Examining the Interchangeability of Two Different Patient‐Reported Global Assessment Measures in an Observational Axial Spondyloarthritis Cohort

Abstract

OBJECTIVE: The goal of this study was to determine if patient global assessment (PGA) scores reported by patients with axial spondyloarthritis (AxSpA) prior to the development of the ASDAS can be used as a substitute for the global item of the ASDAS formula. METHODS: Data from the Prospective Study of Outcomes in Ankylosing Spondylitis (PSOAS) observational cohort were analyzed. Patients completed a questionnaire during visits to assess disease activity. Before the development of ASDAS, global disease activity was rated on a numeric scale with the prompt: "Considering all the ways that your ankylosing spondylitis affects you, rate how you are doing." The ASDAS global prompt instead asks: "How active was your spondylitis on average during the last week?" We calculated the ASDAS based on the original PGA of the PSOAS cohort (PSOAS-PGA) and the standard ASDAS-PGA. Then, we compared ASDAS disease activity categories between the two values and calculated weighted Cohen's Kappa coefficients. We also compared the older vs. newer global single-item scores with intraclass correlation coefficients (ICC). RESULTS: 610 unique patient visits were analyzed. The proportion of agreement of ASDAS calculated with the PSOAS-PGA compared to the ASDAS-PGA was 86.7% based on disease activity categories with Cohen's Kappa Coefficient of 0.818. The ICC was calculated using a two-way mixed-effects model and was 0.980, consistent with excellent agreement. CONCLUSIONS: Prior measures of PGA utilized before the validation of ASDAS are a valid substitution for the PGA in the ASDAS based on calculation of a Kappa Coefficient showing excellent agreement.