ABO blood group and von Willebrand factor levels partially explained the incomplete penetrance of congenital thrombophilia.

Objective: We aimed to study the association among ABO blood group, von Willebrand factor, factor VIII plasma levels, and the risk of venous thrombosis (VT) in a cohort of 1774 relatives from 500 families with inherited thrombophilia.
Methods and Results: One hundred sixty-one of the 1774 relatives had a VT. Different risk groups were formed: no, low-(factor V Leiden or F2G20210A heterozygous carriers), and high-risk thrombophilia (antithrombin, protein C, protein S, factor V Leiden, or F2G20210A homozygous carriers and combined defects). Compared with group O, AB blood group was associated with increased risk of VT: hazard ratio (HR)=3.8 (2.0-7.2). The effect of blood group A and B was milder (HR=1.6 [1.1-2.5] and 1.8 [1.0-3.3], respectively). An increased risk of VT was observed with increasing levels of von Willebrand factor and factor VIII plasma levels (HR=2.96 [1.92-4.56] and HR=2.60 [1.92-4.56] for third versus first tertile of von Willebrand factor and factor VIII plasma levels, respectively). In multivariate analysis, AB group (HR=2.3 [1.1-4.8]), high-risk thrombophilia (HR=2.8 [1.6-4.6]), and high von Willebrand factor levels (HR=2.3 [1.3-4.0]) were significantly associated with increased risk of VT. The risk of VT in individuals with high-risk thrombophilia and AB group was 14.4× higher than in those without thrombophilia and O group (5.0-41.4).
Conclusions: ABO blood group modifies the risk of VT in families with hereditary thrombophilia. Phenotyping of the ABO blood group should be performed to better assess the risk of VT in asymptomatic individuals from thrombophilic families.