Feasibility and Acceptability of Digital Pills to Monitor PrEP Adherence in MSM With Substance Use

Participants' engagement with the digital pill system (DPS) was measured over the 90-day study period. The percentages for the total expected ingestions recorded by DPS each month were compared.

The total number recorded ingestions recorded by the digital pill system (DPS) - which included both the number of Reader-detected and manually-reported ingestions - was collected. The Reader-detected ingestions count was used to reflect the number of times the DPS was operated correctly. Successful DPS operation was defined as ingestion of a digital pill, proper use of the wearable Reader, and confirmation of the ingestion on both the Reader and the app. Cumulative data collected at months 1, 2, and 3 are reported in the data table below.

Accuracy of the digital pill system (DPS) in measuring PrEP adherence. To analyze the performance of the DPS, the ground truth of PrEP ingestion events was defined as the pill counts obtained each month (i.e., the number of unused pills returned, subtracted from the number of pills previously dispensed). The number of DPS-recorded ingestions - which included the number of both Reader-detected and manually-reported ingestions - was compared with the aggregate pill count at each monthly timepoint; this was defined as the overall performance metric for the DPS. Cumulative data collected at months 1, 2, and 3 are reported in the data table below.

The total number of manually reported ingestions was collected. The percentages for instances of manually reported ingestions were calculated. The two instances for manually recording ingestions included, a lack of engagement with the Reader or failure to use the Reader properly, and when reported ingestion was not detected by the Reader despite supporting use metrics indicating proper Reader use. Cumulative data collected at months 1, 2, and 3 are reported in the data table below.

The total number of instances in which ingestions were successfully detected by the Reader (i.e., the radio frequency emitter was activated, and the ingestion was recorded by the DPS) was collected. The total number of instances in which manually reported ingestions were not detected by the Reader, despite supporting use metrics (e.g., accelerometer data) indicating proper Reader use was also collected. These counts were combined and interpreted as the number of times that the DPS was operated correctly. Cumulative data collected at months 1, 2, and 3 are reported in the data table below.

We dichotomized TFV-DP levels using a cutoff of ≥700 fmol/punch to indicate at least four doses of PrEP ingested per week. Using TFV-DP in DBS <700 vs ≥700 fmol/punch as a dichotomous variable, and considering the granular continuous adherence data from the digital pill, we then calculated a point biserial correlation between TFV-DP in DBS and digital pill adherence. Additionally, drug concentrations of tenofovir diphosphate as measured in dried blood spot collection at months one and three were compared to DPS-recorded PrEP adherence.

Acceptability was assessed via individual, semi-structured, qualitative exit interviews conducted at the end of the 90-day study period. The qualitative interview guide was grounded in the Technology Acceptance Model. Questions explored participants' experiences using the digital pill system (DPS), including facilitators and barriers to use, engagement with the technology, and willingness to use the DPS long-term.