Reliability and validity of the Fatigue Severity Scale in Swedish for patients with systemic lupus erythematosus

Objective: The aim was to translate, test, and describe aspects of reliability and validity of the Fatigue Severity Scale (FSS) in Swedish (FSS-Swe) in patients with systemic lupus erythematosus (SLE). Methods: Patients with stable SLE, low or moderate disease activity, and low organ damage were included. Forward and back translations of the FSS were performed. Construct validity was tested with 32 women using a first Swedish translation. Feasibility, ceiling and floor effects, internal consistency, test-retest reliability, and content validity were tested on a slightly modified final version of the FSS-Swe in a non-selected group of patients (n=23). Results: There were correlations (p <= 0.05) between the FSS-Swe and overall disease activity according to the Systemic Lupus Activity Measure (SLAM) (r(s)=0.48) and the SLAM Visual Analogue Scale (SLAM-VAS) (r(s)=0.46); between the FSS-Swe and eight subscales of the Swedish 36-Item Medical Outcomes Study Short-Form Health Survey (SF-36) (r(s)=-0.41 to -0.65) and between the FSS-Swe and age (r(s)=-0.35). All patients answered all FSS-Swe questions at both test and retest. There was one ceiling effect in one question on one occasion. The Kolmogorov-Smirnov test indicated normal distribution. Cronbach's alpha was 0.94 and corrected item-to-total correlation exceeded 0.3. There were no significant systematic test-retest differences, and the median-weighted kappa coefficient was 0.75. Twenty patients understood the questions in FSS-Swe, 18 considered they were relevant, reflected their fatigue, and that none should be excluded. Five items were suggested to be included. Conclusions: The FSS-Swe supports construct validity, is feasible, has no important ceiling or floor effects, has satisfactory internal consistency, substantial test-retest reliability, and satisfactory content validity in the SLE patients studied. However, its sensitivity to change needs to be tested.