) | 178 (55) |
Values are presented as n (%) or median (IQR).
FTC, emtricitabine; b/PI, boosted protease inhibitor.
FTC-TP concentrations in DBS were available in 503 participants at baseline from a total of 514 participants in whom FTC-TP was quantified at any study visit.
Self-reported adherence in the preceding 30 days and 3 months was available for n = 495 and n = 464 participants, respectively.
Table 1 shows the number of subjects reporting <100% adherence for each time period. Overall, a smaller percentage of suppressed individuals reported <100% adherence compared with viraemic participants across all durations. Self-reported adherence was <100% at 3 days for 11% of the suppressed participants versus 21% of the viraemic participants.
FTC-TP and tenofovir diphosphate in DBS
Table 2 shows the quantification of FTC-TP in DBS for the study participants at the enrolment visit, categorized for whether they were viraemic or suppressed. The median (IQR) concentrations of FTC-TP among the viraemic and suppressed groups were 0.366 (0.278–0.459) and 0.367 pmol/punch (0.301–0.470), respectively. Suppressed patients had a higher percentage of quantifiable FTC-TP compared with viraemic patients (P < 0.0001). In an unadjusted analysis, there was no statistically significant difference in whether suppressed or viraemic participants had quantifiable FTC-TP versus BLQ based on gender, race, BMI, ART class, use of a pharmacological booster or duration of ART. Table 3 shows self-reported adherence results by FTC-TP quantification and by viral suppression status. In the viraemic group, the proportion of quantifiable FTC-TP in DBS increased with each adherence category for the 3 day, 30 day and 3 month adherence measures (P < 0.0001). In comparison, the proportion of quantifiable FTC-TP increased significantly with each adherence category for only the 3 day adherence measure (P = 0.004) in the suppressed group.
Table 2.
Proportion of quantifiable FTC-TP based on patient characteristics for participants at baseline
| Variable | Viraemic (n = 170) |
Suppressed (n = 333) |
||||
|---|---|---|---|---|---|---|
| BLQ, n (%) | quantifiable FTC-TP, n (%) | P valuea | BLQ, n (%) | quantifiable FTC-TP, n (%) | P valuea | |
| Sex | ||||||
| male | 34 (22) | 119 (78) | 6 (2) | 272 (98) | ||
| female | 6 (35) | 11 (65) | 0.236 | 2 (4) | 53 (96) | 0.623 |
| Race | ||||||
| black | 9 (26) | 25 (74) | 4 (6) | 59 (94) | ||
| Hispanic | 12 (35) | 22 (65) | 2 (3) | 59 (97) | ||
| other | 1 (12) | 7 (88) | 0 (0) | 16 (100) | ||
| white | 18 (19) | 76 (81) | 0.246 | 2 (1) | 191 (99) | 0.087 |
| BMI (kg/m2) | ||||||
| <18.5 | 1 (14) | 6 (86) | 0 (0) | 14 (100) | ||
| 18.5–25 | 21 (27) | 57 (73) | 4 (3) | 128 (97) | ||
| >25–30 | 12 (23) | 41 (77) | 2 (2) | 115 (98) | ||
| >30 | 6 (19) | 26 (81) | 0.796 | 2 (3) | 66 (97) | 0.81 |
| ART class | ||||||
| INSTI based | 15 (19) | 62 (81) | 3 (3) | 103 (97) | ||
| multiclass | 8 (31) | 18 (69) | 1 (3) | 30 (97) | ||
| NNRTI based | 2 (9) | 20 (91) | 1 (1) | 114 (99) | ||
| b/PI based | 15 (33) | 30 (67) | 0.092 | 3 (4) | 78 (96) | 0.474 |
| Pharmacological booster | ||||||
| no | 9 (15) | 51 (85) | 4 (2) | 183 (98) | ||
| yes | 31 (28) | 79 (72) | 0.06 | 4 (3) | 142 (97) | 0.734 |
| Time on ART (months) | ||||||
| <1 | 3 (18) | 14 (82) | 1 (25) | 3 (75) | ||
| 1–3 | 2 (9) | 20 (91) | 0 (0) | 22 (100) | ||
| >3–6 | 2 (13) | 13 (87) | 0 (0) | 14 (100) | ||
| >6 | 33 (28) | 83 (72) | 0.173 | 7 (3) | 266 (97) | 0.157 |
b/PI: boosted protease inhibitor.
Estimated using Fisher’s exact test.
Table 3.
