Adherence to antiretroviral therapy (ART) is an important determinant of clinical success assessed in many HIV studies. Harmonizing adherence data from studies that use different measures is difficult without a co-calibration equation to convert between validated instruments. Our purpose was to co-calibrate two commonly used adherence measures: the AIDS Clinical Trials Group (ACTG) questionnaire and the Visual Analog Scale (VAS).
We used robust linear regression to develop a co-calibration equation in a clinical care cohort. The outcome was the 30-day VAS percentage of ART taken and the predictors were ACTG questions. We evaluated the equation’s goodness-of-fit in five STTR (Seek, Test, Treat, Retain) consortium studies where individuals completed both measures: 2 criminal justice; 2 international; and 1 other high-risk vulnerable population.
We developed a three-phase decision rule to convert ACTG to VAS in 1045 participants. First, when the last missed dose on the ACTG was reported as >30 days ago the VAS was set to 100% (N=582). Second, if “doses missed” was zero for all items, VAS was 100% (N=104). Third, among remaining participants (N=359), VAS was estimated as 96.8% minus 2.9% times the number of missed doses (“doses per day” was non-significant). Correlation between predicted and reported VAS was r=0.80 in the criminal justice group (N=446), r=0.46 in the international group (N=311), r=0.32 in the other vulnerable population (N=63), and r=0.66 overall. When outliers due to inversion of the VAS scale were excluded (n=25), these correlations were 0.88, 0.78, 0.80, and 0.86, respectively.
We concluded that a simple decision rule and equation allowed us to co-calibrate between two widely used adherence measures thus combining data from studies with different instruments. This study highlighted issues with VAS inversions and its limitations as a single item. Combining studies using different instrument facilitates larger pooled data sets to address key research questions.