Factor V Leiden (activated protein C resistance) is the most common inherited thrombophilia, being present in around 5% of the UK population.
It is due to a gain of function mutation in the Factor V Leiden protein.
= The result of the mis-sense mutation is that activated factor V (a clotting factor) is inactivated 10 times more slowly by activated protein C than normal. This explains the alternative name for factor V Leiden of activated protein C resistance
Heterozygotes have a 4-5 fold risk of venous thrombosis. Homozygotes have a 10 fold risk of venous thrombosis but the prevalence is much lower at 0.05%.
Investigations
Screening for factor V Leiden is not recommended, even after a venous thromboembolism. The logic behind this is that a previous thromboembolism itself is a risk factor for further events and this should dictate specific management in the future, rather than the particular thrombophilia identified.