Thrombophilia means that blood has an increased tendency to form clots.

You're more likely to develop a blood clot in one of the large veins in your leg (deep vein thrombosis) or a pulmonary embolism, where the blood clot breaks off, travels in the circulation and lodges in the arteries supplying the lungs.

How thrombophilia affects blood clotting

When you cut yourself and injure a blood vessel, tiny cells called platelets stick to the damaged vessel wall to form a plug.

Proteins in the blood called clotting factors cause strands called fibrin to form around the plug. These strands get tangled up with the platelet plug to form an even stronger blood clot.

If you have thrombophilia, you have an imbalance in clotting chemicals. You either have too much or too little of the substance that stops clotting (clotting factor).

Symptoms of thrombophilia

Most people with thrombophilia don't have symptoms and never have health problems. Symptoms only occur if thrombophilia causes a blood clot.

If you have thrombophilia, you're at increased risk of developing a DVT or pulmonary embolism. 

Warning signs of DVT include:

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

DVT usually only affects one leg, though not always. 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 can be dangerous because the clot becomes lodged in the lungs.

Known as a pulmonary embolism, this serious and potentially life-threatening condition can prevent blood reaching your lungs.

The symptoms of a pulmonary embolism are:

• chest or upper back pain
• shortness of breath
• coughing – usually dry, but you may cough up blood or mucus containing blood
• lightheadedness or dizziness
• fainting

See your GP immediately if you have any combination of the above symptoms. You can also call NHS 111 or your local out-of-hours service for advice. Dial 999 for an ambulance if your symptoms are severe.

Diagnosing thrombophilia

If you develop a blood clot, you may be tested for thrombophilia a few weeks or months later. A blood sample is taken to look for chemical imbalances.

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

Current tests for thrombophilia have limitations. They may be able to help identify the condition, but they can't always determine the cause of an increased blood clotting tendency.

Types of thrombophilia

There are many different types of thrombophilia. Some types are inherited, while other types develop later in life. The main types of thrombophilia are outlined below.

Factor V Leiden

Factor V Leiden is a type of thrombophilia caused by a faulty gene. It's the most common type of inherited thrombophilia, and tends to be seen in white Europeans and Americans.

It increases the risk of developing a DVT at some point in life, but the majority of carriers of the gene are never affected.

Genetics Home Reference has more information about factor V Leiden thrombophilia.

Prothrombin 20210

Prothrombin 20210, or the prothrombin gene mutation, is another type of thrombophilia caused by inheriting a faulty gene.

Prothrombin is a protein in the blood that helps it clot. People who have the faulty gene produce too much prothrombin. This results 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.

Genetics Home Reference has more information about prothrombin thrombophilia.

Protein C, protein S and antithrombin deficiency

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

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

Low levels of protein C, protein S or antithrombin can be inherited, but are rare.

Genetics Home reference has more information about protein C deficiency, protein S deficiency and antithrombin deficiency.

Antiphospholipid syndrome

Antiphospholipid syndrome, also known as Hughes syndrome, is an immune system disorder that can develop in later life.

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

The antibodies bind to the phospholipids, increasing your risk of a blood clot. Unlike the inherited thrombophilias, blood clots in people with antiphospholipid syndrome can occur in a vein or artery.

Women with antiphospholipid syndrome have an increased risk of complications during pregnancy, such as miscarriage, stillbirth, high blood pressure in pregnancy (pre-eclampsia), and small babies.

Treating thrombophilia

Many people with thrombophilia won't need treatment. You'll only need treatment if you develop a blood clot or you're at risk of developing a clot.

This will depend on the type of thrombophilia you have and factors such as your age, weight, lifestyle and family history.

You may need to take warfarin tablets or have an injection of heparin. Newer oral anticoagulants are also now available, and are sometimes used instead of warfarin to treat DVT and pulmonary embolism.

Warfarin and heparin

Warfarin and heparin are anti-clotting medicines called anticoagulants. They interfere with the clotting process and can be used to treat or prevent DVT and pulmonary embolism.

You may be be prescribed warfarin if you need an anticoagulant to treat a clot and prevent another one occurring. It takes a few days to work properly.

If you have a clot and need immediate treatment, you'll usually be given heparin injections for a few days alongside warfarin – the heparin injections will work straight away.

The injections will either be given in hospital or at home. You no longer need to have an injection when the warfarin tablets start working properly.

A heparin injection may be given on its own to prevent clots forming, and may also be used to treat people with thrombophilia or antiphospholipid syndrome before and after surgery or during pregnancy.

Unlike warfarin, heparin is safe to take in pregnancy. Both warfarin and heparin are safe to use while breastfeeding.

International normalised ratio (INR) test

Your doctor will need to adjust your warfarin dose to just the right amount – enough to stop your blood easily clotting, but not too much that you're at risk of bleeding problems.

You'll need a regular blood test called the international normalised ratio (INR) to measure your blood clotting ability while taking warfarin.

The INR test will be needed less frequently once your ideal dose has been reached – an INR of 2-3 is usually the aim.

New oral anticoagulants

In recent years, a number of new oral anticoagulants have become available for treating and preventing blood clots. They're given in a fixed dose without the monitoring that's necessary with warfarin.

New oral anticoagulants aren't suitable for everyone and shouldn't be used during pregnancy or breastfeeding. They should only be used under the guidance of a specialist to treat people with thrombophilia.

Lifestyle advice

If you have thrombophilia, you need to be aware of the symptoms of a blood clot and see your GP immediately if you think you have one.

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

• lose weight if you're overweight
• stop smoking
• eat a healthy, balanced diet and exercise regularly
• avoid being immobile for long periods – being inactive can cause a DVT

Read more about preventing DVT.

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

You may need to take low-dose aspirin or heparin injections 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 shouldn't take the combined oral contraceptive pill or hormone replacement therapy (HRT) because it further increases the risk of developing a blood clot.

Blood clot risk

Anyone can get a blood clot, but you're most at risk if you've been unwell for long periods and unable to move around much.

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