Factor V Leiden thrombophilia is characterized by a poor anticoagulant response to activated protein C (APC) and an increased risk for venous thromboembolism (VTE). Deep venous thrombosis (DVT) is the most common VTE, with the legs being the most common site. Factor V Leiden thrombophilia is suspected in individuals with a history of venous thromboembolism (VTE) manifest as deep vein thrombosis (DVT) or pulmonary embolism, in women with a history of VTE during pregnancy or in association with oral contraceptive use, and in individuals with a personal or family history of recurrent thrombosis. The variant Factor V Leiden is inactivated by APC at a rate approximately ten times slower than normal factor V and persists longer in the circulation, resulting in increased thrombin generation and a mild hypercoagulable state, reflected by elevated levels of prothrombin fragment F1+2 and other activated coagulation markers. The diagnosis of factor V Leiden thrombophilia is made either using a coagulation screening test (APC resistance assay) or by DNA analysis of the F5 gene which encodes the factor V protein, the only gene associated with factor V Leiden thrombophilia. The term "factor V Leiden" refers to the specific G-to-A substitution at nucleotide 1691 in the gene for factor V that predicts a single amino-acid replacement (Arg506Gln) at one of three APC cleavage sites in the factor V molecule.
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