24th July 2024

Factor V Leiden

FVL is the most commonly inherited thrombophilic condition in caucasian populations and is an autosomal dominant condition. Most patients are heterozygotes. Though many may have the genotype, only around 5-10% develop VTE. Homozygotes have a higher risk of VTE than heterozygotes.

Factor V is a key component of the conversion of fibrinogen to fibrin and, therefore, clot production. Protein C is an endogenous anticoagulant, inhibiting factor V. In FVL a mutation factor V means it cannot be inactivated by protein C and thus, more fibrin can be produced and clots formed.

Screening for FVL is only required in those who present with recurrent VTE, those at a young age with no other obvious risk factor and those with unusual clot location, e.g. cerebral vein. FVL testing includes genetic testing and functional aPC (activated protein C) resistance testing (higher values in FVL). Functional resistance can be affected by DOACs and in these cases genetic testing may be more reliable.

Management of a confirmed VTE is the same in FVL patients as in those without.