• *Since ever OC use was associated with a decreased risk of ACPA-positive RA, the risk category included non-OC users for each interaction analysis, which was separately conducted for smoking, SE alleles and PTPN22.

  • †Adjusted for matching variables (age and residential area) and alcohol consumption.

  • ‡Adjusted for matching variables (age and residential area), pack-years of smoking and alcohol consumption (low (including non-drinkers), medium and high).

  • §The AP estimates the proportion of the excess risk that is due to the interaction per se (factor A + factor B) according to the formula RRAB – RRA – RRB +1/ RRAB (where RR=relative risk).

  • ACPA, anticitrullinated protein antibodies; AP, attributable proportion due to interaction; Ca/Co, number of cases/controls; EIRA, Epidemiological Investigation of Rheumatoid Arthritis; OC, oral contraceptives; RA, rheumatoid arthritis; SE, shared epitope.