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

Giant cell arteritis (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. Cross-sectional study included UK patients with clinician-confirmed GCA. They completed 40 candidate items for the GCA-PRO at time 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 versus other PRO scores, and between participants with 'active disease' versus those 'in remission') and test-retest reliability. Study population: 428 patients, mean age (SD) 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 4-point Likert scales. Factor analysis confirmed four domains: Acute symptoms (8 items), Activities of daily living (7 items), Psychological (7 items), Participation (8 items). The overall scale had adequate Rasch model fit (X2=25.219, DF = 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. The final GCA-PRO is a 30-item, 4-domain scale with robust evidence of validity and reliability in measuring HRQoL in people with GCA.

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© The Author(s) 2023. Published by Oxford University Press on behalf of the British Society for Rheumatology.