Factor V Leiden is a mutation of one of the clotting factors in the blood called factor V. This mutation can increase your chance of developing abnormal blood clots (thrombophilia), usually in your veins. Most people with factor V Leiden never develop abnormal clots. But some do develop clots that lead to long-term health problems or become life-threatening. Both men and women can have factor V Leiden. Women may have an increased tendency to develop blood clots during pregnancy or when taking the hormone estrogen.
Inherited factor V deficiency is a rare condition that is associated with an abnormal factor V plasma level. Acquired factor V deficiency is a rare clinical condition in which the development of antibodies to factor V (factor V inhibitors) leads to hemorrhagic complications of varying severity. Factor V Leiden is a completely different inherited disorder that involves a single point mutation in the factor V gene. Factor V activity levels in patients with factor V Leiden are normal. Proteolytic inactivation of factor Va and factor VIIIa by activated protein C (APC) normally limits clot formation; however, factor V Leiden resists inactivation by APC.
Only 150 cases of congenital factor V deficiency have been reported worldwide since 1943. Homozygous factor V deficiency is rare, occurring in approximately 1 per million population. The severity of factor V deficiency varies from bruising to lethal hemorrhage. Factor V deficiency affects males and females with equal frequency. Factor V deficiency affects all ages. The age at presentation indirectly varies with the severity of disease.