Table 4.
Multivariable GEE models exploring past sick leave as a predictor for recurrence of sick leave over 6 years, stratified by level of education
| Variable | High educational attainment | Low educational attainment | ||||||||||
| Model ASDAS (n=46) | Model BASDAI (n=46) | Model BASFI (n=46) | Model ASDAS (n=82) | Model BASDAI (n=85) | Model BASFI (n=85) | |||||||
| OR | 95% CI | OR | 95% CI | OR | 95% CI | OR | 95% CI | OR | 95% CI | OR | 95% CI | |
| Gender, male | 0.93 | 0.23 to 3.82 | 0.88 | 0.21 to 3.72 | 0.85 | 0.20 to 3.49 | 0.79 | 0.27 to 2.31 | 0.73 | 0.25 to 2.10 | 0.70 | 0.25 to 2.01 |
| Age, years | 0.98 | 0.93 to 1.04 | 0.98 | 0.93 to 1.04 | 0.99 | 0.93 to 1.04 | 0.97 | 0.93 to 1.01 | 0.96 | 0.92 to 1.01 | 0.96 | 0.92 to 1.01 |
| Comorbidity (RDCI) | 1.55 | 0.16 to 14.95 | 1.35 | 0.17 to 10.56 | 1.58 | 0.21 to 11.83 | 1.98 | 1.14 to 3.43 | 2.04 | 1.22 to 3.42 | 1.87 | 1.10 to 3.16 |
| ASDAS-CRP* | ‡ | ‡ | ||||||||||
| BASDAI* | 0.77 | 0.46 to 1.28 | 1.27 | 1.07 to 1.51 | ||||||||
| BASFI* | 0.84 | 0.53 to 1.34 | 1.26 | 1.06 to 1.49 | ||||||||
| BAS-G* | 0.93 | 0.64 to 1.35 | 1.20 | 1.04 to 1.39 | ||||||||
| Early SL, yes† | 8.37 | 1.26 to 55.81 | 8.20 | 1.24 to 54.17 | 7.41 | 1.17 to 46.95 | 2.93 | 1.26 to 6.81 | 2.91 | 1.31 to 6.48 | 2.62 | 1.10 to 6.22 |
Due to correlation/collinearity between ASDAS, BASDAI and BASFI, it was decided to perform three separate analyses for these variables.
Procedure of variable selection: (1) P≥0.20 in univariable analysis and no confounder: symptom duration, HLA-B27, IBD, psoriasis, uveitis, NSAID index, CRP, mSASSS. (2) P<0.20 in univariable analysis and/or a confounder for sex or age, but not significant within variable group: (group 1) country of origin, job type; (group 2) hip involvement, smoking, csDMARD use; (group 3) ESR (BASDAI model), axial pain, peripheral pain, physician global, swollen joint count, tender joint count (ASDAS and BASDAI model); (group 4) BASMI. (3) Statistically significant when tested within variable group, but not when included in final model: part-time worker, ESR (BASFI model), tender joint count (BASFI model). (4) Excluded from analysis: ESR (excluded from ASDAS model due to overlap with ASDAS).
*Time-lagged variables (1 year time-lag).
‡Forced into model, but not statistically significant.
†Defined as occurrence of sick leave during the first year of follow-up.
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; GEE, generalised estimating equation; HLA-B27, human leucocyte antigen B27; IBD, inflammatory bowel disease; NSAID, non-steroidal anti-inflammatory drug; RDCI, Rheumatic Diseases Comorbidity Index; SL, ankylosing spondylitis-related sick leave; csDMARD, conventional synthetic disease-modifying antirheumatic drug; mSASSS, modified Stoke Ankylosing Spondylitis Spine Score.