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A deep vein thrombosis (DVT) is a blood clot in a vein. Blood clots in veins most often occur in the legs but can occur elsewhere in the body, including the arms. This leaflet is about blood clots in leg veins.

The most common cause of a blood clot developing in a vein is immobility. A complication can occur in some cases where part of the blood clot breaks off and travels to the lung (pulmonary embolus). This is usually prevented if you are given anticoagulation treatment.

What is deep vein thrombosis?

Dr Sarah Jarvis MBE

A deep vein thrombosis (DVT) is a blood clot that develops in one of your deep veins. It usually occurs in your leg and makes that leg become very painful and swollen. The clot may travel up to your lungs to cause a pulmonary embolus.

Diagram of leg showing veins and a Deep Vein Thrombosis

Deep leg veins are the larger veins that go through the muscles of the calf and thighs. They are not the veins that you can see just below the skin; neither are they the same as varicose veins. When you have a DVT, the blood flow in the vein is partially or completely blocked by the blood clot.

A calf vein is the common site for a DVT. A thigh vein is less commonly affected. Rarely, other deep veins in the body can be blocked by blood clots.

A DVT is part of a group of problems together known as venous thromboembolism.

Venous means related to veins. A thrombosis is a blockage of a blood vessel by a blood clot (a thrombus). Embolism occurs when the thrombus dislodges from where it formed and travels in the blood. It then becomes stuck in a narrower blood vessel, elsewhere in the body. The thrombus is then called an embolus.

A pulmonary embolus occurs when a thrombus has broken off from a DVT and become stuck in one of the blood vessels in the lung. Pulmonary emboli (plural of 'embolus') are also part of venous thromboembolism. See the separate leaflet called Pulmonary Embolism for more details.

Blood normally flows quickly through veins and does not usually become solid (clot). Blood flow in leg veins is helped along by leg movements, because muscle action squeezes the veins. Sometimes a DVT occurs for no apparent reason. However, the following increase your risk of having a DVT:

  • Immobility which causes blood flow in the veins to be slow. Slow-flowing blood is more likely to clot than normal-flowing blood:
    • A surgical operation where you are asleep (under general anaesthetic) is the most common cause of a DVT. Your legs are still when you are under anaesthetic, because the muscles in your body are temporarily paralysed. Blood flow in the leg veins can become very slow, making a clot more likely to occur. Certain types of surgery (particularly operations on the pelvis or legs) increase the risk of DVT even more.
    • Any illness or injury that causes immobility increases the risk. This includes having a leg in a hard plaster cast after a fracture. People who are admitted to intensive care units are at an increased risk of DVT. This is due to a number of reasons but partly because they are very ill and also because they are immobile (they may even be kept asleep by anaesthetic medications).
    • Long journeys by plane, train or coach/car may cause a slightly increased risk. This is because you are mostly sitting still and not moving around very much.
  • Damage to the inside lining of the vein increases the risk of a blood clot forming. For example, a DVT may damage the lining of the vein. So, if you have a DVT then you have an increased risk of having another one in the future. Some conditions, such as inflammation of the vein wall (vasculitis) and some medicines (for example, some chemotherapy medicines), can damage the vein and increase the risk of having a DVT. Damage to the vein can also happen with injury to the vein caused by a needle. This might occur after treatment with a drip in hospital (where a tube is inserted into a vein to get fluids into you). Drug users who inject drugs, such as heroin, can also damage their veins, making DVTs more common. This is especially the case if they inject illegal drugs into their leg or groin.
  • Conditions that cause the blood to clot more easily than normal (thrombophilia) can increase the risk. Some conditions can cause the blood to clot more easily than usual. Examples include nephrotic syndrome and antiphospholipid syndrome. See the separate leaflets called Nephrotic Syndrome, Antiphospholipid Syndrome and Thrombophilia for more details. Some rare inherited conditions can also cause the blood to clot more easily than normal. An example of an inherited blood disorder which can cause DVTs is factor V Leiden mutation. You are more at risk of a DVT if you have a family history of DVTs - that is, a close relative who has had one.
  • Contraceptive pills, patches and rings and hormone replacement therapy (HRT) which contain oestrogen can cause the blood to clot slightly more easily. Women taking the combined oral contraceptive (COC) pill or HRT have a small increased risk of DVT.
  • People with cancer or heart failure have an increased risk. Sometimes a DVT happens in a person who has not yet been diagnosed with cancer. Investigations looking for the cause of a DVT may show cancer to be the underlying cause.
  • Older people (over 60 years of age) are more likely to have a DVT, particularly if they have poor mobility or have a serious illness such as cancer.
  • Pregnancy increases the risk. About 1 in 1,000 pregnant women have a DVT while they are pregnant, or within about six months after they give birth.
  • Obesity also increases the risk. If your body mass index (BMI) is more than 30 kg/m2, you are more at risk of DVT.
  • Lack of fluid in the body (dehydration) can make a DVT more likely to happen. Effectively, the blood becomes stickier and more liable to clot.

