What is factor V Leiden?

Factor V Leiden is not a disease, but an inherited disorder involving a genetic mutation of the factor V (F5) gene. The F5 gene plays an important role in controlling bleeding. Factor V Leiden gets its name from the Netherlands city (Leiden) where the mutant gene was first identified.

When an injury with bleeding occurs, a protein (coagulation factor V) made by the F5 gene controls the bleeding by forming a clot. Activated protein C keeps the clot from getting too big. In individuals with factor V Leiden, activated protein C is unable to inactivate the coagulation factor, so the clotting process continues unchecked, increasing the risk for abnormal clots such as deep vein thrombosis (a blood clot in a major vein) and pulmonary embolism (a blood clot in the lungs). Factor V Leiden can also increase the risk of miscarriage and complications of pregnancy such as stillbirth and the blood clots in the placenta and umbilical cord. 

Despite these risks, most people with factor V Leiden never develop abnormal clots, and most women with factor V Leiden have normal pregnancies. Nonetheless, if you have a family history of factor V Leiden and plan to get pregnant, you should be tested. Knowing that you have factor V Leiden will help your health care provider ensure that you have a healthy pregnancy. 

Factor V Leiden is among the more common genetic disorders affecting 3–8 percent of Caucasians in the United States and Europe. Some researchers have suggested that all women be screened for factor V Leiden before taking oral contraceptives (birth control pills) or getting pregnant. Those opposed to universal screening argue that it would deny oral contraceptives to a large group of women and prevent only a small number of deaths due to blood clots. Moreover, the risk of fatal bleeding during pregnancy from prophylactic anticoagulation therapy (treatment to prevent blood clots) is likely greater than the risk of death due to blood clots. 

Factor V Leiden should not be confused with factor V deficiency. Factor V deficiency is an inherited disorder characterized by low levels of clotting factor V. Individuals with factor V deficiency have an increased risk for excessive bleeding. Fortunately, factor V deficiency is extremely rare occurring in 1 in 1,000,000. 

Last updated August 8, 2017

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