Optimal cutoff points were identified to define active and structural lesions typical of axial spondyloarthritis (axSpA) in the sacroiliac joint (SIJ), according to study results published in Rheumatology. In addition, the calculated cutoff points were found to have a high accuracy in predicting long-term clinical diagnosis of axSpA.
The Assessment of Spondyloarthritis International Society (ASAS) classification cohort is an inception cohort of adult patients with undiagnosed back pain of at least 3 months with onset at less than age 45 years. Patients underwent MRI of the SIJ and were followed-up for an average of 4.4 years.
In the current study, ASAS experts reviewed baseline and follow-up scans from individuals in the cohort. A total of 2 central reader exercises were conducted. In the first exercise, 7 central readers assessed baseline MRI scans for the number of SIJ quadrants or slices with bone marrow edema (BME), erosion, fat lesion, and/or sclerosis. In the second exercise, 8 readers reviewed follow-up scans for the same characteristics. The sensitivity and specificity of each cutoff point for a majority reader consensus were calculated. Researchers also assessed the predictive utility of baseline MRI data for a clinical diagnosis of axSpA.
Baseline scans were available for 169 patients and follow-up scans were available for 91 patients. A total of 114 (67.4%) patients received a diagnosis of axSpA at baseline. Patients diagnosed with axSpA at baseline had significantly more SpA features on MRI compared with patients without axSpA at baseline (3.4 vs 1.6; P <.001). In addition, the baseline axSpA group had a higher percentage of patients with B27 positivity (52.6% vs 21.8%; P <.001), elevated C-reactive protein levels (37.7% vs 14.5%; P =.002), and definite radiographic sacroiliitis, according to the modified New York criteria (28.8% vs 1.9%; P <.001).
The cutoff points that best identified active lesions typical of axSpA were either 4 or more SIJ quadrants with BME at any location or at the same location in at least3 consecutive slices. Each of these cutoffs attained a specificity of 95% or more for a definite active lesion. For structural lesions, the optimal cutoffs for number of affected SIJ quadrants were 3 or more for erosion, 5 or more for fat lesion, and 7 more for sclerosis. Each of these cutoffs displayed 95% or more specificity for structural lesions typical of axSpA.
Baseline detection of active and structural lesions typical of axSpA was strongly correlated with later diagnosis of clinical axSpA. The positive predictive values (PPVs) for both active and structural lesions exceeded 95%.
A study limitation included the high attrition during follow-up that may have led to an overrepresentation of patients with clinical axSpA.
Overall, the MRI cutoff values determined for active and structural lesions may have high prognostic capacity for future development of clinical axSpA. Further study is necessary to confirm the accuracy of these cutoff values in clinical practice.
Even so, “[these data]… are highly relevant to the interpretation of MRI scans for both diagnosis and classification of axSpA,” the researchers concluded. “[I]t is our hope that this work will finally lay to rest the frequent misuse of the quantitative component of the ASAS definition of a positive MRI, comprising BME in 2 [or more] locations on  slice or in  location on 2 or more consecutive slices, as a diagnostic criterion for axSpA.”
Maksymowych WP, Lambert RG, Baraliakos X, et al. Data-driven definitions for active and structural MRI lesions in the sacroiliac joint in spondyloarthritis and their predictive utility. Rheumatology (Oxford). Published online February 1, 2021. doi:10.1093/rheumatology/keab099