Interaction between oral contraceptive use and coagulation factor levels in deep venous thrombosis

Deep venous thrombosis is a multicausal disease, i.e. more than one risk factor needs to be present to cause the disease. Oral contraceptive use increases the risk of venous thrombosis but since not all women using oral contraceptives develop thrombosis, the presence of additional risk factors in patients is likely. The aim of this study was to assess the joint effect of oral contraceptive use and the levels of procoagulant factors (F)(FII, FV, FVII, FVIII, FIX, FX, FXI, FXII, FXIII and fibrinogen). Data of premenopausal women were re-analyzed in the Leiden Thrombophilia Study. The highest relative risks were observed for the combination of oral contraceptive use and high levels (>90th percentile) of FII (Odds Ratio [OR](OC+FII) 10.1; 95% confidence interval [CI] 3.5-29.0), FV (OROC+FV 12.6; 95% CI 3.8-41.5), and FXI (OROC+FXI 11.9; 95% Cl 3.6-39.2) and low levels (< 10th percentile) of FXII (OROC+FXII 12.3; 95% CI 2.4-63.0). No interaction was observed between oral contraceptive use and high levels of the other coagulation factors, i.e. the joint effect of these risk factors did not exceed the sum of the separate effects. The results of this study indicate that the risk for the joint effects of oral contraceptive use and coagulation factor levels are minor compared with the joint effect of oral contraceptive use and the FV Leiden mutation (RR > 30).