Functional Assessment of Chronic Illness Therapy-Fatigue is a reliable and valid measure in patients with active ankylosing spondylitis

Background: The Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F) scale has demonstrated good internal consistency and responsiveness to changes in clinical status among patients with ankylosing spondylitis (AS). We aimed to further evaluate the psychometric properties of the FACIT-F scale in adult patients with AS. Methods: Measurement properties of the FACIT-F scale were evaluated using data from tofacitinib phase 2/3 (NCT01786668/NCT03502616) studies in adult patients with active AS. Results: Second-order confirmatory factor modeling supported the measurement structure of the FACIT-F scale (Bentler's comparative fit index >= 0.91), and FACIT-F demonstrated excellent internal consistency (Cronbach's coefficient alpha >= 0.88) and test-retest reliability (Intraclass Correlation Coefficient >= 0.75). Correlation coefficients between FACIT-F and other patient-reported outcomes generally exceeded 0.40, supporting convergent validity. Meaningful within-patient change was estimated as 3.1-6.3 for FACIT-F total score, and 1.4-2.8 and 1.7-3.6 for FACIT-F Experience and Impact domain scores, respectively. Large (effect size >= 1.17 standard deviation units), statistically significant differences in FACIT-F domain/total scores between 'no disease activity' (Patient Global Assessment of Disease Activity [PtGA] = 0) and 'very active disease' (PtGA =10) patient groups supported known-groups validity. Ability to detect change was evidenced by an approximately linear relationship between changes in FACIT-F and PtGA scores. Conclusions: FACIT-F is a reliable and valid measure for evaluating fatigue in adult patients with active AS.