Psychometric properties of FACIT-Fatigue in systemic lupus erythematosus: a pooled analysis of three phase 3 randomised, double-blind, parallel-group controlled studies (BLISS-SC, BLISS-52, BLISS-76)

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. CLINICAL TRIAL IDENTIFIERS: BLISS-SC (NCT01484496), BLISS-52 (NCT00424476), and BLISS-76 (NCT00410384). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s41687-021-00298-x.