Quantification of FTC-TP according to self-reported adherence and viraemia status at baseline
| Percentage adherence | Viraemic (n = 170) |
Suppressed (n = 333) |
||||
|---|---|---|---|---|---|---|
| BLQ (n = 40) | quantifiable FTC-TP (n = 130) | P value | BLQ (n = 8) | quantifiable FTC-TP (n = 325) | P value | |
| 3 days (n = 503) | ||||||
| <50 | 13 | 2 | 2 | 5 | ||
| 50–84 | 4 | 5 | 0 | 9 | ||
| 85–99 | 4 | 8 | 0 | 20 | ||
| 100 | 19 | 115 | <0.0001 | 6 | 291 | 0.004 |
| 30 days (n = 495)a | ||||||
| <50 | 11 | 1 | 0 | 0 | ||
| 50–84 | 9 | 14 | 1 | 12 | ||
| 85–99 | 12 | 35 | 4 | 115 | ||
| 100 | 8 | 73 | <0.0001 | 3 | 197 | 0.115 |
| 3 months (n = 464)a | ||||||
| <50 | 7 | 4 | 0 | 3 | ||
| 50–84 | 16 | 17 | 3 | 27 | ||
| 85–99 | 15 | 39 | 3 | 142 | ||
| 100 | 1 | 42 | <0.0001 | 2 | 143 | 0.052 |
Statistically significant P values are shown in bold.
Self-reported adherence in the preceding 30 days and 3 months was available for n = 495 and n = 464 participants, respectively.
Table 4 shows the quantification of FTC-TP according to the concentration of tenofovir diphosphate at baseline. Tenofovir diphosphate concentrations were <350 fmol/punch in 20 (4%), 350–699 fmol/punch in 32 (6%), 700–1249 fmol/punch in 103 (20%), 1250–1849 fmol/punch in 156 (31%) and ≥1850 fmol/punch in 192 (38%) of participants at baseline. Overall, the percentage of DBS where FTC-TP was BLQ decreased with every increasing category of tenofovir diphosphate.
Table 4.
Trend in FTC-TP quantification in DBS according to tenofovir diphosphate concentration at baseline
| TFV-DP (fmol/punch) | FTC-TP |
P value | |
|---|---|---|---|
| BLQ, n (%) | quantifiable, n (%) | ||
| <350 | 17 (85) | 3 (15) | — |
| 350–699 | 11 (34) | 21 (66) | — |
| 700–1249 | 15 (15) | 88 (85) | — |
| 1250–1849 | 3 (2) | 153 (98) | — |
| ≥1850 | 2 (1) | 190 (99) | <0.0001 |
TFV-DP, tenofovir diphosphate. Statistically significant P values are shown in bold.
FTC-TP in DBS as a predictor of viral suppression
A total of 1154 person-visits from 514 participants who had DBS obtained while on an emtricitabine-containing regimen at any time during the study were included in this analysis. Regarding association with viral suppression, the unadjusted OR (95% CI) of viral suppression when FTC-TP was quantifiable was 6.7 (4.3–10.5; P < 0.0001). After adjusting for age, gender, race, BMI, ART class, duration of ART, eGFR and CD4+ T cell count, the aOR for viral suppression when FTC-TP was quantifiable increased to 7.2 (4.3–12.0; P < 0.0001). After including tenofovir diphosphate in the multivariable analysis, the aOR of viral suppression when FTC-TP was quantifiable was 2.1 (1.2–4.0; P < 0.015).
Self-reported adherence as predictor of FTC-TP quantification
The sensitivity and specificity of self-reported adherence as a predictor of FTC-TP quantification in DBS is shown in Figure 1. The area under the ROC curve for 3 day self-reported adherence was 0.82 (95% CI 0.75–0.88, P = 0.011), which compares to 3 month self-reported adherence at 0.70 (95% CI 0.62–0.77, P = 0.004) and 30 day self-reported adherence at 0.79 (95% CI 0.71–0.86; P = 0.32), though the latter did not reach statistical significance.
Figure 1.
Baseline ROC curves for self-reported adherence as a predictor of FTC-TP quantification in DBS. Lines represent the area under the ROC curves (AUC) for 3 day (0.82), 30 day (0.79) and 3 month (0.70) self-reported adherence. SR, self-reported.
Discussion
In this study, we demonstrated that the quantification of FTC-TP in DBS is predictive of virological suppression in PLWH and is an effective biomarker to determine recent adherence to ART among patients on an emtricitabine-containing regimen. To our knowledge, this is the first documentation of the use of FTC-TP in DBS in a cohort of participants living with HIV, as prior studies involved only HIV-negative volunteers. The multivariable adjusted analysis showed that quantifiable FTC-TP was significantly associated with virological suppression, with an adjusted odds of suppression >7-fold higher in those with quantifiable FTC-TP compared with those with BLQ FTC-TP. This remained significant after adjusting for tenofovir diphosphate [aOR 2.1 (95% CI 1.2–4.0); P < 0.015]. Therefore, short-term adherence monitoring for FTC-TP is critical and provides additional information on adherence patterns, even after accounting for cumulative adherence with tenofovir diphosphate. Combined, these two markers could be utilized to obtain a complete picture of adherence to optimize adherence counselling and virological suppression among PLWH.
