| Parameter | All patients (n = 71) | SpA (n = 36) | No SpA (n = 19) | Unclassifiable patients (n = 16) |
| Age (mean, SD) | 38.6 (10.7) | 36.1 (9.0) | 44.6 (12.1) | 37.2 (10.2) |
| Male gender (%) | 29.6 | 47.2 | 15.8 | 6.3 |
| Duration of back pain, years (mean, SD) | 6.5 (6.2) | 5.5 (5.1) | 9.5 (8.7) | 4.9 (3.6) |
| Age at onset ⩽45 years (%) | 90.1 | 97.2 | 78.9 | 87.5 |
| Age at onset <40 years (%) | 80.0 | 88.6 | 68.4 | 75.0 |
| IBP Calin (%)* | 74.2 | 84.4 | 61.1 | 68.8 |
| IBP experts (%)† | 38.6 | 57.1 | 21.4 | 37.5 |
| IBP Rudwaleit (%)‡ | 68.6 | 85.3 | 42.1 | 68.8 |
| Good response to NSAID (%) | 66.2 | 69.4 | 57.9 | 68.8 |
| Enthesitis (%) | 28.2 | 27.8 | 15.8 | 43.8 |
| Peripheral oligoarthritis (%) | 25.4 | 36.1 | 5.3 | 25 |
| Uveitis (%) | 2.8 | 2.8 | 5.3 | 0 |
| Dactylitis (%) | 4.2 | 5.6 | 0 | 6.3 |
| Psoriasis (%) | 14.1 | 16.7 | 5.3 | 18.8 |
| IBD (%) | 2.8 | 5.6 | 0 | 0 |
| Family history of SpA (AS, reactive arthritis, uveitis, psoriasis, IBD) (%) | 25.4 | 33.3 | 15.8 | 18.8 |
| HLA-B27 (%) | 54.9 | 72.2 | 21.2 | 56.3 |
| Elevated CRP (%) | 28.2 | 27.8 | 42.1 | 12.5 |
| Active inflammation MRI (sacroiliac joints) (%) | 38.0 | 72.2 | 5.3 | 0 |
*Inflammatory back pain (IBP) Calin (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 experts (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).16 ‡IBP Rudwaleit (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). AS, ankylosing spondylitis; CRP, C-reactive protein; IBD, inflammatory bowel disease; MRI, magnetic resonance imaging; NSAID, non-steroidal anti-inflammatory drug; SpA, spondyloarthritis.