Introduction 

Thrombophilia is where the blood has an increased tendency to form clots.

It's not a condition or a disorder itself, and is sometimes associated with blood clots (venous thromboses) that are caused by genetic changes (mutations).

If you have thrombophilia, you're more likely to develop a blood clot in one of the large veins in your leg. This is known as deep vein thrombosis (DVT)

It's important to be aware of the signs of a DVT, such as unexplained pain, swelling or tenderness in your leg (usually in your calf). See your GP as soon as possible if you have these symptoms (see below).

The blood clotting process

When you cut yourself, the bleeding usually triggers a number of chemical reactions in the blood.

The chemical reactions cause a blood clot to form, which sticks to the injured part of the blood vessel along with blood cells called platelets.

Clotting substances, known as clotting factors, help control bleeding and work with platelets to ensure the blood clots effectively.

However, in thrombophilia, there's an imbalance in clotting chemicals and a person with the disorder either has too much clotting factor, or too little of the substance that prevents clotting.

Types of thrombophilia

There are many different types of thrombophilia. Some of the main types are outlined below.

Factor V Leiden

Factor V Leiden is a type of thrombophilia caused by a specific gene mutation.

It's the most common type of inherited thrombophilia, with 3-8% of Europeans having one copy of the factor V Leiden mutation in each cell, and about one in 5,000 people having two copies of the mutation. It's less common in other populations.

If you have Factor V Leiden, your risk of developing a DVT is increased (see above). There's also a higher risk of blood clots breaking away from their original location and travelling to other areas of your body through your bloodstream.

This can be particularly serious if the blood clot becomes stuck in the main artery of your lungs (the pulmonary artery). This is known as a pulmonary embolism and it can be life-threatening (see below).

Prothrombin 20210 mutation

Prothrombin 20210 mutation, also known as Factor II mutation, is another genetic blood disorder that increases the risk of clots.

Prothrombin is a protein in the blood which helps it to clot (thicken). However, in some people, a mutation in the gene that produces prothrombin causes too much to be produced, resulting in an increased tendency for blood clots, such as DVTs, to form.

As with Factor V Leiden, prothrombin 20210 is more common in white people, particularly Europeans.

Protein C, protein S or antithrombin deficiency

Protein C, protein S, and antithrombin are natural anticoagulants (substances that prevent the blood from clotting).

If you have low levels of these anticoagulants, or if they don't work properly, your risk of developing a DVT or pulmonary embolism is increased.

Problems with protein C, protein S or antithrombin can be inherited but are rare.

Antiphospholipid syndrome

Antiphospholipid syndrome, also known as Hughes syndrome, is an immune system disorder that increases the risk of blood clots.

Your body produces antibodies that attack phospholipids, a fat molecule thought to keep blood at the right consistency.

The antibodies bind to the phospholipids, increasing your risk of a blood clot developing in a vein or artery.

Symptoms of thrombophilia

Thrombophilia is often mild and many people don't experience any health problems. Symptoms will only occur if the disorder causes a blood clot.

People with thrombophilia are particularly at risk of developing a DVT. 

Warning signs of a DVT include:

  • pain, swelling and tenderness in one of your legs (usually your calf)
  • a heavy ache in the affected area
  • warm skin in the area of the clot
  • redness of your skin, particularly at the back of your leg below the knee

DVT usually, although not always, affects one leg. The pain may be worse when you bend your foot up towards your knee. 

Part of the blood clot can sometimes break away and travel through the bloodstream. This is known as an embolism.

Embolisms can be dangerous if they become lodged in the lungs (pulmonary embolism). This is a serious and potentially life-threatening condition because it can prevent blood from reaching your lungs.

The symptoms of a pulmonary embolism are:

See your GP immediately if you experience any combination of the above symptoms. Alternatively, you can call NHS 111 or contact your local out-of-hours service for advice. Dial 999 to request an ambulance if your symptoms are severe.

Diagnosing thrombophilia

Thrombophilia testing isn't routinely carried out on everyone who has a blood clot.

Testing for inherited thrombophilia in people with venous thrombosis isn't usually needed because the results don't usually influence clinical decisions.

For example, decisions regarding the length of treatment should be made by taking into consideration whether or not venous thrombosis was provoked, whether other risk factors are present, and the risk of bleeding due to treatment with anticoagulants, regardless of whether inherited thrombophilia is present or not.

Thrombophilia is diagnosed by having blood tests weeks or months after your blood clot. The tests look for anticoagulant deficiencies. Before having the tests, you'll usually have to wait until you've stopped taking anticoagulant medicine, such as warfarin, for four to six weeks.

If the blood test results indicate you have thrombophilia, you may be referred to a haematologist (a specialist in diagnosing and treating blood disorders).

Treating thrombophilia

Treatment may not be needed if you have mild thrombophilia.

If you develop a blood clot, you'll need treatment for the blood clot and treatment to prevent further clots. You may need to take warfarin tablets or have an injection of heparin.

Warfarin and heparin

Warfarin and heparin are anticoagulants (anti-clotting medicine). They interfere with the clotting process and are commonly used to treat or prevent DVT and pulmonary embolisms.

If you need an anticoagulant to prevent a clot, you'll be prescribed warfarin. It will take a few days to work properly.

If you need instant treatment for an existing clot, you'll usually be given a heparin injection (which works straight away), as well as warfarin tablets for the first few days. The injection will either be given in hospital or at home.

A heparin injection may also be given to people with antiphospholipid syndrome, before and after surgery or during pregnancy. Unlike warfarin, heparin is safe to take in pregnancy.

International Normalised Ratio (INR) test

Your doctor will need to adjust your warfarin dose so your blood doesn't clot easily, but isn't too high to put you at risk of bleeding problems.

You'll need to have a regular blood test called the International Normalised Ratio (INR) to measure your blood clotting ability while taking warfarin.

You'll have the INR test less frequently once your ideal dose has been reached (an INR of two to three is usually the aim).

Lifestyle advice

If you have thrombophilia, it's important you're aware of the symptoms of a blood clot (see above), and that you see your GP immediately if you think you have one.

You should also take the following precautions to lower your risk of blood clots:

If you're pregnant, or planning to get pregnant, discuss this with your GP and inform the midwife and obstetrician about your condition.

You may need to take low-dose aspirin or an anticoagulant medicine while you're pregnant to prevent problems occurring during pregnancy or miscarriage.

If you're having a major operation, make sure you tell the healthcare professionals treating you about your condition. You may need a heparin injection to prevent a blood clot forming.

Women with thrombophilia should avoid taking the combined oral contraceptive pill or hormone replacement therapy (HRT) because they'll further increase your risk of developing a blood clot.




Formation of a blood clot (thrombus) 

Who gets blood clots?

Anyone can get a blood clot but people most at risk are those who are unwell for long periods and unable to move around very much.

Blood clots are often linked to long-haul plane journeys or the contraceptive pill, but you're much more likely to develop one after going into hospital. About two-thirds of all blood clots occur during or just after a stay in hospital.

Read more about how and why blood clots occur.

You can also use the venous thromboembolism (VTE) self-assessment tool to assess your risk of blood clots.

Deep vein thrombosis

Your risk of DVT is higher during pregnancy. Learn about the symptoms, risk factors and how it's treated

Page last reviewed: 08/09/2014

Next review due: 08/05/2017