Background: Fatigue is a key symptom in patients with systemic lupus erythematosus (SLE), and regulatory bodies recommend its assessment in clinical trials of SLE therapies.
Methods: This post hoc pooled analysis of the three BeLimumab In Subjects with Systemic lupus erythematosus (BLISS) Phase 3 randomised, double-blind, parallel-group controlled trials evaluated the measurement properties of the Functional Assessment of Chronic Illness Therapy (FACIT)-Fatigue. Patients (N = 2520) completed the FACIT-Fatigue every 4 weeks from baseline until the end of each study period. Internal consistency, test-retest reliability, convergent validity, and ability to detect changes in SLE were evaluated for the FACIT-Fatigue.
Results: The FACIT-Fatigue showed good internal consistency reliability (Cronbach's alpha > 0.90), very good test-retest reliability (0.76 ≤ intraclass correlation coefficient ≤ 0.92), and moderate-strong convergent validity (0.49 ≤ |r| ≤ 0.86) against scale and summary measure scores from the Short Form 36 Health Survey Version 2. Correlations between FACIT-Fatigue and British Isles Lupus Assessment Group (BILAG) General/Musculoskeletal scores (0.24 ≤ |r| ≤ 0.43) supported convergent validity. Correlations between FACIT-Fatigue and the Safety of Estrogens in Lupus Erythematosus National Assessment-Systemic Lupus Erythematosus Disease Activity Index (SELENA-SLEDAI) scores and SLE annualised flare rate were weak but in the expected direction (ranging from - 0.02 to - 0.25). Known-groups validity testing showed that the FACIT-Fatigue can significantly discriminate between patient groups with differing scores for SELENA-SLEDAI, BILAG (General and Musculoskeletal) ratings, and Physician's Global Assessment (PGA). Patients showing improvement in PGA and meeting the BILAG responder criteria had significantly higher mean improvement in FACIT-Fatigue scores than those without improvements in either measure (Week 52 mean score difference [95% confidence interval]: - 4.0 [- 5.0, - 3.0] and -2.2 [-3.1, -1.2], respectively; both p < 0.0001). The range of important (i.e. meaningful) change in FACIT-Fatigue, based on multiple anchors, was 3-6 points.
Conclusions: The FACIT-Fatigue demonstrated adequate psychometric properties in patients with SLE. The body of evidence from the three BLISS trials (both pooled and individually) supports the FACIT-Fatigue as a reliable and valid measure of SLE-related fatigue in clinical trials.
Keywords: FACIT-Fatigue; Fatigue; Patient-reported outcomes; Psychometric; Quality of life; Systemic lupus erythematosus; Validity.