Anticoagulant therapy in patients with congenital FXI deficiency.

The bleeding phenotype of factor XI (FXI) deficiency is unpredictable. Bleeding is usually mild and mostly occurs after injury. Although FXI deficiency renders antithrombotic protection, some patients might eventually develop thrombosis or atrial fibrillation, requiring anticoagulant therapy. There is almost no evidence on the bleeding risk in this scenario. Our retrospective study of 269 white FXI-deficient subjects (1995-2021) identified 15 cases requiring anticoagulation. They harbored 8 different F11 variants, mainly in heterozygosis (1 case was homozygote), and had mild to moderate deficiency (FXI:C: 20% to 70%). Two subjects (13.3%) had bleeding history before anticoagulation. Atrial fibrillation was the main indication (12/15; 80%). Fourteen patients started therapy with vitamin K antagonists (VKA), but 4 subjects were on direct oral anticoagulants (DOACs) at the end of follow-up. Over >1000 months of anticoagulation, 2 mild bleeding episodes in 2 patients (13.3%, 95% confidence interval: 3.7% to 37.9%) were recorded. No major/fatal events were reported. "Pre-post" bleeding localization and severity did not change despite treatment. On VKA, drug dosing and management were also standard, unaltered by FXI deficiency. We provide the largest description of anticoagulant use in FXI deficiency, and the first cases receiving DOACs. Although further studies are needed, our observations suggest that moderate FXI deficiency does not interfere with anticoagulant management nor bleeding risk.