Table 1

Basic characteristics of the included cases and controls

Cases N (%)Control N (%)
Total2831 (44)*3570 (56)*
Sex
 Women2024 (71)2564 (72)
 Men807 (29)1006 (28)
Age
 <40549 (19)689 (19)
 40–49482 (17)587 (16)
 50–59889 (31)1093 (31)
 60–70911 (32)1201 (34)
Exposed to infection
 Gastroenteritis640 (23)1026 (29)
 Urinary tract infection400 (14)606 (17)
 Genital infection157 (6)228 (6)
 Prostatitis26 (3)52 (5)
 Sinusitis treated with antibiotics344 (12)425 (12)
 Tonsillitis treated with antibiotics480 (17)579 (16)
 Pneumonia73 (3)89 (2)
Smoking
 Never884 (31)1517 (43)
 Past895 (32)975 (28)
 Current787 (28)713 (20)
 Irregular/other256 (9)333 (9)
Socioeconomic status
 High634 (22)1053 (30)
 Medium1426 (50)1745 (49)
 Low768 (27)757 (21)
Shared epitope
 Any shared epitope genes1915 (74)781 (53)
 No shared epitope genes664 (26)694 (47)
ACPA status
 Positive1782 (64)
 Negative999 (36)
  • In all, 2094 cases (74%) have one control, 735 cases (26%) have two controls and two cases have three controls each. Eight participants have missing information on gastroenteritis, nine participants on genital infections, 19 participants on prostatitis, four participants on urinary tract infection, five participants on sinusitis and three participants on tonsillitis. The question on pneumonia was formulated differently from the other infection questions, and to determine whether the infection occurred during the last 2 years before diagnosis, we needed extra information which had a higher degree of data missing and led to 450 patients having missing information on pneumonia. A total of 18 participants had missing information of socioeconomic status, 41 participants missing smoking information, 252 cases and 2095 controls missing shared epitope status, and 50 cases unknown ACPA status. Participants who lacked information on variables needed for a specific analysis were excluded from that analysis, but still contributed to all other analysis, for which he/she had the required information.