Internal structure evaluations of the fatigue severity scale among Japanese workers with pain in the musculoskeletal regions

Fatigue/low energy is recognized as one of the most important outcome domains in individuals with musculoskeletal diseases. The Fatigue Severity Scale (FSS) is a promising short patient-reported outcome measure in terms of perceived disability due to fatigue. This study aimed to investigate the internal structure of the FSS among workers with musculoskeletal pain. A secondary analysis of an anonymous web survey for Japanese workers with pain in the musculoskeletal regions was conducted. In addition to the 9-item FSS, 39 outcomes from the following five domains were included: (1) fundamental domain, (2) work-related domain, (3) cognitive-emotional domain, (4) lifestyle domain, and (5) disability domain. For the FSS, the Rasch analysis was conducted, and internal consistency was assessed with the Person Separation Reliability (PSR). For construct validity evaluation, 55 hypotheses were tested, and construct validity was assumed if ≥ 75% of the hypotheses were accepted. Consequently, data from 2663 workers were analyzed. The Rasch analysis demonstrated unidimensionality. The PSR was 0.90, indicating acceptable internal consistency. For the hypothesis testing, 50/53 (94.3%) hypotheses were satisfied, indicating the construct validity. In conclusion, the FSS was found to have sufficient unidimensional internal structure in Japanese workers with pain in the musculoskeletal regions.