Perinatal characteristics, older siblings, and risk of ankylosing spondylitis: a case–control study based on national registers

Table 1 Characteristics of the 1960 cases with ankylosing spondylitis and the 8377 matched population controls

Demographics	Cases n = 1960	Controls n = 8377
Women (%)	801 (41)	346 (41)
Year of birth median (Q1,Q3)	1979 (1975, 1984)	1979 (1975, 1984)
Age at first AS diagnosis median (Q1, Q3)	27 (22, 30)	NA
Living in same county at time of matching and at birth	1299 (66)	5571 (67)
AS-related inflammatory diseases^a
Anterior uveitis (%)	337 (17)	22 (0)
Inflammatory bowel disease (%)	108 (6)	58 (1)
Psoriasis (%)	89 (5)	49 (1)
Any of the manifestations above (%)	496 (25)	127 (2)
Rheumatic diagnoses before first AS diagnosis^b
Spondyloarthritis diagnosis^c	627 (32)	NA
Any rheumatic diagnosis^d	630 (32)	NA
Pharmacological treatment in 2011^e
TNF-alpha inhibitor (%)	494 (25)	NA
Methotrexate (%)	161 (8)	NA
Sulphasalazine (%)	217 (11)	NA
Any of the treatments above (%)	700 (36)	NA
Years of formal of education in 2008^f
0–9 years	309 (16)	1160 (14)
10–12 years	880 (45)	3705 (44)
>12 years	679 (35)	2993 (36)
Disposable income in 2008 £ mean (SD)^g	16378 (6797)	16768 (7050)
Maternal diabetes (%)^h	12 (0.6)	36 (0.4)

AS ankylosing spondylitis. NA no data available, TNF tumour necrosis factor
^aAS-related inflammatory diseases are given as cumulative incidence until 2 years after first AS diagnosis for the cases and their matched controls
^bRheumatic diagnoses before first AS diagnosis are based on ICD codes in the National Patient Register
^cPsoriatic arthritis, undifferentiated spondyloarthritis, reactive arthritis and spondyloarthritis associated with inflammatory bowel disease
^dAny rheumatic disease, see Table S1 in Additional file 1
^ePharmacological treatment is based on data in the Prescribed Drug Register for prescriptions during 2011, or in the case of infliximab recoded in the Anti-Rheumatic Therapy in Sweden (ARTIS) register as used during 2011
^fNo statistically significant difference in level of formal education based on chi-square test (p = 0.115)
^gLimited to those ≥30 years old in 2008 and the 0.5^th to the 99.5^th percentiles (cases = 953; controls = 3977), no statistically significant difference based on t test (p = 0.125)
^hMaternal diabetes prior or during pregnancy, as recorded in the Medical Birth Register. No statistically significant difference based on Fisher’s exact test (p = 0.271)

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