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 α ≥ 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. Trial registration: ClinicalTrials.gov; NCT01786668 (registered 6 February 2013, https://clinicaltrials.gov/ct2/show/NCT01786668) and NCT03502616 (registered 11 April 2018, https://clinicaltrials.gov/ct2/show/NCT03502616). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s41687-022-00508-0.