It is estimated that about 1 in 1,000 people have a DVT each year in the UK.

A DVT most commonly develops in a deep vein below the knee in the calf. Typical DVT symptoms include:

  • Pain and tenderness of the calf.
  • Swelling of the calf.
  • Colour and temperature changes of the calf. Blood that would normally go through the blocked vein is diverted to outer veins. The calf may then become warm and red.

Sometimes there are no symptoms and a DVT is only diagnosed if a complication occurs, such as a pulmonary embolus (PE) - see below.

Sometimes it is difficult for a doctor to be sure of the diagnosis from just your symptoms, as there are other causes of a painful and swollen calf. Examples of conditions that can cause similar symptoms are muscle strains or skin infections (cellulitis). Your doctor might calculate something called a Wells' score to work out the likelihood of you having a DVT. It involves looking at your symptoms and risk factors for a DVT.

If you have a suspected DVT, you will normally be advised to have tests done urgently to confirm or rule out the diagnosis. Two commonly used tests are:

  • D-dimer blood test. This detects fragments of breakdown products of a blood clot. The higher the level, the more likely you are to have a blood clot in a vein. Unfortunately, the test can be positive in a number of other situations, such as if you have had recent surgery or if you are pregnant. A positive test does not, therefore, diagnose a DVT. The test may, however, indicate how likely it is that you have a blood clot (the clot can be either a DVT or a PE) and can help decide if further tests are needed.
  • A special type of ultrasound called a duplex Doppler is used to show blood flow in the leg veins and any blockage to blood flow. Ultrasound is useful because it is an easy, non-invasive test.

Sometimes these tests are not conclusive and more detailed tests are necessary. Contrast venography is another test that can be done. In this test a dye is injected into the leg veins. X-ray tests can then detect the dye which is shown not to be flowing if a vein is blocked by a clot. Computerised tomography (CT) scans and magnetic resonance imaging (MRI) scans are also very occasionally needed.

Sometimes, especially if there is a delay in getting a scan, you may be given daily injections to thin your blood. This is, in effect, treating you as if you do have a DVT, even though it has not been proven. This is safer than doing nothing whilst waiting for a scan.

Editor's note

Dr Sarah Jarvis, 17th September 2021

Quality standards for DVT

The National Institute for Health and Care Excellence (NICE) has issued new guidance on what people with a possible DVT can expect on the NHS. They recommend:

  1. If you're over 16 years old and have an assessment which suggests you need medication to prevent a DVT, this should be started as soon as possible (maximum 14 hours after hospital admission).
  2. If you're over 16 and have been in hospital for a problem that prevents you moving your legs much, you should have an assessment to see if you need medication to prevent a DVT.
  3. If you're over 18 and your doctor's assessment suggests you may have DVT in your upper leg, you should have an ultrasound scan within four hours.
  4. If you're over 18 and taking medicine to treat a DVT, you should be reviewed after three months (and at least every year if your doctor advises you to take it long-term).
  5. You should have an agreed follow-up plan in place if you have, or are suspected to have, a clot on the lung (pulmonary embolus) and aren't admitted to hospital.

Editor's note

Dr Sarah Jarvis, 3rd February 2021

New guidance on possible complication of DVT

When you see your doctor with symptoms of a possible DVT, they will check to see if there is any evidence that you have a clot on the lung (a pulmonary embolus, or PE). NICE has updated its guidance on treatment for people with suspected DVT or PE.

