What is Factor V Leiden?

As they have one normal copy of the gene, heterozygous patients have a lower likelihood of recurrent disease and so don’t need...


Please tell me about the Leiden variant of factor V.

I have the mutation, as do two of my three children.


Factor V Leiden is the most frequent genetic predisposition to thickening of the blood, occurring more often than all the other known forms put together.

The variant is the result of a single change, or mutation, in the structure of the factor V gene. This gene encodes a substance involved in controlling blood thickness.

People with the mutation have thicker blood than normal, which means it clots more easily.

A recent study found that you are four times as likely to suffer a blood clot in a deep vein, such as the ones in your calf (deep vein thrombosis or DVT), if you have the mutation.

There are a number of scenarios in which a patient should have a blood test for the Leiden mutation. They are:

  • if they unexpectedly have a DVT without a clear predisposing factor, such as smoking, varicose veins or obesity, especially if under the age of 30

    • if they have a family history of the mutation, as you mention in your question
      • if they have had more than one episode of DVT

        You carry two copies of virtually every gene you have. One copy comes from your mother, the other from your father.

        This means that you can have two copies of the mutant gene (homozygous), one normal copy and one mutant copy (heterozygous), or two normal copies (normal).

        Patients who are homozygous should receive initial treatment with heparin and then a three-month course of warfarin. Both these drugs reduce the likelihood of a DVT.

        As they have one normal copy of the gene, heterozygous patients have a lower likelihood of recurrent disease and so don’t need long-term treatment with Warfarin, unless they have another clot.

        All patients should receive preventative treatment with heparin during surgery or after injury.

        Patients who have a DVT or a blood clot in their lungs during pregnancy, or while using the oral contraceptive pill, have a 30 per cent chance of carrying the mutation and should probably also be screened.

        Women who are homozygous should avoid using oral contraceptives.

        So in summary, your relative risk of DVT will depend upon whether you and your family members are heterozygous or homozygous, and also upon the other variable risk factors mentioned in point 1 above.

        Yours sincerely

        The NetDoctor Medical Team

        Last updated

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