Factor V Leiden, as I found out today (May 4, 2000) during a clinical presentation, is a genetic mutation in clotting factor V. More specifically, it is a single nucleotide substitution of adenine for guanine - which causes an amino acid substitution of glutamine for arginine at position 506.

This mutation causes the factor V in the blood to become resistant to activated protein C. Protein C is an inhibitor of the coagulation pathway, therefore this condition causes the blood to have increased coagulability. This increases the likelihood of thrombus formation (including Deep Venous Thrombosis) and thromboembolism and therefore increases the risk of pulmonary embolus, heart attack and stroke.

Factor V Leiden is the most common hereditary hypercoagulability clotting disorder amongst 'white people', possibly affecting up to 5% of the population of the U.S.

Suspicion of factor V leiden being the cause for any thrombotic event should be considered in any white patient below the age of 45, or in any person with a family history of thrombosis.

Diagnosis is via a genetic assay using PCR.

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