It now recommends that if your doctor suspects you might have a PE, and it is likely to take at least four hours to have tests to check, you should be given a dose of anticoagulant (see below) while you wait to have tests.

It can be. When a blood clot forms in a leg vein it usually remains stuck to the vein wall. The symptoms tend to settle gradually. However, there are two main possible complications:

  • A blood clot that travels to the lung (pulmonary embolus).
  • Persistent calf symptoms (post-thrombotic syndrome).

Pulmonary embolus (PE)

In a small number of people who have a DVT, a part of the blood clot breaks off. This travels in the bloodstream and is called an embolus. An embolus will travel in the bloodstream until it becomes stuck. An embolus that comes from a clot in a leg vein will be carried up the larger leg and body veins to the heart, through the large heart chambers, but will get stuck in a blood vessel going to a lung. This is called a pulmonary embolus.

DVTs and PEs are known collectively as venous thromboembolism
A small PE may not cause any symptoms. A medium-sized PE can cause breathing problems and chest pain. A large PE can cause collapse and sudden death. It is estimated that if people with DVT are not treated, about half of them would develop a PE large enough to cause symptoms or death.

Post-thrombotic syndrome

Without treatment, up to 4 in 10 people who have a DVT develop long-term symptoms in the calf. This is called post-thrombotic syndrome. Symptoms occur because the increased flow and pressure of the diverted blood into other veins can affect the tissues of the calf. Symptoms can range from mild to severe and include calf pain, discomfort, swelling and rashes. An ulcer on the skin of the calf may develop in severe cases.

Post-thrombotic syndrome is more likely to occur if the DVT occurs in a thigh vein or extends up into a thigh vein from a calf vein. It is also more common in people who are overweight and in those who have had more than one DVT in the same leg.

The aims of treatment are:

  • To prevent the clot spreading up the vein and getting larger. This may prevent a large embolus breaking off and travelling to the lungs (a PE).
  • To reduce the risk of post-thrombotic syndrome developing.
  • To reduce the risk of venous ulcers in the leg in future. This can happen to people who have developed post-thrombotic syndrome.
  • To reduce the risk of a further DVT in the future.

Anticoagulation - preventing the clot from becoming larger

Anticoagulation is often called thinning the blood. Medicines which work in this way are called anticoagulants. However, they do not actually thin the blood. They alter certain chemicals in the blood to stop clots forming so easily. Anticoagulants do not dissolve the clot. Anticoagulation prevents a DVT from getting larger and prevents any new clots from forming. The body's own healing mechanisms can then get to work to break up the clot.

If you have a DVT, you will usually need an anticoagulant medicine for at least three months. There are a number of options:

  • Warfarin has been the usual anticoagulant for many years. The aim is to get the dose of warfarin just right so the blood will not clot easily, but not too much, which may cause bleeding problems. You will need regular blood tests (called INRs) whilst you take warfarin. The INR (which stands for International Normalised Ratio) is a blood test that measures your blood clotting ability. You need the tests quite often at first but then less frequently once the correct dose is found. An INR of 2.5 is the aim if you have warfarin for a DVT, although anywhere in the range 2-3 is OK. If you have had recurrent DVTs, or have had a PE whilst on warfarin, you might need a higher INR (even 'thinner' blood). Recently, some alternatives to warfarin have become available, which do not need regular blood tests.
  • Rivaroxaban, apixaban, edoxaban and dabigatran are newer anticoagulant medicines. You do not have to have regular blood tests to monitor your blood clotting. This is an advantage over warfarin. However, there is no antidote (as there is with warfarin) to stop you bleeding too easily. These newer tablets are not suitable for everybody.

NICE recommends:

  • Apixaban or rivaroxaban for confirmed DVT or PE.
  • If neither apixaban nor rivaroxaban is suitable, then either:
    • Low molecular weight heparin (LMWH) injections for at least 5 days followed by dabigatran or edoxaban, or
    • LMWH injections with Warfarin for at least 5 days, followed by Warfarin on its own.

A serious embolus is rare if you start anticoagulation treatment early after a DVT.

