Multimethodological validation of the modified fatigue impact scale in a Danish population of people with Multiple...

To provide a multimethodological validation of MFIS in compliance with the COnsensus-based Standards for the selection of health status Measurement INstruments (COSMIN) guidelines using a Danish population of patients with Multiple Sclerosis (pwMS).

Factor analytic and multidimensional Rasch analytic methods were applied to investigate the structural validity of MFIS. We employed measurement and bi-factor models to address divergence and convergence. McDonalds ω was used to assess reliability. Cross-cultural and nomological validity was assessed in relation to relevant external populations and constructs, respectively. Responsiveness was evaluated following one month of multi-disciplinary rehabilitation (MDR).

Based on data from 424 Danish MS patients, the MFIS showed an acceptable fit to both factor analytic and multidimensional Rasch models. MFIS was found to be dominated by a general factor but with considerable substructure present. Sufficient divergence was found between cognitive and physical subdomains, but not the psychosocial subscale. Mc Donald's ω_t of 0.96 indicated good reliability of the scale, however, with low specific reliability of the psychosocial subscale. Homogeneity across Danish and European populations of pwMS supported cross-cultural validity. Explorative factor analysis of the nomological network of MFIS revealed expected convergence with cognitive and physical aspects. MFIS showed good responsiveness as indicated by moderate to large effect sizes following MDR.

The Danish version of the MFIS showed good reliability, a good structural, cross-cultural, and nomological validity an acceptable fit to a multidimensional Rasch analysis, and a good responsiveness. The psychosocial subscale of the MFIS however should be interpreted with caution.