title: Development and validation of a magnetic resonance imaging reference criterion for defining a positive sacroiliac joint magnetic resonance imaging finding in spondyloarthritis.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Balgrist University Hospital, Zurich, Switzerland. ulrich.weber02@bluewin.ch
authors.forename: Ulrich
authors.initials: U
authors.lastname: Weber
authors.forename: Veronika
authors.initials: V
authors.lastname: Zubler
authors.forename: Susanne J
authors.initials: SJ
authors.lastname: Pedersen
authors.forename: Kaspar
authors.initials: K
authors.lastname: Rufibach
authors.forename: Robert G W
authors.initials: RG
authors.lastname: Lambert
authors.forename: Stanley M
authors.initials: SM
authors.lastname: Chan
authors.forename: Mikkel
authors.initials: M
authors.lastname: Østergaard
authors.forename: Walter P
authors.initials: WP
authors.lastname: Maksymowych
mesh_terms: D000328:Adult; D001855:Bone Marrow Diseases / Q000473:pathology; D016022:Case-Control Studies; D015331:Cohort Studies; D004487:Edema / Q000473:pathology; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000592:standards*; D008297:Male; D008875:Middle Aged; D012015:Reference Standards; D015203:Reproducibility of Results; D012446:Sacroiliac Joint / Q000473:pathology*; D012680:Sensitivity and Specificity; D025241:Spondylarthritis / Q000145:classification* / Q000473:pathology*