| Axial SpA (n = 391) | No SpA (n = 258) | |
| Age, years (mean, SD) | 31.6 (11.0) | 36.5 (10.9) |
| Duration of back pain, years (mean, SD) | 6.1 (7.6) | 9.1 (10.7) |
| Age at onset, years (mean, SD) | 25.5 (8.4) | 27.4 (10.0) |
| Age at onset <40 years (%) | 93.1 | 87.2 |
| Male gender (%) | 52.4 | 32.6 |
| IBP (according to local rheumatologist) (%)* | 89.0 | 31.0 |
| IBP Calin (%)† | 85.9 | 59.7 |
| IBP experts (%)‡ | 73.4 | 44.6 |
| IBP Berlin (%)§ | 63.2 | 36.0 |
| Good response to NSAID (%) | 59.1 | 26.7 |
| Enthesitis (%) | 40.2 | 30.2 |
| Enthesitis of the heel (%) | 16.9 | 14.0 |
| Peripheral oligoarthritis (%) | 36.3 | 17.8 |
| Uveitis (%) | 11.5 | 7.0 |
| Dactylitis (%) | 6.6 | 1.2 |
| Psoriasis (%) | 8.4 | 5.0 |
| IBD (%) | 3.8 | 1.2 |
| Family history of SpA (AS, reactive arthritis, uveitis, psoriasis, IBD)¶ (%) | 23.3 | 18.6 |
| Schober’s test, cm, mean (SD) | 4.1 (2.3) | 3.9 (2.5) |
| Schober’s test, <4 cm (%) | 33.8 | 33.1 |
| Lateral spinal flexion, cm, mean (SD) | 16.3 (5.6) | 16.0 (5.8) |
| Lateral spinal flexion, <10 cm (%) | 10.9 | 10.9 |
| Chest expansion, cm, mean (SD) | 4.9 (2.0) | 5.0 (1.7) |
| Chest expansion, ⩽2.5 cm (%) | 11.0 | 6.2 |
| HLA-B27 (%) | 65.9 | 27.7 |
| Elevated CRP, above upper normal limit (%) | 38.1 | 14.7 |
| Definite radiographic sacroiliitis (⩾ grade 2 bilateral or ⩾ grade 3 unilateral) (%) | 29.7 | 1.9 |
| Unilateral grade 2 radiographic sacroiliitis (%) | 10.7 | 1.2 |
| Active inflammation of sacroiliac joints, MRI (n = 495) (%) | 64.7 | 2.6 |
| Active inflammation of sacroiliac joints (MRI), in patients with radiographic sacroiliitis ⩽ grade 1 (n = 383) (%) | 61.6 | 2.2 |
| Active inflammation of the spine, MRI (n = 276) (%) | 32.6 | 1.0 |
*Global judgement on the presence or absence of inflammatory back pain (IBP), independent of formal criteria. †IBP Calin et al12 (four or more out of five): (1) age at onset <40 years; (2) back pain >3 months; (3) insidious onset; (4) morning stiffness; (5) improvement with exercise. ‡IBP experts13 (four or more out of five): (1) age at onset <40 years; (2) insidious onset; (3) improvement with exercise; (4) no improvement with rest; (5) pain at night (with improvement upon getting up). §IBP Rudwaleit et al13 (two or more out of four): (1) morning stiffness >30 minutes; (2) improvement with exercise, not with rest; (3) alternating buttock pain; (4) pain at second half of night/early morning (only night pain with improvement upon getting up). ¶Family history of ankylosing spondylitis (AS), psoriasis, reactive arthritis, uveitis, or inflammatory bowel disease (IBD) in a first-degree relative (father, mother, sisters, brothers, children) or second-degree relative (maternal and paternal grandparents, aunts, uncles, nieces and nephews). CRP, C-reactive protein; MRI, magnetic resonance imaging; NSAID, non-steroidal anti-inflammatory drug; SpA, spondyloarthritis.