This study assessed whether a legacy patient global assessment (PGA) measure used in the PSOAS observational cohort — which asks patients to rate overall disease impact — can substitute for the standard ASDAS-PGA prompt (which asks about spondylitis activity in the past week) when calculating the ASDAS disease activity score in axial spondyloarthritis (AxSpA).
Across 610 patient visits, ASDAS calculated with the PSOAS-PGA agreed with ASDAS using the standard PGA in 86.7% of disease activity category assignments; weighted Cohen's Kappa was 0.818 (excellent agreement) and ICC was 0.980 (excellent).
Single observational cohort (PSOAS) limits generalizability; both PGA measures were not collected simultaneously — the older PSOAS-PGA was used before ASDAS was developed, so direct head-to-head comparison within the same visit was not possible; the analysis is cross-sectional at the visit level and cannot assess longitudinal substitution effects.
Researchers using historical AxSpA cohort data collected before ASDAS was validated can confidently substitute legacy PGA scores for the ASDAS-PGA when calculating ASDAS — the agreement is excellent and disease activity categories are preserved in ~87% of cases.
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