ASAS criteria for classification of axial spondyloarthritis (SpA) in patients with back pain ≥3 months and age at onset back pain <45 years (with/without peripheral manifestations)109
Sacroiliitis on imaging* plus ≥1 SpA feature**
or
HLA-B27 plus ≥2 other SpA features**
**SpA features
–Inflammatory back pain
–Arthritis
–Enthesitis (heel)
–Uveitis
–Dactylitis
–Psoriasis
–Crohn's/ulcerative colitis
–Good response to NSAIDs
–Family history for SpA
–HLA-B27
–Elevated CRP
*Sacroiliitis on imaging
–Active (acute) inflammation on MRI highly suggestive of sacroiliitis associated with SpA
–Definite radiographic sacroiliitis according to modified New York criteria
ASAS, The Assessment of SpondyloArthritis international Society; CRP, C-reactive protein; NSAIDs, non-steroidal anti-inflammatory drugs.