Introduction

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.

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 1 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 1 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.

Venogram

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 2 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 (6 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

Causes

Deep vein thrombosis (DVT) sometimes occurs for no apparent reason.

However, the risk of developing DVT is increased in certain circumstances.

Inactivity 

When you're inactive your blood tends to collect in the lower parts of your body, often in your lower legs. This is usually nothing to worry about because when you start to move, your blood flow increases and moves evenly around your body.

However, if you're unable to move for a long period of time – such as after an operation, because of an illness or injury, or during a long journey – your blood flow can slow down considerably. A slow blood flow increases the chances of a blood clot forming.

In hospital

If you have to go into hospital for an operation or procedure, your risk of getting a blood clot increases. This is because DVT is more likely to occur when you're unwell or inactive, or less active than usual.

As a patient, your risk of developing DVT depends on the type of treatment you're having. You may be at increased risk of DVT if any of the following apply:

  • you're having an operation that takes longer than 90 minutes, or 60 minutes if the operation is on your leg, hip or abdomen
  • you're having an operation for an inflammatory or abdominal condition, such as appendicitis
  • you're confined to a bed, unable to walk, or spending a large part of the day in a bed or chair for at least three days

You may also be at increased risk of DVT if you're much less active than usual because of an operation or serious injury and have other DVT risk factors, such as a family history of the condition.

When you're admitted to hospital you'll be assessed for your risk of developing a blood clot and, if necessary, given preventative treatment.

Blood vessel damage

If the wall of a blood vessel is damaged, it may become narrowed or blocked, which can cause a blood clot to form.

Blood vessels can be damaged by injuries such as broken bones or severe muscle damage. Sometimes, blood vessel damage that occurs during surgery can cause a blood clot, particularly in operations on the lower half of your body.

Conditions such as vasculitis (inflammation of the blood vessels), varicose veins and some forms of medication, such as chemotherapy, can also damage blood vessels.

Medical and genetic conditions

Your risk of getting DVT is increased if you have a condition that causes your blood to clot more easily than normal. These conditions include:

  • cancer – cancer treatments such as chemotherapy and radiotherapy can increase this risk further
  • heart disease and lung disease
  • infectious conditions, such as hepatitis
  • inflammatory conditions, such as rheumatoid arthritis
  • thrombophilia – a genetic condition where your blood has an increased tendency to clot
  • antiphospholipid syndrome – an immune system disorder that causes an increased risk of blood clots 

Pregnancy

During pregnancy, blood clots more easily. It's the body's way of preventing too much blood being lost during childbirth.

Venous thromboembolism (VTE) – DVT and pulmonary embolism – affects about 1 in 100,000 women of childbearing age.

DVTs are also rare in pregnancy, although pregnant women are up to 10 times more likely to develop thrombosis than non-pregnant women of the same age. A clot can form at any stage of pregnancy and up to 6 weeks after the birth.

Having thrombophilia (a condition where the blood has an increased tendency to clot), or having a parent, or brother or sister, who's had a thrombosis, increase your risk of developing a DVT during pregnancy.

Other risk factors during pregnancy include:

  • being over 35 years old
  • being obese (with a BMI of 30 or more)
  • expecting 2 or more babies
  • having recently had a caesarean section
  • being immobile for long periods of time
  • smoking (find out how to stop smoking)
  • having severe varicose veins
  • dehydration

Low molecular weight heparin (LMWH) is usually used to treat pregnant women with DVT. LMWH is an anticoagulant, which means it prevents the blood clot getting bigger. It's given by injection and doesn't affect your developing baby.

Contraceptive pill and HRT

The combined contraceptive pill and hormone replacement therapy (HRT) both contain the female hormone oestrogen. Oestrogen causes the blood to clot a bit more easily than normal, so your risk of getting DVT is slightly increased. There's no increased risk from the progestogen-only contraceptive pill.

Other causes

Your risk of getting DVT is also increased if you or a close relative have previously had DVT and:

  • you're overweight or obese
  • you smoke
  • you're dehydrated
  • you're over 60 – particularly if you have a condition that restricts your mobility

Treatment

If you have deep vein thrombosis (DVT), you'll need to take a medicine called an anticoagulant.

Anticoagulation

Anticoagulant medicines prevent blood clots getting bigger. They can also help stop part of the blood clot breaking off and becoming lodged in another part of your bloodstream (an embolism).

Although they're often referred to as "blood-thinning" medicines, anticoagulants don't actually thin the blood. They alter proteins within it, which prevents clots forming so easily.

Heparin and warfarin are 2 types of anticoagulants that are used to treat DVT. Heparin is usually prescribed first because it works immediately to prevent further clotting. After this initial treatment, you may also need to take warfarin to prevent another blood clot forming.

