ABO blood groups and the risk of venous thrombosis in patients with inherited thrombophilia.

BACKGROUND Although having a non-O blood type is now regarded as a risk factor for venous thromboembolism, the strength of this association is poorly defined, as is its interaction with inherited thrombophilia. MATERIALS AND METHODS The prevalence of non-O blood group and inherited thrombophilia (deficiencies of natural anticoagulants, factor V Leiden and prothrombin G20210A mutation) was assessed in a series of 712 consecutive patients with proximal deep vein thrombosis of the lower limbs who were referred to our Institution between 2004 and 2010, and in 712 age- and gender-matched healthy volunteers. Odds ratios (OR) of deep vein thrombosis and their 95% confidence intervals (CI) were computed for non-O group and thrombophilia, both separately and in combination. RESULTS A non-O blood group was present in 492 cases and 358 controls (OR 2.21; 95% CI, 1.78 to 2.75). A thrombophilic abnormality was present in 237 cases and 105 controls (OR 2.82; 2.18 to 3.66). The combination of non-O group and thrombophilia was present in 152 cases and 51 controls (OR 7.06; 4.85 to 10.28). DISCUSSION Having a non-O blood group is associated with an increased risk of proximal deep vein thrombosis of the lower limbs with or without pulmonary embolism. The addition of inherited thrombophilia increases the thrombotic risk conferred by non-O group alone by almost 3-fold.