OBJECTIVES: Fatigue in multiple sclerosis (MS) is a key symptom associated with work-related problems and poor quality of life outcomes. The 5-item Modified Fatigue Impact Scale (MFIS-5) is a brief self-assessment tool for measuring the impact of fatigue on cognitive, physical and psychosocial function. However, limited information is available on the psychometric performance of the MFIS-5 in patients with MS. The aim of this study was to assess the factor structure and construct validity of the MFIS-5 in the management of MS.

METHODS: A multicenter, non-interventional, cross-sectional study in adult patients with MS (McDonald 2010 criteria) was conducted. A non-parametric item response theory (IRT) procedure, Mokken analysis, and confirmatory factor analysis (CFA) were performed to assess the factor structure of the MFIS-5. A graded response model (GRM or Samejima’s model) was conducted to determine items characteristics.

RESULTS: A total of 302 patients were studied (mean age = 42.3 ± 10 years, 64.2% female, 90.4% with relapsing-remitting MS). Mean Expanded Disability Status Scale (EDSS) score: 2.6 ± 1.9. The Mokken analysis found the MFIS-5 is a strong one-dimensional scale (overall scalability index H = 0.67) with high reliability (Cronbach’s alpha = 0.90). The CFA model confirmed the one-dimensional structure (Comparative fit index = 1.0, RMSEA = 0.035). Samejima’s model fitted well an unconstrained model with different item difficulties. The item characteristic curve showed all items presented appropriate shape and difficulty parameters. Items #2 (“limited to do things away from home”), #3 (“had trouble maintaining physical effort for long periods”), and #4 (“less able to complete tasks requiring physical effort”) represent 80.4% of the total information.

CONCLUSIONS: The MFIS-5 shows appropriate psychometric characteristics as a patient-reported outcome. MFIS-5 may constitute a valuable and easy-to-implement addition to measure the impact of fatigue in patients with MS in clinical practice.