Patients with Severe Factor XI Deficiency Have a Reduced Incidence of Venous Thromboembolism.

Abstract 3491

Poster Board III-428

Patients with severe factor XI deficiency (level ≤15U/dL) were recently found to be protected against ischemic stroke but not against myocardial infarction (Blood 2008, 111:4113). Regarding venous thrombosis, several clinical and experimental observations suggest that factor XI deficiency might confer protection. Thus, only 2 patients with severe factor XI deficiency have been reported with unprovoked venous thromboembolism, and protection against FeCl3 Cinduced thrombosis in the vena cava was observed in factor XI deficient mice (J Thromb Haemost 2006, 4:1982). Other studies established that an increased level of factor XI is associated with an increased risk of venous thromboembolism. In the present study, we investigated whether patients with severe factor XI deficiency are protected against venous thromboembolism. We interviewed 222 unrelated patients with severe FXI deficiency and reviewed their medical charts. In the cohort, there were 126 females and 96 males aged 20-94 years of whom 137 were 50 years or older. The expected incidence rates of venous thromboembolism in the general population for males and females at different age groups were extracted from a population-based study of venous thromboembolism (J Thromb Haemost 2007,5:692). The expected incidence rate of venous thromboembolism in the factor XI deficiency cohort was 4.67 (2.37 in females and 2.30 in males), while the observed incidence rate was 0/222. This difference was statistically significant taking into account age and sex by the Poisson test (p=0.036). The prevalence of the common inherited thrombophilias was examined in 194 of the 222 patients. Seven (3.6%) patients harbored factor V Leiden, and 17 (8.8%) harbored the prothrombin G20210A mutation; the corresponding proportions in the general Israeli population are 3.9% for factor V Leiden, and 5.4% for prothrombin G20210A mutation. Thus, there was no decreased prevalence of the common thrombophilias in the factor XI deficient cohort. Collectively, the data suggest that patients with severe factor XI deficiency are protected against venous thromboembolism.