Factor V Leiden

Reviewed by Dr Patrick Davey, cardiologist and Dr Rachel Green, consultant haematologist

What is Factor V Leiden?

Factor V Leiden is not a disease, it is the presence of a particular gene that is passed on from your parents.

Factor V Leiden is a variant of the protein Factor V (5) which is needed for blood clotting. People who have a Factor V deficiency are more likely to bleed badly while people with Factor V Leiden have blood that has an increased tendency to clot.

People carrying the Factor V Leiden gene have a five times greater risk of developing a blood clot (thrombosis) than the rest of the population. However, many people with the gene will never suffer from blood clots.

In Britain, 5 per cent of the population carry one or more genes for Factor V Leiden, which is far more than the number of people who will actually suffer from thrombosis.

How do you get Factor V Leiden?

The genes for the Factor V are passed on from our parents. As with all inherited characteristics, we inherit one gene from our mother and one from our father. So, it is possible to inherit:

two normal genes or one Factor V Leiden gene and one normal gene

or two Factor V Leiden genes.

Having one Factor V Leiden gene will result in a slightly higher risk of developing a thrombosis, but having two genes makes the risk much greater.

What are the symptoms of Factor V Leiden?

There are no signs, unless you have a blood clot (thrombosis).

What are the danger signals?

The most common problem is a blood clot in the leg. This is indicated by the leg becoming swollen, painful and red.

In rarer cases a blood clot in the lungs (pulmonary thrombosis) may develop, making it hard to breathe. Depending on the size of the blood clot this can range from being barely noticeable to the patient experiencing severe respiratory difficulty.

In even rarer cases the clot might occur in an arm or another part of the body. Since these clots are formed in the veins that take blood to the heart and not in the arteries (which take blood from the heart), Factor V Leiden does not increase the risk of coronary thrombosis.

What can be done to avoid blood clots?

Factor V Leiden only slightly increases the risk of getting a blood clot and many people with this condition will never experience thrombosis.

There are many things you can do to avoid getting blood clots.

Avoid standing or sitting in the same position for long periods of time.

When travelling long distances it is important to exercise regularly - the blood must not 'stand still'.

Being overweight or smoking will greatly increase the risk of blood clots.

Women carrying the Factor V Leiden gene should not take the contraceptive pill as this will significantly increase the chance of getting thrombosis.

Women carrying the Factor V Leiden gene should also consult their doctor before becoming pregnant as this can also increase the risk of thrombosis.

How does the doctor find out if you have Factor V Leiden?

The gene for Factor V Leiden can be found in a blood sample.

A blood clot in the leg or the arm can usually be detected by an ultrasound examination.

Clots can also be detected by X-ray after injecting a substance into the blood to make the clot stand out.

A blood clot in the lung is harder to find, but normally a doctor will use a radioactive substance to test the distribution of blood flow in the lung, and the distribution of air to the lungs. The two patterns should match - a mismatch indicates the presence of a clot.

How is Factor V Leiden treated?

People with Factor V Leiden do not need treatment unless their blood starts to clot, in which case the doctor will prescribe blood-thinning (anticoagulant) medicines such as warfarin or heparin to prevent further clots.

Treatment will usually last for three to six months, but if there are several clots it could take longer. In severe cases the course of drug treatment may be continued indefinitely; in very rare cases the blood clots may need to be surgically removed.

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