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). Plain English summary: Ankylosing spondylitis (AS) is a disease that causes pain and stiffness in the spine. AS is a chronic disease. Most people with AS experience fatigue (a feeling of being very tired or exhausted). People with more severe AS tend to have more fatigue and a lower level of well-being. Because of this, fatigue is an important symptom to measure in studies of AS. The Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F) is a questionnaire that measures fatigue. It has 13 questions that assess level of fatigue over a week's time. The FACIT-F has not been studied in depth in people with AS. Therefore, we analyzed results from two clinical trials. We found that FACIT-F was a reliable and valid way to measure fatigue in adults with AS. This makes it a suitable tool to use in AS clinical trials. [ABSTRACT FROM AUTHOR]