Fig. 1

Classification of nr-axSpA and AS using the ASAS criteria for axSpA in patients with back pain lasting ≥3 months and age at onset < 45 years [14]. AS = ankylosing spondylitis; ASAS = Assessment of SpondyloArthritis international Society; CRP = C-reactive protein; HLA = human leukocyte antigen; MRI = magnetic resonance imaging; nr-axSpA = nonradiographic axial spondyloarthritis; NSAID = nonsteroidal anti-inflammatory drug; SpA = spondyloarthritis. * The modified New York criteria for sacroiliitis are: grade ≥ 2 bilaterally or grade ≥ 3–4 unilaterally [13]. .^† Active acute inflammation is defined by clearly present bone marrow edema (on short-T1 inversion recovery) or osteitis (on T1 post gadolinium) highly suggestive of SpA, located in subchondral or periarticular bone marrow [15]. ^‡ ASAS criteria for inflammatory back pain criteria include 4 of the 5 following: improvement with exercise, pain at night, insidious onset, age at onset < 40 years, and no improvement with rest [16, 17]