Background: While Factor V Leiden is associated with an increased risk of thrombosis, the magnitude of that risk among homozygous carriers is uncertain.

Methods: In a cross-sectional study of Mayo Clinic patients referred to the Special Coagulation Laboratory for a thrombophilia profile (n=379) over the 18-year period, 1996-2013, the prevalence of venous and arterial thrombosis was estimated for patients found to be Factor V Leiden carriers based on leukocyte genomic DNA genotyping. T tests and chi-square tests were used for continuous and categorical variables, respectively.

Results: Among homozygous (n=111) and heterozygous (n=268) carriers, the mean age at testing (50.3 and 50.5 years, respectively; p=0.9) and the proportion of females (50% and 54%, respectively; p=0.5) did not differ significantly. Of 29 asymptomatic carriers at the time of testing, 14 were homozygotes. Of the remaining 350 carriers, the mean age at first venous or arterial thrombosis (42 vs 43.8 years; p=0.5) among homozygous and heterozygous carriers, respectively, did not differ significantly. Thirty-five (33%) and 83 (31%) homozygous and heterozygous carriers, respectively, had at least one recurrent venous or arterial thrombosis (p=0.72). The prevalence of deep venous thrombosis (DVT) was 61% and 46% in homozygous and heterozygous carriers, respectively (p=0.008); the prevalence of pulmonary embolism was 18% and 20%, respectively (p=0.69). The prevalence of arterial thrombosis (mainly stroke and/or TIA) was higher in heterozygous (23%) compared to homozygous carriers (8%).

Conclusion: In the largest sample of homozygous Factor V Leiden carriers reported to date, thrombosis penetrance and phenotype did not appear to differ among homozygous vs. heterozygous carriers with the exception of a higher prevalence of DVT among homozygous carriers. These data should be interpreted with caution due to the potential for referral bias.


No relevant conflicts of interest to declare.

Author notes


Asterisk with author names denotes non-ASH members.

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