Validity and Reliability of the Fatigue Severity Scale in a Real-World Interstitial Lung Disease Cohort

Rationale: Fatigue is a common and debilitating symptom for people living with interstitial lung disease (ILD). Studies on fatigue in ILD are limited, and little headway has been made toward developing interventions targeting the alleviation of fatigue. A barrier to progress is a lack of knowledge around the performance characteristics of a patient-reported outcome measure to assess fatigue in patients with ILD. Objectives: To assess the validity and reliability of the Fatigue Severity Scale (FSS) for measuring fatigue in a national cohort of patients with ILD. Methods: FSS scores and several anchors were measured in 1,881 patients from the Pulmonary Fibrosis Foundation Patient Registry. Anchors included the Short Form 6D Health Utility Index (SF-6D) score and a single vitality question from the SF-6D; the University of California, San Diego, Shortness of Breath Questionnaire; FVC; DlCO; and 6-minute-walk distance. Internal consistency reliability, concurrent validity, and known-groups validity were assessed. Structural validity was assessed using confirmatory factor analysis. Measurements and Main Results: The FSS demonstrated high internal consistency (Cronbach's α = 0.96). There were moderate to strong correlations between FSS score and patient-reported anchors (vitality question from the SF-6D [r = 0.55] and University of California, San Diego, Shortness of Breath Questionnaire total score [r = 0.70]) and weak correlations between FSS score and physiological measures (FVC [r = -0.24], percentage predicted DlCO [r = -0.23], and 6-minute-walk distance [r = -0.29]). Higher mean FSS scores, indicating greater fatigue, were observed among patients using supplemental oxygen, those prescribed steroids, and those with lower percentage predicted FVC and percentage predicted DlCO. The confirmatory factor analysis results suggest that the nine questions of the FSS reflect one dimension of fatigue. Conclusions: Fatigue is an important patient-centered outcome in ILD that is poorly correlated with physiological measures of disease severity, including lung function and walk distance. These findings further support the need for a reliable and valid measure of patient-reported fatigue in ILD. The FSS possesses acceptable performance characteristics for assessing fatigue and distinguishing different degrees of fatigue among patients with ILD.