If you are pregnant, regular heparin injections rather than anticoagulant tablets may be used. This is because anticoagulant medicines can potentially cause harm (birth defects) to the unborn child.

The length of time you will be advised to take anticoagulation depends on various factors. If you have a DVT during pregnancy or after an operation, then after the birth, or when you are fit again, the increased risk is much reduced. So, anticoagulation may be only for a few months. On the other hand, some people continue to have an increased risk of having a DVT. In this case, the anticoagulation may be long-term.

As a guide, for a DVT that happens below the knee, you will need at least six weeks of anticoagulant treatment. You will need at least three months of treatment if the DVT is above the knee. Some people at higher risk of another DVT may need longer than this. The length of time of anticoagulation varies from person to person. Your doctor or anticoagulant clinic will advise you how long your treatment will be for.

Compression stockings

Most people who develop a DVT are advised to wear compression stockings. Compression stockings are also known as compression hosiery or thromboembolic deterrent stockings ('TEDs'). This treatment has been shown to reduce the risk of developing another DVT and can also reduce the risk of developing post-thrombotic syndrome. You should wear the stockings each day, for at least two years. If you do develop post-thrombotic syndrome, you may be advised to wear the stockings for more than two years.

The stockings work by applying pressure from outside the veins. This helps squeeze the blood in the veins back up the legs. This then makes the pressure inside the veins less. In turn this prevents leakage of blood into the surrounding tissues.

Before compression stockings are advised you will need to have a test to check that the circulation through the arteries of your legs is normal. This is usually done by a nurse, with a handheld machine called a Doppler. This measures the pressure in your arteries.

Compression stockings come in light, medium or strong material (class 1, 2 or 3) and are worn below the knee. Most people can tolerate class 2. If class 2 is too uncomfortable or difficult, class 1 is prescribed. If class 2 stockings do not work it may be necessary to try class 3, although many people find these difficult to wear for any length of time. These can be prescribed by your doctor, and obtained at the chemist. People often do not like wearing these stockings, because they are uncomfortable, or difficult to put on, or don't look very nice. But they really help your circulation, and can prevent serious problems such as ulcers.

Further tips on support stockings

  • They are available in different colours, so do ask for the one that suits you best. That way, you are more likely to use them.
  • They should be removed at bedtime and put on first thing in the morning. It is important to put them on before your legs start to swell in the mornings.
  • There are open or closed toe options. Open toe stockings may be useful if:
    • You have painful toes due to arthritis or infection.
    • You have large feet.
    • You want to wear socks over your support stockings.
    • You prefer them.
  • Stockings should be replaced every 3-6 months. Each time you should be measured again, just in case the size needs to be changed.
  • You should always have at least two pairs prescribed so that one pair can be washed and dried while the other is worn.
  • Do not tumble dry support stockings, as this may damage the elastic.
  • They can be made to measure if none of the standard sizes fit you. This can still be done with a prescription.
  • Support stocking applicator aids are available if you are unable to get them on. You can discuss this with your pharmacist or nurse.

Walking regularly but raising your leg whilst resting

  • Unless your doctor advises against this, you should walk regularly after you are discharged from hospital. Walking is thought to improve circulation in the affected leg and may help to reduce your risk of further DVT.
  • When you are resting, as much as possible - raise your leg. This reduces the pressure in the calf veins and helps to prevent blood and fluid from pooling in the calves. Raised means that your foot is higher than your hip so gravity helps with blood flow returning from the calf. The easiest way to raise your leg is to recline on a sofa with your leg up on a cushion. Sitting on the sofa or in a chair, with your feet on a footstool or pouffe, is not keeping your feet up - the feet are well below your hips in this position.
  • Raise the foot of the bed a few inches if it is comfortable to sleep like this. This is so your foot and calf are slightly higher than your hip when you are asleep.

