The ASAS criteria developed in 2009 utilize the presence of bone marrow edema to define a positive MRI of the sacroiliac joints in axSpA [79]. Using solely the imaging arm of the ASAS algorithm (which includes positive radiographs or positive MRI with one clinical axSpA feature), the ASAS criteria were initially shown to have a sensitivity of 66.2% and specificity of 97.3%, although by combining both imaging and clinical arms of the algorithm the sensitivity was 82.9% and specificity was 84.4% for axSpA [19]. Subsequently, in an inception cohort followed for 8 years, the ASAS definition of a positive MRI demonstrated a sensitivity of 79% and a specificity of 89% for the diagnosis of axSpA [80]. Although the presence of bone marrow edema provides a sensitive evaluation for inflammatory changes in axSpA, bone marrow edema can also be seen in noninflammatory conditions such as in age-related degenerative changes, postpartum patients, patients with chronic back pain, athletes, and in up to 30% of healthy controls [81-85]. As a result, the ASAS MRI criteria have been criticized for decreased specificity of bone marrow edema as the sole MRI diagnostic criterion [82]. Importantly, although bone marrow edema may be nonspecific in some cases, one study found that the presence of a deep bone marrow edema lesion which extends at least 1 cm deep to the articular surface is more specific and found almost exclusively in axSpA [86]. Structural changes of axSpA including erosions, sclerosis, fatty deposition, and ankylosis are not accounted for in the ASAS definition of a positive MRI, although they are important findings in axSpA and should be identified in clinical practice [38].
Inflammatory Back Pain Known or Suspected Axial Spondyloarthropathy PCAs. The ASAS criteria developed in 2009 utilize the presence of bone marrow edema to define a positive MRI of the sacroiliac joints in axSpA [79]. Using solely the imaging arm of the ASAS algorithm (which includes positive radiographs or positive MRI with one clinical axSpA feature), the ASAS criteria were initially shown to have a sensitivity of 66.2% and specificity of 97.3%, although by combining both imaging and clinical arms of the algorithm the sensitivity was 82.9% and specificity was 84.4% for axSpA [19]. Subsequently, in an inception cohort followed for 8 years, the ASAS definition of a positive MRI demonstrated a sensitivity of 79% and a specificity of 89% for the diagnosis of axSpA [80]. Although the presence of bone marrow edema provides a sensitive evaluation for inflammatory changes in axSpA, bone marrow edema can also be seen in noninflammatory conditions such as in age-related degenerative changes, postpartum patients, patients with chronic back pain, athletes, and in up to 30% of healthy controls [81-85]. As a result, the ASAS MRI criteria have been criticized for decreased specificity of bone marrow edema as the sole MRI diagnostic criterion [82]. Importantly, although bone marrow edema may be nonspecific in some cases, one study found that the presence of a deep bone marrow edema lesion which extends at least 1 cm deep to the articular surface is more specific and found almost exclusively in axSpA [86]. Structural changes of axSpA including erosions, sclerosis, fatty deposition, and ankylosis are not accounted for in the ASAS definition of a positive MRI, although they are important findings in axSpA and should be identified in clinical practice [38].