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Written by Dr Sarah Davis Haematology Consultant. Adapted from previous document written by Davina Gallagher, Anticoagulant Nurse Specialist and Dr Denise White, Haematology Consultant.
What is factor V Leiden?
Factor V Leiden is a common inherited condition.
Factor V Leiden is a variant of the protein Factor V (5), which is needed for blood clotting.
Factor V Leiden is harder to ‘turn off’ than normal Factor V so people who carry the Factor V Leiden gene have a greater risk of developing a blood clot in the veins (thrombosis) than the rest of the population.
Factor V Leiden only slightly increases the risk of getting a blood clot and many people with this condition will never experience thrombosis.
How do you get Factor V Leiden?
The abnormal Factor V Leiden gene is passed on from our parents. We inherit one gene from our mother and one gene from our father.
So it is possible to inherit:
Two normal genes
One Factor V Leiden gene and one normal gene (heterozygote)
Two Factor V Leiden genes (homozygote) Approximately 1 in 25 people will inherit one factor V Leiden gene and fewer than 1 in 1000 people will inherit 2
How do you know if you have Factor V Leiden?
The gene for Factor V Leiden can be found in a blood sample.
What are the symptoms of Factor V Leiden?
There are no signs, unless you have a blood clot in the veins.
How is Factor V Leiden treated?
People with Factor V Leiden do not need treatment unless there is a blood clot. In this case the doctor will prescribe medication (anticoagulant) to make the blood take longer to clot. Treatment will usually last 3-6 months but may be longer depending on the individual circumstances.
What are the signs of a blood clot?
The most common problem is a blood clot in the veins of the leg (Deep Vein Thrombosis). Symptoms of this include the leg becoming swollen, tense and painful.
Rarely a blood clot in the lungs can develop (Pulmonary Embolism). Depending on the size of the blood clot symptoms range from being barely noticeable to severe difficulty in breathing and chest pain on breathing in.
Even more rarely a blood clot might occur in the veins of the arm or another part of the body What is my risk of having a blood clot?
In the general population around 1 person in 10,000 will have either a DVT or PE every year.
The risk increases with age:
1 in 100,000 children will have a venous thrombosis every year
1 in 10,000 20-30 year olds
1 in 1,000 40-50 year olds
1 in 100 >80 year olds
Most people with factor V Leiden have additional risk factors for developing a blood clot but having one Factor V Leiden gene will increase your risk of developing blood clots by 5 times and having two genes increases your risk 80 times.
What can be done to avoid blood clots?
Avoid standing or sitting in the same position for long periods of time.
When travelling long distances (by any method of transport) take short walks every few hours and keep well hydrated.
Keep your weight in a healthy range.
If you require surgery or are immobile/bed bound tell your doctor you carry the Factor V Leiden gene as treatment to prevent thrombosis is likely to be recommended.
Information for women
The risk of thrombosis on the combined pill is small but four times that of the general population so women who also carry the Factor V Leiden gene should use an alternative contraceptive method. This is particularly important if they or a first degree relative have had a thrombosis.
Hormone replacement therapy approximately doubles the risk of thrombosis. This remains a small risk so in many women the benefits of HRT outweigh the risks but this should be discussed with your doctor. There is some evidence that patches have a lower risk of thrombosis than tablets. As pregnancy is a risk factor for thrombosis, if you have had a clot it is likely you will have low molecular weight heparin injections throughout pregnancy and the 6 weeks after birth. It is therefore important that you tell your doctor/midwife early that you are pregnant, or ideally before.
If you have not had a clot your factor V Leiden status, family history and other risk factors for a thrombosis will be assessed. You may have no preventative treatment, treatment for 10 days after birth (the riskiest time for clots) or throughout pregnancy and the 6 weeks after birth. If you are not on treatment it is important that you let your doctor know your Factor V Leiden status if you are hospitalised
Can I pass the condition on?
A heterozygote has a 50/50 chance of passing the abnormal gene to their children. A child of a homozygote will inherit at least one factor V Leiden gene.
Children have a very small risk of thrombosis so testing for factor V Leiden isn’t recommended but it is worth discussing your status with your child when they reach teenage years, especially girls who may go on the pill or get pregnant.