Comparison of three medication adherence measures in patients taking warfarin

In order to improve warfarin adherence, we must first know the rate of non-adherence and the reasons for it. Assessment of warfarin adherence is important in improving patients' warfarin-taking behavior and International Normalized Ratio (INR) control. This study aimed to compare three medication adherence measures in patients taking warfarin: the 8-item Morisky Medication Adherence Scale (MMAS), the 100-point Visual Analogue Scale (VAS) and medication refill adherence (MRA). A cross-sectional observational study was conducted in a convenience sample of 174 patients taking warfarin at an anticoagulation clinic. A survey questionnaire that included the MMAS, VAS, and demographic and warfarin-related clinical questions was administered to patients in English or Chinese depending on their preferences. The MRA and INR values were retrieved from hospital electronic databases. Most participants were adherent to warfarin as indicated by the three measures. A weak association was found between the MMAS and the other two adherence measures. The MMAS and MRA were associated with the percentage of INRs within range in the past 2 weeks. Moreover, the MRA was weakly associated with time within the therapeutic INR range in the past 3 months and 2 weeks. The findings provide insights into the differences among three medication adherence measures and may assist healthcare providers to select the most suitable measure for the assessment of warfarin adherence.