Anticoagulation UK

Thrombosis & Genetic Disorders

Inherited thrombophilia’s

Factor V Leiden

This gene affects the factor V part of the clotting cascade, making the clotting process go on longer. This increases the risk of a blood clot.

Protein C deficiency

Protein C is a natural anticoagulant chemical in the blood. The deficiency can be genetic, or due to other conditions such as kidney disease. With inherited protein C deficiency, the risk of a blood clot forming varies between families. This is because each family has a different alteration in the protein C gene.

Protein S deficiency

Protein S is also a natural anticoagulant chemical in the blood. Protein S deficiency is rare. The risk of a blood clot forming varies between families.

Antithrombin deficiency

Antithrombin is another of the natural anticoagulant chemicals in the blood. There are different types of antithrombin deficiency, both genetic and due to other diseases. The inherited form is rare, affecting about 1 in 2,000 people.

The risk of having a blood clot varies between families but can be increased by 25-50 times the normal risk. Others, however, may reach elderly age without problems.

Combined inherited thrombophilias

Some people inherit more than one thrombophilia. With combined thrombophilias, the risk of developing a blood clot is multiplied and there is a much greater risk than with either condition alone.

Acquired thrombophilias

Acquired thrombophilias are not inherited, and usually start in adulthood.

Antiphospholipid  syndrome (APS)

Also known as Hughes' syndrome. It is caused by certain immune system chemicals (antibodies) in the blood, which are called antiphospholipid antibodies. APS can affect pregnancy in some cases. Many women with APS do not have problems in pregnancy. However, APS may cause miscarriage, or other problems.


This is a raised level of a chemical called homocysteine in the blood, which is thought to increase the risk of arterial and venous blood clots. It damages blood vessels. This is a relatively mild condition but it can add to the risk factors for having a blood clot. Vitamin B12 and folic acid may help.

Paroxysmal nocturnal haemoglobinuria

This is a rare condition affecting the bone marrow. It can lead to venous blood clots, often in unusual sites such as the veins of the gut, liver or brain. Anticoagulation may be needed and there are other treatments too for this condition.

Elevated factor VIII

This is pronounced 'factor 8'. It is due to abnormally high levels of factor VIII, which is one of the blood chemicals that promotes clotting. Depending on the actual level of factor VIII, it can increase the clotting risk by about six times - this is a relatively low risk for developing a blood clot.