The GCA-PRO was designed and developed by researchers at the Centre for Health and Clinical Research, University of the West of England, and Nuffield Department of Population Health (HSRU), University of Oxford, Oxford, UK.
The Outcome Measures in Rheumatology (OMERACT)
Large Vessel Vasculitis Working Group initially identified
the need for a disease-specific patient-reported outcome measure (PROM) for GCA. Generic PROMs, which can be used across a range of different diseases, may not always involve content specific enough for use in GCA.
A cross-sectional study was conducted to validate the GCA-PRO and included UK patients (n=428, mean age (SD) of 74.2 (7.2), 285 (67%) female) with clinician-confirmed GCA; diagnosed within the last three years or flaring within the last year. Patients completed the original 40 candidate GCA-PRO items, the EQ5D-5L, CAT-PROM5 and self-report of disease activity. Rasch and factor analysis were used to determine internal validity and factor structure. Item reductions were based on clinical importance, Rasch model fit, and redundancy. Tests of validity included comparison of the GCA-PRO (i) in participants with ‘active disease’ versus patients ‘in remission’ (known groups validity) and (ii) with EQ5D-5L and CAT-PROM5 scores (convergent validity).
After the initial analysis (40 items), ten items were deleted, and two response categories collapsed to ensure overall fit to the Rasch model. This resulted in a final PRO comprising a 30-item scale with a 4-response category structure. Factor analysis confirmed four factors (domains): Acute symptoms (8 items), Activities of daily living (7 items), Psychological (7 items) and Participation (8 items), all of which individually fitted the Rasch model (X2 = 25.219, DF=24, p=0.394 including reliability [Person Separation Index, PSI=0.828]), (construct validity). Each domain correlated, at least moderately, with EQ5D-5L and CAT-PROM5 scores (Spearman’s Correlation Coefficients 0.44 to 0.78) (convergent validity). The new GCA-PRO discriminated between patients with active disease and remission (known groups validity).
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