Fatigue in Parkinson's disease: Italian validation of the Parkinson Fatigue Scale and the Fatigue Severity Scale using a Rasch analysis approach

Introduction: The Fatigue Severity Scale (FSS-9) and the Parkinson Fatigue Scale (PFS-16) are commonly used for assessing fatigue in Parkinson's disease (PD). Here we validated the Italian version of these scales, assessed their psychometric properties by Rasch analysis, and computed their optimal cut-off scores using clinical diagnosis of PD-related fatigue as the gold standard. Methods: PD patients (n = 167) completed the Italian versions of FSS-9 and PFS-16. Each item of PFS-16 was scored both on a 5-point (PFS-16polytomous) and on a 2-point scale (PFS-16dichotomous). Results: All scales showed an adequate overall Rasch model fit, high reliability, and good discriminant, convergent, and concurrent validity, but were less accurate in measuring very high and very low fatigue levels. No evidence of differential item functioning with respect to age, sex, and severity of parkinsonian symptoms was found. Some items of FSS-9 (item 1), PFS-16polytomous (items 1 and 13), and PFS-16dichotomous (items 1, 8, and 13) showed misfit, possibly due to their content concerning sleep and motivation disorders. When FSS-9 and PFS-16polytomous' responses were rescored on a 3-point scale, the discriminability across response categories improved. The optimal cut-off score in detecting clinically-diagnosed fatigue (observed in 20% of the sample) was 3.09 for PFS-16polytomous, 8.00 for PFS-16dichotomous, and 4.67 for FSS-9. Conclusions: The Italian version of PFS-16 and FSS-9 showed sound psychometric properties and can be confidently used to quantify fatigue symptoms in PD, although clinical diagnosis of fatigue should rely on validated criteria. The PFS-16polytomous exhibited advantages with respect to PFS-16dichotomous.