Heparin

Heparin is available in 2 different forms:

  • standard (unfractioned) heparin
  • low molecular weight heparin (LMWH)

Standard (unfractioned) heparin can be given as:

  • an intravenous injection – an injection straight into one of your veins
  • an intravenous infusion – where a continuous drip of heparin (via a pump) is fed through a narrow tube into a vein in your arm (this must be done in hospital)
  • a subcutaneous injection – an injection under your skin

LMWH is usually given as a subcutaneous injection.

The dose of standard (unfractionated) heparin to treat a blood clot varies significantly from person to person, so the dosage must be carefully monitored and adjusted if necessary. You may need to stay in hospital for 5 to 10 days and have frequent blood tests to ensure you receive the right dose.

LMWH works differently from standard heparin. It contains small molecules, which means its effects are more reliable and you won't have to stay in hospital and be monitored.

Both standard heparin and LMWH can cause side effects, including:

  • a skin rash and other allergic reactions
  • bleeding 
  • weakening of the bones if taken for a long time (although rare with LMWH)

In rare cases, heparin can also cause an extreme reaction that makes existing blood clots worse and causes new clots to develop. This reaction, and weakening of your bones, is less likely to occur when taking LMWH.

In most cases, you'll be given LMWH because it's easier to use and causes fewer side effects.

Warfarin

Warfarin is taken as a tablet. You may need to take it after initial treatment with heparin to prevent further blood clots occurring. Your doctor may recommend that you take warfarin for 3 to 6 months. In some cases, it may need to be taken for longer, even for life.

As with standard heparin, the effects of warfarin vary from person to person. You'll need to be closely monitored by having frequent blood tests to ensure you're taking the right dosage.

When you first start taking warfarin, you may need to have 2 to 3 blood tests a week until your regular dose is decided. After this, you should only need to have a blood test every 4 weeks at an anticoagulant outpatient clinic.

Warfarin can be affected by your diet, any other medicines you're taking, and by how well your liver is working.

If you're taking warfarin you should:

  • keep your diet consistent
  • limit the amount of alcohol you drink (no more than 14 units of alcohol a week)
  • take your dose of warfarin at the same time every day
  • not start to take any other medicine without checking with your GP, pharmacist or anticoagulant specialist
  • not take herbal medicines

Warfarin isn't recommended for pregnant women who are given heparin injections for the full length of treatment.

Rivaroxaban

Rivaroxaban is a medication recommended by the National Institute for Health and Care Excellence (NICE) as a possible treatment for adults with DVT, or to help prevent recurrent DVT and pulmonary embolism.

Rivaroxaban comes in tablet form. It's a type of anticoagulant known as a directly acting oral anticoagulant (DOAC). It prevents blood clots forming by inhibiting a substance called factor Xa and restricting the formation of thrombin (an enzyme that helps blood clot).

Treatment usually lasts 3 months and involves taking rivaroxaban twice a day for the first 21 days and then once a day until the end of the course.

Read the NICE guidance about rivaroxaban

Apixaban

NICE also recommends apixaban as a possible method of treatment and prevention for DVT and pulmonary embolism.

Like rivaroxaban, apixaban is a DOAC that's taken orally as a tablet, and prevents blood clots forming by hindering factor Xa and restricting the formation of thrombin.

Treatment usually lasts at least 3 months and involves taking apixaban twice a day.

Read the NICE guidance about apixaban

Compression stockings

Wearing compression stockings helps prevent calf pain and swelling, and lowers the risk of ulcers developing after having DVT.

They can also help prevent post-thrombotic syndrome. This is damage to leg tissue caused by the increase in venous pressure that occurs when a vein is blocked by a clot and blood is diverted to the outer veins.

After having DVT, stockings should be worn every day for at least 2 years. This is because symptoms of post-thrombotic syndrome may develop several months or even years after having a DVT.

Compression stockings should be fitted professionally and your prescription should be reviewed every 3 to 6 months. The stockings need to be worn all day but can be taken off before going to bed or in the evening while you rest with your leg raised. A spare pair of compression stockings should also be provided.

Exercise

Your healthcare team will usually advise you to walk regularly once compression stockings have been prescribed. This can help prevent symptoms of DVT returning and may help to improve or prevent complications of DVT, such as post-thrombotic syndrome.

Raising your leg

As well as wearing compression stockings, you might be advised to raise your leg whenever you're resting. This helps to relieve the pressure in the veins of the calf and stops blood and fluid pooling in the calf itself.

When raising your leg, make sure your foot is higher than your hip. This will help the returning blood flow from your calf. Putting a cushion underneath your leg while you're lying down should help raise your leg above the level of your hip.

You can also slightly raise the end of your bed to ensure that your foot and calf are slightly higher than your hip.

Read more about preventing DVT

Inferior vena cava filters

Although anticoagulant medicines and compression stockings are usually the only treatments needed for DVT, inferior vena cava (IVC) filters may be used as an alternative. This is usually because anticoagulant treatment needs to be stopped, isn't suitable, or isn't working.

