Factor XI (FIX) was only first recognized in 1953. The incidence of FXI deficiency is estimated at 1 in 100,000, and it is inherited in an autosomal dominant fashion, affecting men and women equally. It can occur with greater frequency in people of Ashkenazi Jewish descent because intermarriage among this group has been more prevalent. In Israel, FXI deficiency has been estimated to be around 8% among Ashkenazi Jews, making it one of the most common genetic disorders in this group.
FXI is important in the propagation phase of blood coagulation, and its absence will markedly delay thrombin generation. Some people with FXI deficiency may have milder symptoms, while others may resembe hemophilia in the severity of their bleeding. Thus, there can be quite a bit of variability with this condition. However, most individuals do not bleed spontaneously, and hemorrhage normally occurs only after trauma or surgery. Certain procedures carry an increased risk of bleeding. These include dental extractions, tonsillectomies, surgery in the urinary and genital tracts and nasal surgery. Joint bleeds are uncommon. Patients are more prone to bruising, nosebleeds, or blood in the urine. Woman may experience menorrhagia and prolonged bleeding after childbirth.
Diagnosis is made through the finding of a normal prothrombin time (PT) and prolonged activated partial thromboplastin time (aPTT). Mixing studies will show correction of the prolonged APTT with normal plasma. Confirmation of FXI deficiency is achieved with a specific FXI assay based on the APTT.
