Objectives
In patients with axial spondyloarthritis (axSpA) initiating their first tumor necrosis factor alpha-inhibitor (TNFi), we aimed to identify common baseline predictors of Ankylosing Spondylitis Disease Activity Score (ASDAS-CRP) inactive disease (primary objective) and clinically important improvement (CII) at 6 months, and drug retention at 12-months across 15 European registries.

Methods
Baseline demographic and clinical characteristics were collected. Outcomes were investigated per registry and in pooled data using logistic regression analyses on multiply imputed data.

Results
The consistency of baseline predictors in individual registries justified pooling the data. In the pooled dataset (n = 21,196), the 6-month rates for ASDAS inactive disease and ASDAS CII were 26% and 51%, and the 12-month drug retention rate 65% in patients with available data (n = 9,845, n = 6,948 and n = 21,196, respectively). Nine common baseline predictors of ASDAS inactive disease, ASDAS CII and 12-month drug retention were identified, and the odds ratios (95%-confidence interval) for ASDAS inactive disease were: age, per year: 0.97 (0.97–0.98), men vs. women: 1.88 (1.60–2.22), current vs. non-smoking: 0.76 (0.63–0.91), HLA-B27 positive vs. negative: 1.51 (1.20–1.91), TNF start year 2015–2018 vs. 2009–2014: 1.24 (1.06–1.45), CRP>10 vs. ≤10 mg/l: 1.49 (1.25–1.77), one unit increase in health assessment questionnaire (HAQ): 0.77 (0.58–1.03), one-millimeter (mm) increase in Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) fatigue and spinal pain: 0.99 (0.99–1.00) and 0.99 (0.99–1.99), respectively

Conclusion
Common baseline predictors of treatment response and adherence to TNFi could be identified across data from 15 European registries, indicating that they may be universal across different axSpA populations.