In the unadjusted analysis, there was no statistically significant difference in baseline characteristics according to whether participants had quantifiable FTC-TP versus BLQ for either the suppressed or the viraemic groups. In both, the proportion of participants with FTC-TP in DBS increased with each adherence category in the 3 day, 30 day and 3 month adherence measures. For viraemic participants, there was a statistically significant difference noted in all three adherence categories, but only 3 day self-reported adherence remained significant for those who were suppressed. This suggests that small changes in variability in adherence over 30 days and 3 months may be harder to detect using a short-term adherence marker in a virologically suppressed individual. Similar findings were observed when the ROC curves for self-reported adherence and FTC-TP were evaluated, in which the largest estimated ROC for quantifiable FTC-TP was observed with 3 day self-reported adherence. Collectively, these findings demonstrate that FTC-TP is a better measure of short-term adherence, as previously observed in individuals without HIV infection.11
Prior studies in HIV-negative volunteers have verified the association between the plasma half-life of emtricitabine and quantification of FTC-TP in DBS.14 The Cell-PrEP study found similar half-lives of FTC-TP in DBS between PLWH and HIV-negative volunteers, with almost all at BLQ at washout 5 days after the last dose.11 Similar findings were observed in the DOT-DBS study, in which 100% FTC-TP detection was found in participants taking daily doses.18 The iPrEx OLE and ATN 110 studies demonstrated a strong association between tenofovir diphosphate and FTC-TP in 746 paired plasma and DBS samples.11 These results were similar to those found in our study, showing a strong association between FTC-TP quantification and higher concentrations of tenofovir diphosphate. However, in our study, 85% of participants had quantifiable FTC-TP at tenofovir diphosphate concentrations >700 fmol/punch (the expected concentration for four or more doses per week in people without HIV), which was lower than the 96% of HIV-negative volunteers with quantifiable FTC-TP at the same tenofovir diphosphate concentrations.11 This likely reflects the higher variability in adherence patterns among PLWH in clinical practice, as opposed to the expected higher levels of adherence among participants in a clinical trial setting. However, other factors inherent to HIV infection, such as drug–drug interactions, pharmacokinetic differences and/or residual inflammation, could have also influenced these findings and require further study.
The major clinical implication of this study is that FTC-TP measured in DBS can be utilized as an objective measurement of short-term adherence to ART among PLWH. Given that the majority of current first-line regimens include emtricitabine as a major component, typically combined with tenofovir disoproxil fumarate or tenofovir alafenamide,22,23 FTC-TP in DBS has the potential to complement tenofovir diphosphate as a highly useful biomarker of ART adherence in clinical practice. We found that 3 day self-reported adherence to emtricitabine best predicted the quantification of FTC-TP in DBS, which likely reflects the short half-life of FTC-TP at 35 h.11 Thus, this can distinguish a patient’s recent adherence patterns prior to a clinical visit.24 Our results also showed a strong association between FTC-TP quantification and higher tenofovir diphosphate concentrations. A cumulative adherence assessment is critical to exclude transient improved adherence prior to a clinic visit, termed ‘white coat’ adherence,24 and could overcome the limitations of self-reported adherence or pill counts to detect recent episodes of suboptimal adherence and prevent virological failure. For example, we observed quantifiable FTC-TP in 15% (3/20) of subjects with low tenofovir diphosphate (<350 fmol/punch), suggesting ‘white coat’ dosing.
The strengths of this study include the prospective analysis of a large clinical cohort in a clinical setting. In addition, FTC-TP was associated with self-reported adherence in various time frames, allowing analysis of the utility of this biomarker over different time periods. Limitations include the possible overestimation of self-reported adherence,3 which was particularly evident in the larger proportion of participants who reported 100% 3 day adherence and had FTC-TP concentrations BLQ, a discrepancy which could have skewed our results. Despite this, self-report continues to be widely used in routine clinical practice and thus our focus on comparing FTC-TP with this adherence assessment. In addition, although the outcome-dependent sampling approach could have influenced the strength of our association, the focus on viral suppression as a reasonably rare outcome allowed us to longitudinally extend our case–control design, while maintaining statistical power.
In summary, we determined that FTC-TP in DBS is associated with virological suppression in PLWH and that it is best predicted by 3 day self-reported adherence. Future studies that can prospectively evaluate this biomarker (alone and in conjunction with tenofovir diphosphate) to identify early changes in adherence in clinical practice are required.
Acknowledgements
We would like to thank the study participants, the director of the HIV programme (Steven Johnson, MD) and the medical assistants (Nancy Olague, Brittany Limon, Ariel Cates, Maureen Sullivan and Missy Sorrell) at the University of Colorado Hospital-Infectious Disease Group Practice for their invaluable contributions and support of this study. We would also like to thank the personnel at the Colorado Antiviral Pharmacology Laboratory for their invaluable assistance and support of this study.
Funding
This work was supported by the National Institutes of Health (K23 AI104315 to J. C.-M., R21 AI124859 to J. C.-M. and R01 AI122298 to P. L. A.). P. L. A. receives support (paid to their institution) for investigator-initiated studies from Gilead Sciences.
Transparency declarations
None to declare.
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