Italian adaptation and psychometric validation of the Fatigue Impact Scale (FIS) and its modified versions in adults with multiple sclerosis: a Rasch analysis study

Purpose: Several outcome measures are available to assess the severity of fatigue in people with multiple sclerosis (MS). The aim of this study was to adapt the Italian version of the Fatigue Impact Scale (FIS-40) and its modified versions: a 21-item Modified scale (MFIS-21), its 5-item short version (MFIS-5), and an 8-item version for daily use (DFIS-8) and investigate their measurement properties through classical theory-test (CTT) and Rasch analysis (RA).Methods: 229 Italian-speaking adults with MS were included. Questionnaires were cross-culturally translated and subjected to CTT (i.e. internal consistency through Cronbach's alpha and unidimensionality through confirmatory factor analysis [CFA]) and RA. (i.e. internal construct validity, reliability, and targeting).Results: Internal consistency was high for all scales (>0.850). Final CFAs reported issues in the unidimensionality for all scales except for FIS-40. Baseline RA revealed a misfit for all scales. After adjusting for local dependency, FIS-40, MFIS-21, and MFIS-5 fitted the Rasch model (RM). MFIS-21 and D-FIS-8 required a structural modification, i.e. item deletions to satisfy the RM.Conclusion: The FIS-40, MFIS-21, MFIS-5, and DFIS-8 achieved the fit to the RM after statistical and structural modifications. The fit to the RM allowed for providing ordinal-to-interval measurement conversion tables for all the questionnaires.