Background/Purpose: The ASAS definition of a positive MRI for inflammation (ASAS-MRI+) is intended for classification of patients as having axSpA but is often misused for diagnostic purposes. This is problematic because bone marrow edema (BME) in the sacroiliac joints (SIJ) may occur in 20-40% of both healthy individuals and those with mechanical back disorders when only the quantitative aspect of the definition is applied (presence of BME in ≥2 locations on one slice or one location in ≥2 slices). The definition also requires that BME is considered “highly suggestive” of SpA but interpretation of this qualitative component may vary according to expertise.  Revised definitions of MRI lesions in the SIJ have been validated by 7 readers from the ASAS-MRI group on images from the ASAS Classification Cohort^1-3. We aimed to identify quantitative cut-offs based on numbers of slices and SIJ quadrants that define a positive MRI for active lesions typical of axSpA, the gold standard being majority central reader decision as to the presence of a definite active lesion typical of axSpA with high confidence.

Methods: MRI active lesions meeting ASAS definitions from 160 cases were recorded by 7 ASAS-MRI readers in an eCRF that comprises global assessment (Is there an active lesion typical of axSpA (yes/no) and degree of confidence (-4 (lesion absent) to +4 (lesion present)), and detailed scoring of lesions per SIJ quadrant and per slice. Detailed scoring was based only on assessment of DICOM images. We calculated sensitivity and specificity for numbers of SIJ quadrants and consecutive slices with BME where a majority of readers (≥4/7) agreed as to the presence of an active lesion typical of axSpA with high confidence (≥ +3). We selected cut-offs with ≥95% specificity. These cut-offs were analyzed for their predictive utility for rheumatologist diagnosis of axSpA at follow up (average of 4.4 years) by calculating positive and negative predictive values (PPV, NPV) and selecting those cut-offs with PPV of ≥95% and comparable predictive utility with global MRI assessment. Both specificity of ≥95% for a definite lesion as well as a PPV of ≥95% were considered requirements for preliminary designation of MRI lesion cut-offs defining a positive MRI for active lesions.

Results: Active lesions typical of axSpA were observed by majority read in 39 (35.8%) of 109 cases diagnosed with axSpA, and 1 (2.0%) of 49 cases without axSpA and 26 cases were assigned a high degree of confidence (≥ +3) by a majority of readers. Cut-offs achieving specificity of ≥95% for a definite active lesion were BME at the same location in ≥3 consecutive slices (sensitivity 100%) and BME at any location in ≥4 SIJ quadrants (sensitivity 100%) (Table 1). Both of these cut-offs had very high positive predictive values (≥95%) for diagnosis of axSpA in cases diagnosed by the rheumatologist after 4.4 years follow up (Table 2).

Conclusion: ASAS-defined BME at the same location in ≥3 consecutive slices or in ≥4 SIJ quadrants at any location, are high priority candidates for defining an MRI active lesion typical of axSpA. This will require similar assessment in additional axSpA cohorts.

1. Maksymowych et al. Ann Rheum Dis 2019; 78:1550-8.
2. Rudwaleit et al. Ann Rheum Dis 2009;68: 777-83
3. Maksymowych et al. Ann Rheum Dis May 05 2020

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/preliminary-definition-of-a-positive-mri-for-active-lesions-in-the-sacroiliac-joints-typical-of-axial-spondyloarthritis/