Validation of the fatigue severity scale in Croatian population of patients with multiple sclerosis disease: Factor structure, internal consistency, and correlates.

BACKGROUND: Fatigue is a common symptom in people with multiple sclerosis (MS) and is evaluated and monitored with self-report questionnaires. The objective of this study was to determine the psychometric properties of the Croatian version of the Fatigue Severity Scale (FSS) in people with MS. MATERIAL AND METHODS: This is a retrospective cohort study conducted as an online survey from December 16, 2020, until January 13, 2021. A total of 179 people with MS and 999 control subjects completed FSS and self-administered questionnaires capturing information of demographic, education level, disease-related variables (duration of the disease, MS type, the expanded disability status scale (EDSS), and Multiple Sclerosis Impact Scale-29 (MSIS-29). Psychometric properties were examined by estimating the validity, reliability, and factor structure of the FSS scale in people with MS. RESULTS: The Croatian version of the FSS had excellent internal consistency (Cronbach's α value 0.93). Factor analysis demonstrated a unidimensional structure. The concurrent validity of the FSS appeared to be satisfactory due to the significant differences between people with MS and control subjects (p < .05). The correlations between FSS and MSIS-29 physical (r = 0.60) and psychological (r = 0.50) subscale results confirmed the convergent validity of the FSS scale. Results also indicated that the best cut-off score is between 4 and 5 with a relatively high sensitivity and specificity. CONCLUSIONS: The Croatian version of FSS was shown to have excellent psychometric properties in people with MS and can be used in the research and clinical settings evaluating fatigue in people with MS in Croatia.