Figure 1

Two sets of candidate criteria for the classification of axial spondyloarthritis (SpA) that differ only in their clinical arms (in set 1 any three or more of typical SpA features are required for the fulfilment, whereas in set 2 inflammatory back pain (IBP) is obligatory in addition to at least two other typical SpA features) (in patients with back pain for ⩾3 months and age at onset <45 years). CRP, C-reactive protein; ES, extraspinal manifestation; ESR, erythrocyte sedimentation rate; MRI, magnetic resonance imaging; NSAID, non-steroidal anti-inflammatory drug.