Thrombophilia is an inherited or acquired susceptibility to thrombosis due to an abnormality of blood coagulation. Factor V Leiden thrombophilia is the most common form of inherited thrombophilia and is characterized by poor anticoagulant response to activated protein C,thus imparting an increased risk to develop venous thrombosis. Individuals with factor V Leiden have a 3-8 fold increased risk for deep venous thrombosis compared to the general population. Thrombotic risks associated with factor V Leiden thrombophilia can include deep-vein thrombosis, with the leg being the most common site along with increased risk for pulmonary embolism. Also, factor V Leiden thrombophilia has been associated with an increased risk for pregnancy loss, and possibly other pregnancy complications such as preeclampsia, fetal growth retardation, and placental abruption.

The F5 gene encodes the protein factor V, and a specific disease-causing mutation in this gene is associated with factor V Leiden thrombophilia. Individuals with factor V Leiden thrombophilia may be either heterozygous or homozygous for a specific mutation, 1691G>A (R506Q), in the F5 gene. Heterozygosity occurs in 3-8% of the Caucasian population in the United States and Europe, and is less common in other ethnic groups. Molecular testing of additional genes associated with thrombotic risk, including factor II and MTHFR, is also available at the Genetics Center Molecular Diagnostic Laboratory.