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Thrombosis is a blood clot within a blood vessel. It happens when a blood clot forms and blocks a vein or an artery, obstructing or stopping the flow of blood.

Some types of blood clot often have few or no symptoms, and are referred to as a 'silent' condition. Therefore it is important to be aware of the signs and risk factors of blood clots. There are several things that you can do to help prevent the condition from occurring.

Read more about the symptoms of blood clots.

Blood clots can be treated if they are spotted in time. However, preventing them from happening in the first place is often the preferred approach.

Making lifestyle changes, such as improving your diet, increasing the amount of exercise you do and stopping smoking if you smoke, can also help to significantly lower your risk.

Read more about preventing blood clots.

Types of blood clot

A blood clot can occur anywhere in the body’s bloodstream. There are two main types of blood clot:

  • venous thromboembolism (VTE), which is a blood clot that develops in a vein
  • arterial thrombosis, which is a blood clot that develops in an artery

Venous thromboembolism (VTE)

One of the most common types of venous thromboembolism is deep vein thrombosis (DVT). This is a blood clot in one of the deep veins of the body, usually one of the larger veins in your leg. Read more about DVT.

Sometimes, part of a newly formed blood clot can come away from its original site and travel through the bloodstream. If this occurs, the clot can become lodged in another part of the body. This is known as an embolism. A blood clot that lodges in one of the lungs is called a pulmonary embolism (PE). Read more about PE.

The term venous thromboembolism (VTE) refers to both deep vein thrombosis (DVT) and pulmonary embolism (PE).

Arterial thrombosis

Arterial thrombosis is usually associated with a condition called atherosclerosis, which causes narrowing of the arteries. Read more about atherosclerosis.

If a blood clot blocks the arteries leading to the heart and the blood supply to part of the heart is lost, this is known as a heart attack. If a blood clot blocks an artery in the brain, this will cause a stroke.

Who is at risk of blood clots?

Thrombosis is a widespread condition and a leading cause of death in the UK. Every year, 1 in every 1,000 people in the UK is affected by a blood clot in a vein.

Anyone can be affected by blood clots, although it is more common in adults over 40 years old. As well as age, a number of other factors make developing blood clots more likely. They include: 

  • your family history
  • being inactive for long periods of time
  • having damaged blood vessels as a result of infection, surgery or inflammation
  • having an increased tendency for the blood to clot due to certain medical conditions or taking the contraceptive pill

Read more about the causes of blood clots.

Going into hospital is the biggest risk factor for blood clots in a vein. Although being in an aeroplane is often considered the major cause of deep vein thrombosis (DVT), two thirds of blood clots happen in hospital. 

Pregnant woman also have an increased risk of developing a blood clot in a vein, depending on their age, weight and number of previous pregnancies. This risk continues for at least six weeks after giving birth.

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Blood clots have a number of symptoms, but in many cases you may have no symptoms at all. This is why doctors often refer to it as a silent killer. The symptoms that you have will depend on the type of blood clot and where it is located.

Venous thromboembolism (VTE)

If you have a blood clot in a vein (venous thromboembolism), your symptoms may include:

  • pain, swelling and redness or blueness in the affected leg; it may be difficult to stand or walk
  • a heavy ache in the affected area
  • an itchy skin rash in the area of the clot
  • warm skin in the area of the clot
  • prominent veins that stand out from your skin
  • a mild fever – usually a temperature no higher than 38ºC (100.4ºF)

Depending on the type of venous thromboembolism that you have, you may have more specific symptoms as well as those listed above.

Deep vein thrombosis (DVT)

Deep vein thrombosis (DVT) is a blood clot in one of the deep veins in your body, usually in your leg. However, most cases of DVT only cause pain, without a change in colour or swelling.

Read more about the symptoms of DVT.

Pulmonary embolism (PE)

If DVT is not treated, a pulmonary embolism (a blood clot that has moved away from its original site and become lodged in one of your lungs) may occur. The three most common symptoms of pulmonary embolism (PE) are:

  • shortness of breath, which comes on suddenly and continues even when you are resting
  • pain in your chest, which may be worse when you breathe in 
  • coughing up blood

Other symptoms of pulmonary embolism (PE) include:

  • fainting or collapse
  • a bluish tinge to your skin (cyanosis)
  • rapid breathing
  • rapid heartbeat (tachycardia)
  • gradual shortness of breath

Read more about the symptoms of pulmonary embolism (PE).

