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. A blood clot (thrombus) normally forms to stop the bleeding when an artery or vein is damaged, such as when you experience a cut. Clots are formed by chemical reactions between specialized blood cells (platelets) and proteins in your blood (clotting factors). Anti-clotting factors prevent an excessive formation of blood clots. 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 populations. Two types of tests can be done Activated protein C resistance test, Genetic test. Testing babies before or soon after birth isn't recommended. Children who don't have symptoms of clotting problems also don't need screening. Blood clots are rare, even in children with factor V Leiden. Testing isn't recommended until adulthood. Doctors generally use blood-thinning (anticoagulant) medications, such as warfarin (Coumadin, Jantoven), heparin or low molecular weight heparin to treat people who develop blood clots.