Which Magnetic Resonance Imaging Lesions in the Sacroiliac Joints Are Most Relevant for Diagnosing Axial Spondyloarthritis? A Prospective Study Comparing Rheumatologists’ Evaluations With Radiologists’ Findings

Objective Pathologic sacroiliac (SI) joint changes on magnetic resonance imaging (MRI) are important for the classification of axial spondyloarthritis (SpA). In daily practice, radiologists play a major role in interpreting imaging findings. This study was undertaken to evaluate the impact of MRI SI joint findings on the identification of axial SpA by radiologists, in comparison to diagnosis by rheumatologists. Methods Patients age ≤45 years were prospectively included when referred for clinical suspicion of axial SpA and underwent a complete diagnostic evaluation including STIR‐ and T1‐weighted MRI of the SI joint. Diagnosis made by an experienced rheumatologist with access to all relevant information was considered the gold standard. MRIs were evaluated by 2 experienced radiologists who were unaware of the clinical data, who indicated which MRI lesions were “critical” to the decision for or against axial SpA. Results Of the 300 patients included, 132 (44%) were diagnosed as having axial SpA. Mean age was comparable between the 2 groups, but patients with axial SpA and those with non–axial SpA differed with regard to symptom duration (58.6 ± 69.5 versus 33.9 ± 45.1 months, respectively; P = 0.003) and HLA–B27 positivity (75.6% versus 19%, respectively; P < 0.001). Rheumatologists and radiologists agreed on the diagnosis in 262 cases (87.3%), while 34 patients (11.3%) were diagnosed as having axial SpA by rheumatologists only (clinically), and 4 cases (1.3%) were judged as suggestive of axial SpA by radiologists only. Bone marrow edema (BME) and sclerosis showed the highest sensitivity, while erosions and fatty lesions showed the highest specificity, for axial SpA diagnosis. The combination of BME with erosions had the highest positive predictive value (86.5%). Conclusion The MRI findings with the highest diagnostic value in patients in whom axial SpA is suspected are structural changes in the SI joint, alone or in combination with BME. Our findings indicate that while the absence of BME is usually not compatible with a diagnosis of axial SpA, the presence of BME does not necessarily confirm a diagnosis of axial SpA.