Baseline characteristics of the study population, according to smoking status
| Smoker | Non-smoker | p Value | ||
|---|---|---|---|---|
| Male sex (N=647) | 123 (51.0%) | 174 (42.9%) | 0.04 | |
| Mean age at onset of IBP (years) (N=628) | 31.1±8.3 | 32.6±9.0 | 0.04 | |
| Mean duration of axial symptoms (years) (N=628) | 1.6±1.0 | 1.5±0.9 | 0.44 | |
| Mean age at onset of peripheral arthritis (years) (N=359) | 31.3±8.8 | 33.1±9.7 | 0.08 | |
| Mean age at onset of enthesitis (years) (N=324) | 31.6±8.5 | 33.4±9.0 | 0.08 | |
| Caucasian race (N=646) | 220 (91.7%) | 360 (88.7%) | 0.22 | |
| Drinker (N=644) | 55 (23.1%) | 40 (9.9%) | <0.001 | |
| Family history of ankylosing spondyloarthritis (N=640) | 63 (26.4%) | 106 (26.4%) | 0.98 | |
| HLA-B27 positive (N=641) | 157 (65.7%) | 244 (60.7%) | 0.21 | |
| History of peripheral arthritis (N=645) | 137 (57.3%) | 241 (59.4%) | 0.61 | |
| Signs of peripheral arthritis (N=386) | 46 (32.9%) | 99 (40.2%) | 0.15 | |
| History of enthesitis (N=647) | 125 (51.9%) | 218 (53.7%) | 0.65 | |
| NSAID user (N=647) | 271 (66.7%) | 173 (71.8%) | 0.18 | |
| Steroid user (N=647) | 53 (13.1%) | 29 (12.0%) | 0.71 | |
| DMARD user (N=648) | 40 (9.9%) | 19 (7.9%) | 0.40 | |
| Analgesics user (N=647) | 258 (63.5%) | 152 (63.1%) | 0.90 | |
| Mean CRP (mg/l) (N=626) | 8.0±13.7 | 7.7±14.0 | 0.78 | |
| Patients with elevated CRP (N=626) | 106 (27.0%) | 80 (34.3%) | 0.051 | |
| Mean ESR (mm/h) (621) | 12.9±14.7 | 14.8±16.6 | 0.14 | |
| Patients with elevated ESR (N=618) | 79 (20.5%) | 45 (19.4%) | 0.75 | |
| Patients with elevated CRP or ESR | 131 (34.4%) | 89 (38.9%) | 0.26 | |
| Patients with extra-articular features (N=647) | 116 (28.6%) | 63 (26.1%) | 0.50 | |
| BASDAI (N=641) | 4.6±1.9 | 4.3±2.1 | 0.06 | |
| BASDAI ≥4 (N=641) | 150 (63.3%) | 238 (58.9%) | 0.27 | |
| ASDAS–CRP (N=618) | 2.6±1.0 | 2.4±1.1 | 0.051 | |
| BASFI (N=634) | 3.4±2.2 | 2.8±2.3 | 0.001 | |
| BASFI ≥4 (N=634) | 96 (40.5%) | 121 (30.5%) | 0.01 | |
| Intensity of axial pain in last 2 days (NRS) (N=644) | 5.2±2.8 | 4.7±2.7 | 0.03 | |
| Intensity of peripheral joints pain in last 2 days (NRS) (N=643) | 3.3±2.8 | 3.2±2.8 | 0.76 | |
| Tender joint count (out of 53 joints) (N=647) | 5.0±9.5 | 4.1±7.9 | 0.22 | |
| Swollen joint count (out of 28 joints) (N=645) | 0.2±1.0 | 0.2±0.8 | 0.56 | |
| BASMI (N=616) | 1.6±1.2 | 1.5±1.1 | 0.12 | |
| Chest expansion (cm) (N=645) | 5.7±2.0 | 5.7±2.2 | 0.99 | |
| Euro-quality of life questionnaire (N=645) | 10.4±4.8 | 8.7±5.0 | <0.001 | |
| HAQ–AS – disability index (N=644) | 1.0±0.7 | 0.9±0.7 | 0.06 | |
| SF-36 mental health component score (N=642) | 46.2±20.2 | 52.7±20.5 | <0.001 | |
| SF-36 physical health component score (N=642) | 35.6±15.8 | 40.5±16.9 | <0.001 | |
| Patients with MRI inflammatory lesions in sacroiliac joints (N=610) | 104 (46.8%) | 123 (31.7%) | <0.001 | |
| Patients with MRI inflammatory lesions in spine (N=605) | 72 (32.9%) | 64 (16.6%) | <0.001 | |
| Patients with MRI structural lesions in sacroiliac joints (N=610) | 78 (35.1%) | 92 (23.7%) | 0.002 | |
| Patients with MRI structural lesions in spine (N=601) | 28 (12.8%) | 26 (6.8%) | 0.01 | |
| Patients with MRI inflammation, spine or sacroiliac joints (N=608) | 125 (56.6%) | 149 (38.5%) | <0.001 | |
| Patients with MRI structural lesions, spine or sacroiliac joints (N=604) | 88 (40.2%) | 108 (28.1%) | 0.002 | |
| mSASSS (N=623) | 1.4±3.3 | 0.9±2.6 | 0.09 | |
| Patients with mSASSS >0 (N=623) | 66 (28.6%) | 92 (23.5%) | 0.16 | |
| Patients with radiographic sacroiliitis (N=625) | 78 (33.8%) | 103 (26.1%) | 0.04 | |
| Missing workdays (N=555) | 45.6±79.6 | 27.3±61.4 | 0.003 | |
ASDAS–CRP, ankylosing spondylitis disease activity score, C-reactive protein based; BASDAI, Bath ankylosing spondylitis disease activity index; BASFI, Bath ankylosing spondylitis functional index; BASMI, Bath ankylosing spondylitis metrology index; CRP, C-reactive protein; DMARD, disease-modifying antirheumatic drug; ESR, erythrocyte sedimentation rate; HAQ–AS, health assessment questionnaire for ankylosing spondylitis; HLA, human leucocyte antigen; IBP, inflammatory back pain; mSASSS, modified Stoke ankylosing spondylitis spine score; NRS, numerical rating scale; NSAID, non-steroidal anti-inflammatory drug; SF-36, short form 36.