OBJECTIVES: To determine quantitative sacroiliac joint (SIJ) MRI lesion cut-offs that optimally define a positive MRI for inflammatory and structural lesions typical of axial spondyloarthritis (axSpA) and that predict clinical diagnosis. METHODS: The ASAS MRI group assessed MRIs from the ASAS Classification Cohort in two reading exercises: A. 169 cases and 7 central readers; B. 107 cases and 8 central readers. We calculated sensitivity/specificity for the number of SIJ quadrants or slices with bone marrow edema (BME), erosion, fat lesion, where a majority of central readers had high confidence there was a definite active or structural lesion. Cut-offs with ≥95% specificity were analyzed for their predictive utility for follow-up rheumatologist diagnosis of axSpA by calculating positive/negative predictive values (PPV/NPV) and selecting cut-offs with PPV≥95%. RESULTS: Active or structural lesions typical of axSpA on MRI had PPV≥95% for clinical diagnosis of axSpA. Cut-offs that best reflect definite active lesion typical of axSpA were either ≥4 SIJ quadrants with BME at any location or at the same location in ≥ 3 consecutive slices. For definite structural lesion, the optimal cut-offs were any one of ≥ 3 SIJ quadrants with erosion or ≥ 5 with fat lesion, erosion at the same location for ≥2 consecutive slices, fat lesion at the same location for ≥3 consecutive slices, or presence of a 'deep' (>1cm) fat lesion. CONCLUSION: We propose cut-offs for definite active and structural lesions typical of axSpA that have high PPV for a long-term clinical diagnosis of axSpA for application in disease classification and clinical research
