↵*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 RR_AB – RR_A – RR_B +1/ R_RAB (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.