IVC filters are small mesh devices that can be placed in a vein. They trap large fragments of a blood clot and stop it travelling to the heart and lungs. They can be used to help prevent blood clots developing in the legs of people diagnosed with:

  • DVT
  • pulmonary embolism
  • multiple severe injuries

IVCs can be placed in the vein permanently, or newer types of filters can be placed temporarily and removed after the risk of a blood clot has decreased.

The procedure to insert an IVC filter is carried out using a local anaesthetic (where you're awake but the area is numb). A small cut is made in the skin and a catheter (thin, flexible tube) is inserted into a vein in the neck or groin area. The catheter is guided using an ultrasound scan. The IVC filter is then inserted through the catheter and into the vein.

Complications

The two main complications of deep vein thrombosis (DVT) are pulmonary embolism and post-thrombotic syndrome.

Pulmonary embolism

A pulmonary embolism is the most serious complication of DVT. It happens when a piece of blood clot (DVT) breaks off and travels through your bloodstream to your lungs, where it blocks one of the blood vessels. In severe cases this can be fatal.

If the clot is small, it might not cause any symptoms. If it's medium-sized, it can cause chest pain and breathing difficulties. A large clot can cause the lungs to collapse, resulting in heart failure, which can be fatal.

About one in 10 people with an untreated DVT develops a severe pulmonary embolism.

Post-thrombotic syndrome

If you've had a DVT, you may develop long-term symptoms in your calf known as post-thrombotic syndrome. This affects around 20-40% of people with a history of DVT.

If you have DVT, the blood clot in the vein of your calf can divert the flow of blood to other veins, causing an increase in pressure. This can affect the tissues of your calf and lead to symptoms, including:

When a DVT develops in your thigh vein, there's an increased risk of post-thrombotic syndrome occurring. It's also more likely to occur if you're overweight or if you've had more than one DVT in the same leg.

Prevention

If you're admitted to hospital or planning to go into hospital for surgery, your risk of developing a blood clot while you're there will be assessed.

Surgery and some medical treatments can increase your risk of developing DVT – see causes of DVT for more information.

If you're thought to be at risk of developing DVT, your healthcare team can take a number of measures to prevent a blood clot forming.

Before going into hospital

If you're going into hospital to have an operation, and you're taking the combined contraceptive pill or hormone replacement therapy (HRT), you'll be advised to temporarily stop taking your medication four weeks before your operation.

Similarly, if you're taking medication to prevent blood clots, such as aspirin, you may be advised to stop taking it one week before your operation.

There's less risk of getting DVT when having a local anaesthetic compared with a general anaesthetic. Your doctor will discuss whether it's possible for you to have a local anaesthetic.

While you're in hospital

There are a number of things your healthcare team can do to help reduce your risk of getting DVT while you're in hospital.

For example, they'll make sure you have enough to drink so you don't become dehydrated, and they'll also encourage you to move around as soon as you're able to.

Depending on your risk factors and individual circumstances, a number of different medications can be used to help prevent DVT. For example:

  • anticoagulant medicines – such as dabigatran etexilate or fondaparinux sodium, which are often used to help prevent blood clots after certain types of surgery, including orthopaedic surgery
  • low molecular weight heparin (LMWH) – often used in many cases to help prevent blood clots, including during and shortly after pregnancy
  • unfractionated heparin (UFH) – often used in people with severe kidney impairment or established kidney failure 

Compression stockings or compression devices are also commonly used to help keep the blood in your legs circulating.

Compression stockings are worn around your feet, lower legs and thighs, and fit tightly to encourage your blood to flow more quickly around your body.

Compression devices are inflatable and work in the same way as compression stockings, inflating at regular intervals to squeeze your legs and encourage blood flow.

Your healthcare team will usually advise you to walk regularly after you've been prescribed compression stockings. Keeping mobile can help prevent the symptoms of DVT returning and may help prevent or improve complications of DVT, such as post-thrombotic syndrome.

Read more about treating DVT

When you leave hospital

You may need to continue to take anticoagulant medicine and wear compression stockings when you leave hospital.

Before you leave, your healthcare team should advise you about how to use your treatment, how long to continue using it for, and who to contact if you experience any problems.

Lifestyle changes

You can reduce your risk of getting DVT by making changes to your lifestyle, such as:

Travelling

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

If you're planning a long-distance plane, train or car journey (journeys of six hours or more), make sure you:

  • drink plenty of water
  • avoid drinking excessive amounts of alcohol because it can cause dehydration
  • avoid taking sleeping pills because they can cause immobility
  • perform simple leg exercises, such as regularly flexing your ankles
  • take occasional short walks when possible – for example, during refuelling stopovers
  • wear elastic compression stockings

Read more about preventing DVT when you travel

Travel insurance

If you're travelling abroad, it's very important to ensure you're prepared should you or a family member fall ill.

Make sure you have full travel insurance to cover the cost of any healthcare you may need while abroad. This is particularly important if you have a pre-existing medical condition, such as cancer or heart disease, which may increase your risk of developing DVT.

DVT can be a very serious condition, and it's important you receive medical assistance as soon as possible. Treating DVT promptly will help minimise the risk of complications.