The Functional Assessment of Chronic Illness Therapy–Fatigue (FACIT-Fatigue) scale in patients with axial spondyloarthritis: psychometric properties and clinically meaningful thresholds for interpretation.

Background: Fatigue is an important symptom for most patients with axial spondyloarthritis (axSpA). The FACIT-Fatigue is a 13-item patient-reported outcome (PRO) instrument that has been used in axSpA clinical trials to measure fatigue severity and impact on daily activities. However, the psychometric properties of the FACIT-Fatigue are not fully evaluated across the entire spectrum of axSpA including non-radiographic axSpA (nr-axSpA) and radiographic axSpA (r-axSpA). This study determined: (1) the psychometric properties of the FACIT-Fatigue in nr-axSpA, r-axSpA, and the broad axSpA population and (2) FACIT-Fatigue scores representing meaningful within-patient change (MWPC), meaningful between-group differences, and cross-sectional severity bands. Methods: Data from two Phase 3 trials in adults with nr-axSpA (BE MOBILE 1; N = 254) and r-axSpA (BE MOBILE 2; N = 332) were analyzed pooled and separately to assess the psychometric properties of the FACIT-Fatigue. MWPC and meaningful between-group difference estimates were derived using anchor-based and distribution-based methods. Cross-sectional fatigue severity bands were estimated using logistic regression analysis. Results: The FACIT-Fatigue presented good internal consistency, adequate convergent and known-groups validity, and was sensitive to change over time across the full axSpA spectrum. A 5–11-point increase in FACIT-Fatigue score was estimated to represent a MWPC, with an 8-point increase selected as the responder definition. A 2.14–5.34-point difference in FACIT-Fatigue score change over a 16-week period was estimated to represent a small-to-medium meaningful between-group difference. FACIT-Fatigue score severity bands were defined as: none or minimal (>40), mild (>30 to ≤40), moderate (>21 to ≤30), and severe (≤21). Conclusions: These findings support the use of the FACIT-Fatigue as a fit-for-purpose measure to assess fatigue-related treatment benefit in axSpA clinical trials. The proposed score estimates and thresholds can guide FACIT-Fatigue score interpretation across the full axSpA spectrum. Trial registration: ClinicalTrials.Gov, NCT03928704. Registered 26 April 2019—Retrospectively registered, https://classic.clinicaltrials.gov/ct2/show/NCT03928704. ClinicalTrials.Gov, NCT03928743. Registered 26 April 2019—Retrospectively registered, https://classic.clinicaltrials.gov/ct2/show/NCT03928743. Plain English summary: Fatigue is common in patients with axial spondyloarthritis (axSpA)—a painful, chronic disease that mainly affects the joints of the spine. Measuring fatigue in clinical trials is important to see if new treatments for axSpA help reduce this symptom. This study investigated whether a patient-completed measure of fatigue, the 'FACIT-Fatigue', is suitable for use by patients with either of the two main types of axSpA—radiographic axSpA (r-axSpA) and non-radiographic axSpA (nr-axSpA). This study was needed because we did not know if the FACIT-Fatigue is valid and reliable in both axSpA subtypes and because we are lacking information to interpret the FACIT-Fatigue score in axSpA. Psychometric analysis of clinical trial data showed that the FACIT-Fatigue accurately measures fatigue severity in both types of axSpA. The study additionally pinpointed what change in FACIT-Fatigue score can be used to define whether a treatment has reduced fatigue in a patient, or what constitutes a meaningful between-group difference. It also identified severity score bands that can be used to classify patients into those with no, mild, moderate, or severe fatigue. These findings support using the FACIT-Fatigue in axSpA clinical trials to measure how well treatments reduce fatigue. [ABSTRACT FROM AUTHOR]