When MRI would be useful in patients without evidence of sacroiliitis on radiographs?

We aimed to identify when magnetic resonance imaging (MRI) would be useful to diagnose patients with suspected axial spondyloarthropathy (AxSpA) without evidence of sacroiliitis on radiographs. We retrospectively reviewed electronic medical records of patients who underwent pelvis MRI after radiographs at the rheumatology clinic in a single tertiary center in Korea. Patients underwent imaging from January 2020 to July 2022. We collected data including complete blood count, erythrocyte sedimentation rate, C-reactive protein (CRP), human leukocyte antigen (HLA)-B27, history of acute anterior uveitis (AAU), peripheral arthritis, dactylitis, inflammatory bowel disease (IBD), enthesopathy, and psoriasis. A total of 105 patients who showed no evidence of sacroiliitis on radiographs were included. The median age of patients was 41.0 years, and 44.8% were male. Of them, 34 showed sacroiliitis on MRI (group 1), and 71 showed no evidence of sacroiliitis even on MRI (group 2). Known AxSpA-related clinical features including AAU, peripheral arthritis, dactylitis, IBD, enthesopathy, and psoriasis were not different between the two groups. HLA-B27 positivity (79.4% vs. 40.0%, p < 0.001), median white blood cell count (7700 vs. 6300, p = 0.007), mean platelet count (307.7 ± 69.7 vs. 265.3 ± 68.9 × 10³/µL, p = 0.005), and median CRP level (0.38 vs. 0.10, p = 0.001) showed significant differences between the two groups. In a multivariate analysis, HLA-B27 positivity and platelet count were significantly associated with sacroiliitis on MRI. In our cohort, sacroiliitis was observed on MRI in one-third of patients without radiographic evidence. MRI could be recommended to evaluate sacroiliitis in patients with positive HLA-B27 and a high platelet count.