Validation of a patient-reported outcome measure for giant cell arteritis

Objectives GCA is systemic vasculitis manifesting as cranial, ocular or large vessel vasculitis. A prior qualitative study developed 40 candidate items to assess the impact of GCA on health-related quality of life (HRQoL). This study aimed to determine final scale structure and measurement properties of the GCA patient reported outcome (GCA-PRO) measure. Methods Cross-sectional study included UK patients with clinician-confirmed GCA. They completed 40 candidate items for the GCA-PRO at times 1 and 2 (3 days apart), EQ-5D-5L, ICECAP-A, CAT-PROM5 and self-report of disease activity. Rasch and exploratory factor analyses informed item reduction and established structural validity, reliability and unidimensionality of the final GCA-PRO. Evidence of validity was also established with hypothesis testing (GCA-PRO vs other PRO scores, and between participants with 'active disease' vs those 'in remission') and test-retest reliability. Results The study population consisted of 428 patients: mean (s.d.) age 74.2 (7.2), 285 (67%) female; 327 (76%) cranial GCA, 114 (26.6%) large vessel vasculitis and 142 (33.2%) ocular involvement. Rasch analysis eliminated 10 candidate GCA items and informed restructuring of response categories into four-point Likert scales. Factor analysis confirmed four domains: acute symptoms (eight items), activities of daily living (seven items), psychological (seven items) and participation (eight items). The overall scale had adequate Rasch model fit (chi(2) = 25.219, degrees of freedom = 24, P = 0.394). Convergent validity with EQ5D-5L, ICECAP-A and Cat-PROM5 was confirmed through hypothesis testing. Internal consistency and test-retest reliability were excellent. Conclusion The final GCA-PRO is a 30-item, four-domain scale with robust evidence of validity and reliability in measuring HRQoL in people with GCA.