Validity and reliability of a self-reported measure of medication adherence in patients with Type2 diabetes mellitus in Singapore

Aims A reliable and valid measure is essential for the assessment of medication adherence. Until now, no patient-reported medication adherence measure has been validated in Singapore. The aim of this study was to validate a modified 4-item Morisky-Green-Levine Medication Adherence Scale in patients with Type2 diabetes in Singapore. Methods A cross-sectional survey was conducted in a sample of outpatients with Type2 diabetes in Singapore from September to December in 2009. Respondents completed either an English or Chinese version of the modified 4-item Morisky-Green-Levine Medication Adherence Scale. The scale scores ranged from 0 to 4, with higher scores indicating better medication adherence. Reliability was assessed using Cronbach's alpha. Content validity was assessed by expert review. Construct validity was examined using factor analysis and hypothesis testing. Results Of the 294 respondents who completed the modified Morisky-Green-Levine Medication Adherence Scale, 13.3, 21.4, 35.7 and 29.6% had a score of 0-1, 2, 3 and 4, respectively. The internal consistency of the scale was moderate (Cronbach's alpha=0.62). Principal component analysis showed that the four items loaded onto one factor (eigenvalue=1.95). Respondents with higher scores were older (P<0.001), had lower HbA_1c levels (P<0.001) and had better adherence to physician-recommended diet (P<0.001) and physical exercise (P=0.02). Conclusions The psychometric properties of the modified Morisky-Green-Levine Medication Adherence Scale were less than satisfactory. A ceiling effect was observed. The scale may not be an adequate measure to assess medication adherence in patients with Type2 diabetes in Singapore. Future research could target refining the scale and investigating its use in other patient populations.