Factor structure and longitudinal invariance of the Medical Adherence Report Scale-Asthma.

Adequate assessment of adherence to medical treatment is critical for both research purposes and clinical practice. This study examined the factor structure and longitudinal invariance of the Medication Adherence Report Scale (MARS-A10) in a sample of asthmatic patients. We examined longitudinal data from 294 inner-city, adult participants with moderate to severe asthma. Because of ambiguous evidence regarding the dimensionality of the MARS-A10, the data was analysed with exploratory structural equation modelling. We first proceeded by determining the dimensionality of the scale at baseline and examined whether the structure, loadings, intercepts and errors were invariant over the four assessments points. Results indicated that a two-factor structure (factor 1: non-adherence based on experiential changes; factor 2: non-adherence based on intentional medication avoidance) had the best fit to the data (χ(2)(25)=37.69, p=0.05). Longitudinal analyses revealed that the nine items assessing intentional non-adherence were invariant over time. The evidence from the factor analysis suggests that intentional non-adherence is a multidimensional construct. Additionally, longitudinal data provided strong evidence that the items examining intentional non-adherence are invariant over time, indicating that changes in non-adherence scores can be validly attributed to changes in behaviour.