Development and validation of treatment adherence scale in older people with Type 2 diabetes: a sequential exploratory mixed method

Background Type 2 diabetes (T2D) is one of the most common chronic diseases in older people and is associated with numerous health complications. Adherence to prescribed treatments plays a crucial role in effective diabetes management and the prevention of secondary diseases. Therefore, the development of a valid and reliable instrument for assessing treatment adherence in older people with T2D is of paramount importance. The aim of the present study was to develop and validate a comprehensive scale for assessing treatment adherence (Treatment Adherence Scale, TAS) specifically for this population group. Methods This methodological investigation was conducted using a sequential exploratory mixed-method design consisting of qualitative and quantitative phases. In the qualitative phase, the concept of treatment adherence in older people with T2D was extracted using inductive content analysis through individual, semi-structured in-depth interviews with 20 older patients. Based on the qualitative results, a first version of the scale was developed. In the quantitative phase, the psychometric evaluation of the TAS included the assessment of face, content, and construct validity as well as the testing of reliability. Exploratory factor analysis (EFA) and correlation with an existing adherence scale were used to confirm construct validity, while internal consistency and test-retest methods were used to assess reliability. Results In the qualitative phase, 67 items were initially identified and reduced to 51 through content and face validity checks. EFA reduced the scale to 37 items, which were grouped into four factors: adherence to medication and treatment, adherence to care and nutrition, adherence to activity plan, and helpfulness. These factors accounted for 58.37% of the total variance. The scale showed acceptable convergent validity (p < 0.001, r = 0.483) and high internal consistency (Cronbach's α = 0.93) and stability (ICC = 0.93). Additional metrics such as SEM = 4.596, MDC = 5.94, and MIC = 8.68 showed satisfactory reliability and interpretability. Conclusions The TAS is a valid and reliable scale for assessing treatment adherence in older people with T2D. It provides researchers, clinicians, and policymakers seeking to improve diabetes self-management and health outcomes in older adults with a practical and multidimensional scale.