Patient characteristics
| Characteristic | N=702 | |
| Baseline | 2-year follow-up | |
| Age, years | 30 (8) | n.a. |
| Male | 271 (39%) | n.a. |
| Body mass index (kg/m2) | 25 (5) | n.a. |
| Duration of back pain (months) | 13 (7) | n.a. |
| Imaging features of SpA | ||
| Sacroiliitis on MRI-SIJ (ASAS) | 140 (20%) | 179 (25%) |
| Definitive damage on X-SIJ (mNY) | 24 (3%) | 32 (5%) |
| BME on MRI-Spine (≥5 lesions) | 15 (2%) | 15 (2%) |
| ≥1 syndesmophyte on X-Spine | 64 (9%) | 83 (12%) |
| Clinical features of SpA | ||
| Peripheral arthritis ever | 104 (15%) | 123 (18%) |
| Dactylitis ever | 43 (6%) | 70 (10%) |
| Heel pain ever | 147 (21%) | 175 (25%) |
| Psoriasis ever | 85 (12%) | 100 (14%) |
| Uveitis ever | 57 (8%) | 76 (11%) |
| Inflammatory bowel disease ever | 47 (7%) | 53 (8%) |
| Inflammatory back pain ever | 490 (70%) | 547 (78%) |
| Elevated CRP (>5 mg/L) | 191 (27%) | 261 (37%) |
| HLA-B27 positive | 314 (45%) | 314 (45%) |
| Family history of SpA (ASAS) | 312 (44%) | 333 (47%) |
| Good response to NSAID ever | 250 (36%) | 414 (59%) |
Mean (SD) or n (%).
Data for BMI available for 673 (96%) participants, for duration of back pain for 696 (99%) participants.
Missing values for imaging and clinical features of SpA have been imputed as described in the ‘Methods’ section.
ASAS, Assessment of SpondyloArthritis international Society; BME, bone marrow oedema; CRP, C reactive protein; HLA-B27, human leucocyte antigen B27; mNY, modified New York criteria; n.a., not applicable; NSAID, non-steroidal anti-inflammatory drug; SIJ, sacroiliac joints; SpA, spondyloarthritis; X-SIJ, radiograph of the sacroiliac joints; X-Spine, radiograph of the spine.