How should we measure medication adherence in clinical trials and practice?

OBJECTIVE: To determine if simple adherence measures, such as twenty-four hour recall and refill history, are accurate for routine use, compared to more time-consuming measures such as pill counts. DESIGN: Randomized, double-blind, placebo-controlled trial. SETTING: Walter Reed Army Medical Center, a tertiary medical center in Washington. PATIENTS: Men and women >30 years old with known coronary heart disease and taking a statin medication. INTERVENTION: Clinical pharmacists met with patients for adherence assessments. MAIN OUTCOME MEASURES: Adherence was measured by pill counts, twenty-four hour recall by patient, and refill history per computer record. Temporal changes in these adherence measures were assessed using general linear models for repeated measures. RESULTS: Adherence was consistently greater for the experimental agent than for the statin therapy (n = 148). Mean pill count adherence for statin drug was 78.7 ± 25.2% compared to 93.5 ± 11.6% (P < 0.001) for the study agent. Refill history and twenty-four hour recall inaccurately measured adherence when compared to pill counts. Adherence, as determined by pill count, for both experimental (P = 0.029) and statin therapy (P = 0.015) showed significant variability across time in general linear models. Neither refill history nor twenty-four hour recall was sensitive to temporal changes. CONCLUSIONS: Twenty-four hour recall and refill history inaccurately measure medication adherence for both clinical trial and clinical practice pharmacotherapies. Further, these measures are insensitive to changes in adherence. For a single or multiple assessments across time, pill count more accurately measures medication adherence. Pill count should be the standard for monitoring medication adherence for both clinical trials and clinical practice.