A cultural adaptation and validation study of the Intentional Nonadherence Scale (INAS) among people with type 2 diabetes in Singapore

Abstract

Aims

To examine the psychometric properties of the Intentional Non-adherence Scale (INAS) among people with type 2 diabetes mellitus (PwT2D) in Singapore.

Methods

This study consisted of Phase 1: translation and adaptation of the questionnaire into local Mandarin and English and Phase 2: a longitudinal validation study at the outpatient clinics of a hospital in Singapore. In Phase 1, cognitive interviews were conducted with 20 PwT2D and healthcare providers to examine the content validity of the INAS. In Phase 2, 290 PwT2D were recruited. Fifty-three of them were involved in test-retest reliability analysis, while 185 were followed-up in 3–6 months to assess the predictive validity of the INAS. The INAS was also evaluated for its structural validity, construct validity and internal reliability.

Results

Exploratory factor analysis revealed four factors, namely “Resisting illness and medication”, “Sensitivity to medication”, “Testing treatment” and “Inconvenience”. All INAS factors showed good internal consistency (Cronbach's alpha = 0.84–0.94) and moderate test-retest reliability (intraclass correlation coefficient = 0.50–0.62). Construct validity of the INAS was demonstrated in its relationship with medication adherence, glycated haemoglobin (HbA1c), beliefs about medications, illness perception and mood. Quantile and linear regression for medication adherence and HbA1c in 3–6 months did not show statistical associations with the INAS after adjusting for potential confounders.

Conclusions

Our study supports the reliability and most aspects of validity of the INAS, which revealed new factors that may affect medication adherence and HbA1c. In clinical settings, healthcare providers may consider using this questionnaire to evaluate potential intentional nonadherence.