Abstract

Abstract Objective Ankylosing spondylitis (AS) is diagnosed late, because radiographs of the sacroiliac joints often do not show definite sacroiliitis at the time of disease onset. The aim of this study was to investigate whether patients without definite radiographically defined sacroiliitis, referred to as nonradiographic axial spondylarthritis (SpA), are different from patients with AS with regard to clinical manifestations and disease activity measures. Moreover, we sought to identify determinants of the development of radiographic sacroiliitis. Methods In a cross‐sectional analysis of 462 patients, we compared 226 patients with nonradiographic axial SpA (symptom duration ≤5 years) and 236 patients with AS (symptom duration ≤10 years) who are participants in the German Spondyloarthritis Inception Cohort. Radiographs of the sacroiliac joints and the spine were assessed by 2 readers in a blinded manner. Logistic regression analysis was applied to identify parameters associated with structural damage. Results The 2 groups did not differ in the frequency of HLA–B27 positivity, inflammatory back pain, arthritis, enthesitis, and uveitis and had similar levels of disease activity, using measures such as the Bath Ankylosing Spondylitis Disease Activity Index. In both groups, HLA–B27 positivity determined the age at disease onset. Male sex (adjusted odds ratio [OR] 2.38, 95% confidence interval [95% CI] 1.19–4.73 [ P = 0.014]) and an elevated C‐reactive protein (CRP) level (adjusted OR 1.85, 95% CI 0.96–3.56 [ P = 0.066]) were associated with radiographic sacroiliitis. In patients with AS, male sex and an elevated CRP level were also associated with the presence of syndesmophytes. Conclusion Clinical manifestations and disease activity measures are highly comparable between patients with early nonradiographic axial SpA and those with early AS, suggesting that these 2 entities are part of the same disease. Male sex and an elevated CRP level are associated with structural damage on radiographs, whereas HLA–B27 positivity determines the age at disease onset.