Baseline characteristics at first radiograph
| Parameter | N | |
| Male sex, % | 432 | 65.7 |
| HLA-B27 positive, % | 391 | 80.6 |
| Age, years | 432 | 40.3 (11.0) |
| Symptom duration, years | 424 | 13.8 (9.7) |
| BASDAI | 369 | 4.2 (2.3) |
| ASDAS | 351 | 2.8 (1.1) |
| CRP (mg/L), median (IQR) | 365 | 8.0 (3.0; 11.0) |
| Elevated CRP, % | 364 | 40.4 |
| BASFI | 373 | 3.1 (2.6) |
| BASMI | 375 | 2.2 (2.0) |
| mSASSS, median (IQR) | 432 | 1.0 (0.0; 6.0) |
| mean (SD) | 6.6 (12.5) | |
| Syndesmophytes present, % | 432 | 34.3 |
| EQ-5D | 370 | 65.1 (21.6) |
| Current peripheral arthritis, % | 378 | 28.6 |
| Current enthesitis, % | 381 | 54.3 |
| BMI 25–30, % | 373 | 29.5 |
| BMI >30, % | 373 | 15.6 |
| On NSAID treatment, % | 341 | 83.9 |
| On TNFi treatment, % | 432 | 37.7 |
| Ever TNFi treatment, % | 432 | 43.1 |
| Years of TNFi treatment in treated patients | 163 | 2.1 (1.7) |
| Current smokers, % | 365 | 38.4 |
| Number exercise sessions per week, median (IQR) | 366 | 2.0 (0.0; 2.0) |
| Patients with different number of radiographic intervals*, % | 432 | 100 |
| One interval | 304 | 70.4 |
| Two intervals | 83 | 19.2 |
| Three intervals | 35 | 8.1 |
| Four intervals | 9 | 2.1 |
| Five intervals | 1 | 0.2 |
Except where indicated otherwise, values are the mean (SD).
*Differences in baseline characteristics in patients with different number of radiographic intervals are provided in the online supplementary table S8.
ASDAS, Ankylosing Spondylitis Disease Activity Score; BASDAI, Bath Ankylosing Spondylitis Disease Activity Index; BASFI, Bath Ankylosing Spondylitis Functional Index; BASMI, Bath Ankylosing Spondylitis Metrology Index; BMI, body mass index; CRP, C reactive protein; EQ-5D, EuroQol 5-domain; HLA-B27, human leucocyte antigen B27; mSASSS, modified Stoke Ankylosing Spondylitis Spine Score; NSAID, non-steroidal anti-inflammatory drug; TNFi, tumour necrosis factor inhibitor.