Deep vein thrombosis (DVT) is a blood clot that develops within a deep vein in the body, usually in the leg. 

Blood clots that develop in a vein are also known as venous thrombosis.

DVT usually occurs in a deep leg vein, a larger vein that runs through the muscles of the calf and the thigh.

It can cause pain and swelling in the leg and may lead to complications such as pulmonary embolism. This is a serious condition that occurs when a piece of blood clot breaks off into the bloodstream and blocks one of the blood vessels in the lungs (see below).

DVT and pulmonary embolism together are known as venous thromboembolism (VTE).

Symptoms of DVT

In some cases, there may be no symptoms of DVT. If symptoms do occur they can 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
  • red 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 upward towards your knee.

Pulmonary embolism

If left untreated, about one in 10 people with a DVT will develop a pulmonary embolism. A pulmonary embolism is a very serious condition which causes:

  • breathlessness – which may come on gradually or suddenly
  • chest pain – which may become worse when you breathe in
  • sudden collapse 

Both DVT and pulmonary embolism require urgent investigation and treatment.

Seek immediate medical attention if you have pain, swelling and tenderness in your leg and you develop breathlessness and chest pain.

Read more about the complications of DVT.

What causes DVT?

Each year, DVT affects around one person in every 1,000 in the UK.

Anyone can develop DVT, but it becomes more common over the age of 40. As well as age, there are also a number of other risk factors, including:

  • having a history of DVT or pulmonary embolism
  • having a family history of blood clots
  • being inactive for long periods – such as after an operation or during a long journey
  • blood vessel damage – a damaged blood vessel wall can result in the formation of a blood clot
  • having certain conditions or treatments that cause your blood to clot more easily than normal – such as cancer (including chemotherapy and radiotherapy treatment), heart and lung disease, thrombophilia and Hughes syndrome
  • being pregnant – your blood also clots more easily during pregnancy
  • being overweight or obese

The combined contraceptive pill and hormone replacement therapy (HRT) both contain the female hormone oestrogen, which causes the blood to clot more easily. If you're taking either of these, your risk of developing DVT is slightly increased.

Read more about the causes of DVT.

Diagnosing DVT

See your GP as soon as possible if you think you may have DVT – for example, if you have pain, swelling and a heavy ache in your leg. They'll ask you about your symptoms and medical history.

D-dimer test

It can be difficult to diagnose DVT from symptoms alone, so your GP may advise that you have a specialised blood test called a D-dimer test.

This test detects pieces of blood clot that have been broken down and are loose in your bloodstream. The larger the number of fragments found, the more likely it is that you have a blood clot in your vein.

However, the D-dimer test isn't always reliable because blood clot fragments can increase after an operation, injury or during pregnancy. Additional tests, such as an ultrasound scan, will need to be carried out to confirm DVT.

Ultrasound scan

An ultrasound scan can be used to detect clots in your veins. A special type of ultrasound called a Doppler ultrasound can also be used to find out how fast the blood is flowing through a blood vessel. This helps doctors identify when blood flow is slowed or blocked, which could be caused by a blood clot.


A venogram may be used if the results of a D-dimer test and ultrasound scan can't confirm a diagnosis of DVT.

During a venogram, a liquid called a contrast dye is injected into a vein in your foot. The dye travels up the leg and can be detected by X-ray, which will highlight a gap in the blood vessel where a clot is stopping the flow of blood.

Treating DVT

Treatment for DVT usually involves taking anticoagulant medicines, which reduce the blood's ability to clot and stop existing clots getting bigger.

Heparin and warfarin are two types of anticoagulant often used to treat DVT. Heparin is usually prescribed first because it works immediately to prevent further clotting. After initial treatment, you may also need to take warfarin to prevent another blood clot forming.

A number of anticoagulants, known as directly acting oral anticoagulants (DOACs), may also be used to treat conditions such as DVT. These medications include rivaroxaban and apixaban, and they've been shown to be as effective as heparin and warfarin with less serious side effects.

You'll also be prescribed compression stockings to wear every day, which will improve your symptoms and help prevent complications.

Read more about treating DVT.

Preventing DVT

If you need to go into hospital for surgery, a member of your care team will assess your risk of developing a blood clot while you're there.

If you're at risk of developing DVT, there are a number of things you can do to prevent a blood clot occurring, both before you go into hospital, such as temporarily stopping taking the combined contraceptive pill, and while you're in hospital, such as wearing compression stockings.

When you leave hospital, your care team may also make a number of recommendations to help prevent DVT returning or complications developing. These may include:

There's no evidence to suggest that taking aspirin reduces your risk of developing DVT.

See your GP before embarking on long-distance travel if you're at risk of getting a DVT, or if you've had a DVT in the past.

When taking a long-distance journey (six hours or more) by plane, train or car, you should take steps to avoid getting DVT, such as drinking plenty of water, performing simple leg exercises and taking regular, short walking breaks.

Read more about preventing DVT.

Assessing risk

Surgery and some medical treatments can increase your risk of getting DVT. It's estimated that around 25,000 people who are admitted to hospital die from preventable blood clots each year.

The Department of Health has made the prevention of DVT a priority across the NHS. All patients admitted to hospital should be assessed for their risk of developing a blood clot and, if necessary, given preventative treatment. This recommendation was made by the National Institute for Health and Care Excellence (NICE) in 2010.

For more information, see the NICE guidance about venous thromboembolism: reducing the risk for patients in hospital.

Page last reviewed: 27/04/2016

Next review due: 27/04/2018