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Objective

To validate a magnetic resonance imaging (MRI) reference criterion for a positive sacroiliac (SI) joint MRI finding based on the level of confidence in the classification of spondyloarthritis (SpA) by expert MRI readers.

Methods

Four readers assessed SI joint MRIs in 2 inception cohorts (cohorts A and B) of 157 consecutive patients with back pain ages ≤50 years and 20 age-matched healthy controls. Patients were classified according to clinical examination and pelvic radiography as having nonradiographic axial SpA (n = 51), ankylosing spondylitis (n = 34), or nonspecific back pain (n = 72). Readers indicated their level of confidence in their classification of SpA on a 0–10 scale, where 0 = definitely not SpA and 10 = definite SpA. The MRI reference criterion was prespecified by consensus as the majority of readers indicating a confidence score of 8–10; the absence of SpA required all readers to indicate non-SpA (a confidence score of 0–4). We calculated interreader reliability and agreement between MRI-based and clinical classification using kappa statistics. We estimated cutoff values for MRI lesions attaining a specificity of ≥0.90 for SpA.

Results

In cohorts A and B, 76.4% and 71.6% of subjects met the MRI criterion, respectively. The kappa values for interreader agreement were 0.76 for cohort A and 0.80 for cohort B and between MRI-based and clinical assessment were 0.93 for cohort A and 0.57 for cohort B. Using this MRI reference criterion, the cutoff for the number of affected SI joint quadrants needed to reach a predefined specificity of ≥0.90 was ≥2 for bone marrow edema (BME) in both cohorts and ≥1 for erosion in both cohorts, and the BME and/or erosion lesions increased sensitivity without reducing specificity.

Conclusion

This data-driven study using 2 inception cohorts and comparing clinical and MRI-based classification supports the case for including both erosion and BME to define a positive SI joint MRI finding for the classification of axial SpA.