Validation of the ASDAS with a quick quantitative CRP assay (ASDAS-Q) in patients with axial SpA: a prospective multicentre cross-sectional study

Objectives: The objective of the study was to validate the Ankylosing Spondylitis Disease Activity Score (ASDAS) based on a quick quantitative C-reactive protein (qCRP) assay (ASDAS-Q) in a multicentre, prospective, cross-sectional study in patients with axial spondyloarthritis (axial SpA). Methods: Disease activity assessment was performed in prospectively recruited patients with axial SpA. Routine laboratory CRP was determined in the central laboratory of each study centre, while quick qCRP and erythrocyte sedimentation rate (ESR) were measured locally. Consequently, ASDAS-CRP, ASDAS-Q using the qCRP and ASDAS-ESR were calculated. The absolute agreement on the disease activity category ascertainment was analysed with cross-tabulations and weighted Cohen's kappa. Bland-Altman plots and intraclass correlation coefficients (ICCs) were used to analyse the criterion validity. Results: Overall, 251 axial SpA patients were included in the analysis. The mean qCRP value (6.34 +/- 11.13 mg/l) was higher than that of routine laboratory CRP (5.26 +/- 9.35 mg/l). The ICC for routine laboratory CRP versus qCRP was 0.985 [95% confidence interval (CI): 0.972-0.991]. Comparing ASDAS-Q with ASDAS-CRP, 242 of 251 (96.4%) patients were assigned to the same disease activity categories with a weighted Cohen's kappa of 0.966 (95% CI: 0.943-0.988) and ICC of 0.997 (95% CI: 0.994-0.999). Conclusions: ASDAS-Q showed an almost perfect agreement with ASDAS-CRP in the assignment to specific disease activity categories. Consequently, ASDAS-Q using the qCRP value can be applied as an accurate and quickly available alternative to ASDAS-CRP, thus facilitating the implementation of the treat-to-target concept in clinical trials and clinical routine.