Other treatments

Sometimes other treatments may be considered - for example:

  • Clot-busting (thrombolytic) therapy with medicines such as streptokinase or urokinase. These may help to dissolve a blood clot. The medicine is given directly into the vein and sometimes directly to the clot by a tube (catheter). Studies report this treatment results in fewer people developing post-thrombotic syndrome. It is not yet known if it makes people less likely to have a PE, or another DVT.
  • Sometimes an operation (called an embolectomy) is done to remove a blood clot from the leg vein or pulmonary artery. These operations are not routine and it is not clear if they are an effective treatment in most cases.
  • Occasionally, an operation is done to place a filter in the large vein above the blocked leg vein. The aim is to stop any blood clots from travelling up to the lungs. This may be considered if anticoagulation cannot be given (for various reasons), or if anticoagulation fails to prevent clots breaking off and travelling up into the larger veins and up to the lungs.

A DVT is often just a one-off event after a major operation.

However, some people who develop a DVT have an ongoing risk of a further DVT - for example, if you have a blood clotting problem, or continued immobility. As mentioned above, you may be advised to take anticoagulation medicine long-term. Your doctor will advise you about this.

Other things that may help to prevent a first or recurrent DVT include the following:

  • If possible, avoid long periods of immobility, such as sitting in a chair for many hours. If you are able to, get up and walk around now and then. A daily brisk walk for 30-60 minutes is even better if you can do this. The aim is to stop the blood pooling and to get the circulation in the legs moving. Regular exercise of the calf muscles also helps. You can do some calf exercises even when you are sitting.
  • Major operations are a risk for a DVT - particularly operations to the hip, lower tummy (abdomen) and leg. There are a number of methods to help reduce this risk:
    • To help prevent a DVT, you may be given an anticoagulant such as a heparin injection just before an operation. This is called prophylaxis. Enoxaparin and dalteparin are the most common types of heparin given for prophylaxis against blood clots. The new medicine, fondaparinux sodium, can also be given by injection in some circumstances to prevent DVT in surgical patients or immobile medical patients in hospital.
    • The newer anticoagulant medicines discussed above can be used to prevent DVT or PE after hip or knee replacement surgery. Rivaroxaban, apixaban and dabigatran are used in these situations, and can be given by mouth as a tablet, rather than injection.
    • An inflatable sleeve connected to a pump to compress the legs during a long operation may also be used.
    • You may also be given compression stockings to wear whilst you are in hospital.
    • It is now common practice to get you up and walking as soon as possible after an operation.
  • When you travel on long plane, train, car or coach journeys, you should have little walks up and down the aisle every now and then. Try to exercise your calf muscles whilst sitting in your seat. (You can do this by circling your ankles, getting into a 'tiptoe' position and lifting your toes off the floor whilst keeping your heels on the ground.) You should aim to stay well-hydrated and avoid alcohol and sleeping medications. See the separate leaflet called Preventing DVT When You Travel for more details. If you have had a previous DVT, you should see your doctor for advice before you travel on a long journey or fly.
  • People who are overweight have an increased risk of DVT. Therefore, to reduce your risk, you should try to lose weight.

Reducing the risk of having a DVT in hospital

There has been increased awareness of patients in hospital being at risk of a DVT. This has led to National Institute for Health and Care Excellence (NICE) recommendations on reducing the risk of having a DVT in hospital - see Further Reading below. The purpose of the guideline is to protect people in hospital from blood clots by making sure that:

  • Everyone has their risk of blood clots checked when they are admitted to hospital (including mental health units), whatever type of treatment they are having.
  • Each person's risk check is used to decide whether they need treatment to prevent clots - for example, blood-thinning medicines, compression stockings or foot pumps.
  • If people need blood-thinning medicines to stop clots, they have their risk of bleeding checked first.
  • Staff explain how important it is for people to stick to their treatments after leaving hospital, because a blood clot can develop weeks later.
  • The main cause of DVT is immobility - especially during or after surgery.
  • If you are having major surgery, you will usually be given medication with injections and/or tablets, to prevent blood clots forming. This is called thromboprophylaxis.
  • The most serious complication of DVT is a PE, where part of the blood clot breaks off and travels to the lung. PEs can cause death.
  • Persistent calf symptoms may occur after a DVT.
  • With treatment, the risk of the above two complications is much reduced.
  • Treatment includes anticoagulation medicines, compression stockings, leg elevation when sitting, and keeping active.
  • Prevention is important if you have an increased risk of DVT - for example, during long operations or when you travel on long journeys.

Further reading and references