Abstract
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.
Acknowledgments
Full version of the self-report measure will be available on https://www.surgery.wisc.edu/research/researchers-labs/corrine-voils-phd/self-reported-medication-nonadherence-measurement/. The National Healthcare Group Pharmacy Transformation Office (NHGPTO) provided a non-restricted grant for this project. All patients provided written informed consent to participate in this research.
Disclosure
Dr Voils was supported by a Research Career Scientist award from the Department of Veterans Affairs Health Services Research & Development Service (RCS 14-443). The authors report no other conflicts of interest in this work.
Supplementary materials
Table S1 Proportion of extent of nonadherence
Table S2 Cross-cultural validity of the self-report measure (n=393)