Background: This self-report measure is a new instrument to measure the extent of and reasons for medication adherence separately. However, few studies have assessed its psychometric properties in diabetic patients and also in Asian populations.
Objectives: To validate this self-report measure in diabetic patients in Singapore.
Methods: We collected data prospectively using a questionnaire among 393 diabetic patients from hospitals in Singapore from July 2018 to January 2019. Using the COnsensus-based Standards for the selection of health Measurement INstruments framework, we assessed face validity, internal consistency, test-retest reliability, structural validity, and measurement error. We tested four a priori hypotheses on correlation of extent score with patient-reported outcome measures to assess construct validity. We examined cross-cultural validity via measurement invariance across gender, age groups, and languages.
Results: We performed cognitive interviews with 30 consenting English-literate, Chinese-literate, and Malay-literate (10 patients per language) diabetic patients (age range 48-76 years, 53% male, disease duration range 1-30 years) and face validity was supported. Among 393 patients (mean age: 59.4±12.2 years, 50.9% female, 52.4% Chinese), we showed moderate internal consistency (Cronbach's alpha =0.67) and test-retest reliability (intra-class coefficient=0.56 [95% CI 0.37-0.70]). We calculated smallest detectable change as 0.80. We established construct validity by meeting all four hypotheses. We showed structural validity as confirmatory factor analysis confirmed a one-factor model, with excellent fit statistics (Comparative Fit Index=1.0; Tucker-Lewis Index=1.0; Root Mean Square Error of Approximation<0.001; Standardized Root Mean Residuals<0.001). Analysis of cross-cultural validity supported configural invariance model but not metric invariance and scalar invariance model. Caution must be taken against directly comparing extent scores across gender, age groups, and languages.
Conclusion: This self-report measure is valid and reliable in measuring medication adherence in diabetic patients in Singapore.
Keywords: Singapore; adherence; diabetes; patient-reported outcome; psychometric; quality of life.