• Yuan Wei Liau, Celine Cheow, Kenneth Tin Yau Leung, Hejing Tan, Suat Fern Low, CheenHua Heng McVinHHMDepartment of Pharmacy, Singapore General Hospital, Singapore, Singapore., Woan Chyi Lim, Li Ling Tan, Joyce Zhen Yin Tan, Eng Sing Lee, Sandra Jialun Xu, Corrinne Yong Koon Tan, Jie Wen Phang, Jie Kie Phang, Miao Hui Lam, Dan V Blalock, Corrine I Voils, Kai Zhen Yap, and Yu Heng Kwan.
• Department of Pharmacy, Faculty of Science, National University of Singapore, Singapore, Singapore.
• Patient Prefer Adher. 2019 Jan 1; 13: 1241-1252.

BackgroundThis 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.ObjectivesTo validate this self-report measure in diabetic patients in Singapore.MethodsWe 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.ResultsWe 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.ConclusionThis self-report measure is valid and reliable in measuring medication adherence in diabetic patients in Singapore.

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