Imaging of the sacroiliac joints is important for diagnosing early axial spondyloarthritis but not all-decisive.

Objectives: To evaluate the contribution of the results of sacroiliac imaging to diagnosis and to the level of confidence in diagnosis in patients presenting with chronic back pain (CBP) and suspected of having axial spondyloarthritis (axSpA).

Methods: Data from 513 patients from the SPondyloArthritisCaughtEarly cohort with CBP (⩾3 months, ⩽2 years, onset <45 years) were analysed after full diagnostic work-up. Rheumatologists were asked not only to provide a diagnosis before and after the imaging results had been provided to them, but also to provide the level of confidence of this diagnosis on an 11-point numerical scale.

Results: Before imaging, 317/513 patients were diagnosed with axSpA. Of these patients, 178/317 (56%) did not have signs of sacroiliitis on either MRI or radiography, which led to the rheumatologist refuting the initial diagnosis of axSpA in 81/178 (46%) patients. Of the 196/513 patients without axSpA before imaging, 35/196 (18%) had signs of sacroiliitis on imaging. Subsequently, 28/35 (80%) patients were diagnosed with axSpA. Overall, imaging was incongruent with the diagnosis before imaging in 213 patients. This led to a change in diagnosis in 109/213 (51%), which corresponds to 21% (109/513) of all patients in the cohort. In general, diagnostic confidence increased by having imaging results available (from 6.2 to 7.4, P < 0.001).

Conclusion: In patients with CBP suspected of having axSpA, sacroiliac imaging adds to the confidence in the final diagnosis. However, the number of changes in diagnosis suggests that imaging is important but not all-decisive in early axSpA diagnosis.