Factor XI Deficiency

Inheritance is autosomal with expression in both males and females. Although FXI deficiency is sometimes described as a recessive disorder, several observations show that the bleeding risk is not closely related to the factor level.^23-25 While bleeding is more common in individuals with very low factor XI levels (<10 IU/dL), it is clear that heterozygotes are also at risk and that bleeding risk cannot be predicted from the factor level alone.

FXI deficiency should therefore not be described as a recessive disorder which implies an asymptomatic carrier state in individuals with a single mutation. Factor XI deficiency is particularly prevalent in the Ashkenazi Jewish population, with a heterozygote frequency of about 8%.²⁶ This high frequency relates to the presence of two common mutations, type II (Glu117Stop) and type III (Phe283Leu),²⁷ which account for most cases in this population and are due to a founder effect.²⁸ Factor XI deficiency may occur in all racial groups and stem from a variety of mutations. Evidence of founder mutations in other populations, such as the British²⁹ and the French Basques³⁰ has been documented. Genetic testing is of research interest and may be available in many centers but is not generally required for clinical purpose and is not appropriate for prenatal testing.

Two mutation databases are available: one from the factor XI website (http://www.factorxi.org/stats.php; last updated in 2015), and the second from the website of the International Society on Thrombosis and Haemostasis.