The most common genetic risk factor for venous thrombosis is factor V Leiden, present in 5 percent of the general population. Factor V is one of the normal blood clotting factors. Factor V Leiden is a changed or ‘mutated’ form of factor V which is inactivated ten times slower that the normal factor V. This causes it to persist longer in the circulation, resulting in a hypercoagulable state. In other words, the blood continues clotting, resulting in possible obstruction.

One copy of the Factor V Leiden gene increases the risk for venous thrombosis 4 to 8 times, while two copies of the gene increase the risk 80-fold.

Other coexisting coagulation defects can occur with Factor V Leiden, and in general, the risk for thrombosis increases in patients with more than one genetic defect.

The factor V Leiden mutation is involved in 20 to 40 percent of venous thrombosis cases and is suspected in individuals who have a medical history of venous thrombosis or in families with a high incidence of venous thrombosis.

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