About thrombosis
Thrombophilia: congenital thrombophilia

Factor V Leiden

Factor V Leiden is by far the most common congenital thrombophilia. In the UK it is present in 1 in 20 individuals of European origin. It is rare in people of black or Asian origin.

Factor V Leiden is caused by a change in the gene for Factor V, which helps the blood to clot. To stop a clot spreading a natural blood thinner, known as Protein C, breaks down factor V.

If someone has Factor V Leiden, the factor V molecule in the blood is more resistant to being broken down and the clotting process goes on for longer. This makes individuals with this condition more prone to blood clots, leading to venous thrombosis.

The risk of venous thrombosis is about eight times greater in individuals with Factor V Leiden than for those without it. However Factor V Leiden does not increase the risk of arterial thrombosis, so there is no increased risk of heart attacks or strokes.

Inheritance
A single dose is known as heterozygous Factor V Leiden. However as Factor V Leiden is relatively common, some individuals have a double dose, which means that they have inherited it from both parents.

A double dose is known as homozygous Factor V Leiden. It affects 1 in 1600 people of European origin.

Factor V Leiden and venous thrombosis
Most individuals with Factor V Leiden have a low risk of venous thrombosis unless one or more additional risk factors is present.

The risk of having a venous thrombosis is about eight times greater for an individual with heterozygous Factor V Leiden. However this risk is relatively low and most individuals with the condition are not affected.

The risk of venous thrombosis is much higher for individuals with homozygous Factor V Leiden. It is about 80 times greater than for those without the condition.

Preventing venous thrombosis
People who have Factor V Leiden can reduce the risk of venous thrombosis by making lifestyle changes, such as:

• Leading a physically active life
• Eating a healthy, balanced diet
• Avoiding becoming overweight or obese
• Avoiding long periods of immobility during illness or when travelling
• Stopping cigarette smoking

Individuals with Factor V Leiden should also seek medical advice before major surgery, or when they are likely to be immobile for a long period, as this increases the risk of venous thrombosis.

Women with Factor V Leiden should seek medical advice before taking oral contraception or hormone replacement therapy, and when pregnant or planning to become pregnant.

Factor V Leiden and pregnancy
It is important that women with Factor V Leiden who are pregnant discuss this with their obstetrician. Women with Factor V Leiden have an increased risk of venous thrombosis during pregnancy, and some evidence suggests that they may also have a slightly higher risk of miscarriage and placental problems.

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