Psychometric properties of fatigue severity scale in Chinese systemic lupus erythematosus patients

Fatigue is the most common symptom in Systemic Lupus Erythematosus (SLE) patients. Many fatigue instruments have been used in SLE, with Fatigue Severity Scale (FSS) mostly adopted. However, fatigue instruments haven't been tested in the Chinese SLE population. The aim of our study was to test the psychometric properties of FSS in Chinese SLE patients.

A cross-sectional study was conducted. 201 patients diagnosed with SLE were enrolled in the study with convenience sampling. Fatigue score, depression score and vitality subscale score of SF-36 were collected. Floor and ceiling effects were tested. Factor analysis was conducted. Reliability and validity of FSS were also tested.

Floor (4.50%) and ceiling (4.00%) effects were minimal. One factor was extracted, explaining 61.80% of total variance. When item1 and item 2 were deleted, one factor explained 69.54% of variance, and Cronbach's Alpha increased from 0.92 to 0.93. Intraclass correlation coefficient (ICC) was 0.94. Fatigue correlated with both depression (r = 0.52, P < 0.01) and vitality (r = - 0.55, P < 0.01), indicating acceptable construct validity for original FSS. When item 1 and 2 were removed, the correlation coefficient between 7-item FSS and vitality increased (r = - 0.58, P < 0.01), while correlation coefficient between 7-item FSS and depression remained the same (r = 0.52, P < 0.01). Known-groups validity was verified by that patients with depression showed higher fatigue score both for 9-item (Z = -5.56, P < 0.001) and 7-item FSS (Z = -5.70, P < 0.001).

9-item FSS is a reliable instrument and can be used to assess fatigue problem in Chinese SLE patients, and 7-item FSS also demonstrated good psychometric properties in the same participants.