Table 3.

Multivariable GEE models exploring determinants 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	0.40	0.13 to 1.21	0.34	0.11 to 1.04	0.38	0.13 to 1.11	1.14	0.48 to 2.75	1.08	0.48 to 2.46	1.06	0.48 to 2.32
Age, years	0.98	0.94 to 1.03	0.98	0.94 to 1.03	0.99	0.94 to 1.04	1.00	0.96 to 1.04	0.99	0.96 to 1.02	0.98	0.95 to 1.02
Comorbidity (RDCI)			1.58	0.34 to 7.38					1.52	1.00 to 2.29
ASDAS-CRP*	0.74	0.44 to 1.25					1.48	1.07 to 2.03
BASDAI*			0.78	0.58 to 1.06					1.31	1.15 to 1.49
BASFI*					0.85	0.57 to 1.26					1.31	1.16 to 1.47

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: country of origin, symptom duration, HLA-B27, hip involvement, IBD. (2) P<0.20 in univariable analysis and/or a confounder for sex or age, but not significant within variable group: (group 1) job type; (group 2) psoriasis, uveitis, smoking, NSAID index, csDMARD use; (group 3) BAS-G, axial pain (ASDAS and BASDAI model), peripheral pain, physician global (ASDAS and BASDAI model), swollen joint count, tender joint count (ASDAS and BASDAI model); (group 4) BASMI, mSASSS. (3) Statistically significant when tested within variable group, but not when included in final model: part-time worker, comorbidity index (ASDAS and BASFI model), axial pain (BASFI model), physician global (BASFI model), tender joint count (BASFI model). (4) Excluded from analysis: CRP/ESR (excluded from ASDAS model due to overlap with ASDAS).

*Time-lagged variables (1 year time-lag).

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; csDMARD, conventional synthetic disease-modifying antirheumatic drug; mSASSS, modified Stoke Ankylosing Spondylitis Spine Score.