Factor V Leiden and other inherited clotting disorders | CION Cancer Clinics

Factor V Leiden is a change in one gene that makes your blood a little more likely to clot than usual. It is inherited from a parent, it is not cancer, and most people who carry it never have a clot in their life.

What the gene normally does

When you cut yourself, a chain of proteins in the blood switches on to form a clot. Factor V is one link in that chain. Once the bleeding has stopped, another protein, called protein C, switches factor V off again so the clot does not keep growing. In factor V Leiden, factor V has a slightly different shape. Protein C cannot switch it off as easily, so the clotting signal stays on for longer than it should.

Why it matters, and why it often does not

The extra risk is real but modest for most carriers. A clot usually needs a second push as well: a long operation, a plaster cast, a long journey sitting still, pregnancy, the weeks after childbirth, or hormone tablets that contain oestrogen. The gene on its own rarely causes a clot out of nowhere.

Everyone has two copies of the factor V gene, one from each parent. Your report will say which situation you are in, and that word is the single most useful thing on it.

Heterozygous: one changed copy

This is by far the most common result. One copy is changed and one is normal. The raised risk is present but mild, and most carriers go through surgery, pregnancy and travel without a clot, often with some extra care from their doctors at those times.

Homozygous: two changed copies

Both parents passed on the change. This is uncommon. The risk is clearly higher than with one copy, and your haematologist is more likely to plan ahead for pregnancy, operations and hormone treatment.

When two conditions sit together

Carrying factor V Leiden and a second inherited change, such as the prothrombin gene change, raises the risk further than either alone. So does a past clot. A person who has already had one clot is in a very different position from a relative who tested positive and has never had one.

Relatives do not all need testing. Testing helps when the result would change a decision, for example before a sister starts hormone tablets or plans a pregnancy. Testing a child who has never had a clot rarely changes anything and can cause worry for years.

What the test cannot show

A negative result does not mean your blood cannot clot. Many clots happen in people with no inherited change at all, and the common tests do not look for every rare change. A positive result does not tell you whether or when a clot will happen.

What to ask your haematologist

Ask whether you carry one copy or two, whether any other change was found, what to do before an operation or a long journey, and which contraception suits you.