Baseline* characteristics of the total study population at risk for sick leave and for the patients with and without sick leave separately
| Variable | Total group | Ever sick leave§ | Never sick leave§ | P values** |
| (n=139) | (n=88) | (n=51) | ||
| Age, years | 38.7 (10.0) | 37.9 (9.6) | 40.3 (10.6) | 0.18 |
| Gender, male (%) | 105 (75.5) | 68 (77.3) | 37 (72.5) | 0.53 |
| Country of origin | 0.23 | |||
| The Netherlands (%) | 74 (53.2) | 51 (58.0) | 23 (45.1) | |
| Belgium (%) | 45 (32.4) | 24 (27.3) | 21 (41.2) | |
| France (%) | 20 (14.4) | 13 (14.8) | 7 (13.7) | |
| Education, high (%) | 49 (36.0) | 23 (27.1) | 26 (51.0) | <0.01 |
| Job type, white collar (%) | 71 (55.9) | 38 (48.1) | 33 (68.8) | 0.02 |
| Part-time worker (%) | 22 (15.9) | 11 (12.6) | 11 (21.6) | 0.17 |
| Partial work disability (%) | 19 (13.8) | 14 (16.1) | 5 (9.8) | 0.30 |
| Age at onset symptoms, years | 22.1 (7.5) | 21.5 (7.4) | 23.1 (7.7) | 0.25 |
| Duration of symptoms, years | 16.4 (8.9) | 16.2 (8.3) | 16.7 (10.1) | 0.79 |
| HLA-B27 positive (%) | 112 (83.6) | 70 (81.4) | 42 (87.5) | 0.36 |
| Comorbidity index (RDCI)† | 0.2 (0.5) | 0.2 (0.5) | 0.1 (0.3) | 0.11 |
| Cardiovascular disease (%) | 7 (5.0) | 6 (6.8) | 1 (2.0) | |
| Diabetes mellitus (%) | 1 (0.7) | 1 (1.1) | 0 (0.0) | |
| Fracture (%) | 5 (3.6) | 4 (4.5) | 1 (2.0) | |
| Depression (%) | 1 (0.7) | 1 (1.1) | 0 (0.0) | |
| Gastrointestinal disease (%) | 6 (4.3) | 5 (5.7) | 1 (2.0) | |
| History of IBD (%) | 7 (5.1) | 5 (5.7) | 2 (4.0) | 1.00 |
| History of psoriasis (%) | 8 (5.8) | 6 (6.8) | 2 (4.0) | 0.71 |
| History of AAU (%) | 22 (15.9) | 14 (15.9) | 8 (16) | 0.99 |
| Hip involvement (%)† | 26 (18.8) | 16 (18.4) | 10 (19.6) | 0.86 |
| Medication use, current‡ | ||||
| NSAID (%) | 101 (72.7) | 66 (75) | 35 (68.6) | 0.42 |
| ASAS NSAID score | 62.5 (53.8) | 64.6 (53.2) | 58.6 (55.3) | 0.45 |
| csDMARDs (%) | 15 (10.8) | 8 (9.1) | 7 (13.7) | 0.40 |
| Smoking, current (%) | 41 (39.4) | 27 (39.7) | 14 (38.9) | 0.94 |
| CRP, mg/L | 15.3 (19.5) | 16.2 (17.6) | 13.9 (22.3) | 0.07 |
| Elevated CRP (%)¶ | 48 (36.1) | 33 (40.2) | 15 (29.4) | 0.21 |
| ESR, mm/hour | 12.3 (12.1) | 11.3 (10.1) | 14.0 (14.7) | 0.42 |
| ASDAS-CRP | 2.5 (0.9) | 2.6 (0.9) | 2.3 (1.0) | 0.06 |
| BASDAI (0–10) | 2.9 (1.8) | 3.0 (1.9) | 2.7 (1.7) | 0.27 |
| BASFI (0–10) | 2.4 (2.0) | 2.6 (2.1) | 2.1 (1.9) | 0.22 |
| Back pain (0–10) | 3.2 (2.2) | 3.4 (2.1) | 2.9 (2.2) | 0.16 |
| BAS-G (0–10) | 3.4 (2.4) | 3.4 (2.3) | 3.5 (2.6) | 0.84 |
| Swollen joint count (0–44) | 0.5 (1.5) | 0.4 (1.5) | 0.5 (1.5) | 0.60 |
| Tender joint count (0–53) | 1.8 (3.3) | 2.0 (3.5) | 1.5 (2.9) | 0.19 |
| BASMI (0–10) | 3.4 (1.5) | 3.5 (1.5) | 3.2 (1.5) | 0.27 |
| mSASSS (0–72) | 7.0 (11.0) | 7.5 (12.0) | 6.0 (9.3) | 0.87 |
Values expressed as mean (SD) unless otherwise stated.
*Data from the first assessment in which patients reported to be working were used as baseline variables.
†Only assessed at baseline.
‡No patients used biologicals at baseline (first assessment in which they reported to be working).
§Ever sick leave; patients who reported sick leave at least once during upto 12 years of follow-up. Never sick leave; patients who never reported sick leave during upto 12 years of follow-up. Note that some of these patients became lost to follow-up or were censored over time (see figure 1).
¶Elevated CRP defined as CRP>10mg/L.
**Two-tailed ever sick leave vs never sick leave.
AAU, acute anterior uveitis; AS, ankylosing spondylitis; ASAS, Assessment of SpondyloArthritis international Society; ASDAS, AS Disease Activity Score; BASDAI, Bath AS Disease Activity Index; BASFI, Bath AS Functional Index; BAS-G, Bath AS Global Score; BASMI, Bath AS Metrology Index; CRP, C reactive protein; ESR, erythrocyte sedimentation rate; HLA-B27, human leucocyte antigen B27; IBD, inflammatory bowel disease; NSAID, non-steroidal anti-inflammatory drug; RDCI, Rheumatic Diseases Comorbidity Index; csDMARD, conventional synthetic disease-modifying antirheumatic drug; mSASSS, modified Stoke Ankylosing Spondylitis Spine Score.