Arterial thrombosis

Depending on where the clot lies in your body, a blood clot in an artery (arterial thrombosis) can cause: 

Each of the above conditions has its own set of symptoms. For example, the symptoms of angina may include:

  • pain in your chest when you are physically active or stressed 
  • breathlessness or a choking feeling 
  • feeling sick or exhausted

Heart attack

A heart attack can occur when a blood clot completely blocks an artery that pumps blood to your heart. If this occurs, you may experience:

  • a crushing central chest pain or mild chest discomfort
  • shortness of breath
  • a clammy, sweaty and grey complexion
  • dizziness
  • nausea and vomiting

If you suspect that you or someone you know is having a heart attack, dial 999 immediately. Do not wait.

Read more about the symptoms of a heart attack.


Arterial thrombosis can also cause a stroke if a blood clot is blocking an artery that supplies blood to your brain. The symptoms of a stroke usually come on suddenly and may include:

  • numbness or weakness down one side, ranging in severity from weakness in your hand to complete paralysis of the whole side of your body
  • weakness in your face, which can make you drool saliva 
  • dizziness 
  • difficulty talking and understanding what others are saying 
  • problems with balance and co-ordination 
  • difficulty swallowing

If you suspect that you or someone else is having a stroke, dial 999 for an ambulance.

Read more about the symptoms of stroke.

Peripheral vascular disease

If you have a blood clot in an artery in one of your legs (peripheral vascular disease) you may have:

  • pain when exercising, usually in the lower half of your legs 
  • pain that may affect both legs, but develops in one leg before the other  
  • pale, cold skin and numbness in one of your legs
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Thrombosis occurs when a blood clot forms within a vein or an artery, slowing or stopping the flow of blood.

How the blood clots

Your blood contains cells called platelets and proteins, known as clotting factors. Together they make up the blood-clotting mechanism. 

When a blood vessel is cut, the platelets and clotting factors in your blood mesh together to form a solid clot at the site of the wound. This clot acts as a plug to stop the wound from bleeding.

Normally, the blood-clotting mechanism is only triggered when a blood vessel is damaged and bleeds, such as when you cut yourself. However, the blood may sometimes begin to clot even when a blood vessel has not been damaged. If this happens, a blood clot can form within a vein or artery (thrombosis).

Thrombosis can occur for many different reasons. The causes of the two different types of thrombosis – venous thromboembolism and arterial thrombosis – are explained below.     

Venous thromboembolism (VTE)

Venous thromboembolism (VTE) is a blood clot that develops in a vein. The three main causes for VTE are:

  • slow blood flow 
  • blood clotting too easily 
  • blood vessel damage

In some cases, a blood clot may occur due to a combination of these factors. The three main causes of a blood clot in a vein are described in more detail below.

Slow blood flow

When you are 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 around, your blood flow increases and moves evenly around your body.

If you are inactive or immobile (unable to move at all) for a long period of time, such as after an operation or on a very long journey, your blood flow can slow down considerably. A slow blood flow makes it easier for the clotting mechanism to be triggered and for a blood clot to form.

Blood clotting too easily

In some cases, your blood may be ‘stickier’ than usual, which means that it has an increased tendency to clot.

One possible reason why the blood may clot too easily is a condition called thrombophilia. Thrombophilia can be inherited (passed down through family members).

It is also possible to develop thrombophilia later in life. This can happen if you have another condition, such as Hughes syndrome (which occurs when your immune system attacks a particular fat in your blood).

Having cancer can also make your blood more likely to clot, and this can be made worse by chemotherapy and radiotherapy.

Your blood may also be more prone to clotting if you are taking the combined contraceptive pill, or some types of hormone replacement therapy (HRT). Being pregnant also makes your blood ‘stickier’, which is your body’s way of preventing too much blood loss during childbirth.

Blood vessel damage

If the wall of a blood vessel is damaged, it may become narrowed or blocked, which can result in a blood clot forming.

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

People in hospital have a higher chance of developing a blood clot. This is because they are usually confined to a bed, their veins can be damaged after trauma or surgery, and the blood thickens in unwell people and after surgery. 

Risk factors

Other risk factors for a blood clot in a vein include:

  • a previous blood clot
  • being overweight
  • being immobile
  • having an operation
  • significant injury or trauma
  • during and after pregnancy

Arterial thrombosis

Arterial thrombosis is a blood clot that develops in an artery. Most cases of arterial thrombosis are caused when an artery is damaged by atherosclerosis. Fatty deposits build up on the walls of the arteries and cause them to harden and narrow. If the artery ruptures (bursts), a blood clot may develop.

The risk factors for a blood clot in an artery include:

Read more about atherosclerosis.

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If you think you may have a blood clot in a vein or artery (thrombosis), see your GP or go to your nearest accident and emergency (A&E) department as soon as possible. You will be asked about your medical history and your symptoms.

Alternatively, as many cases of deep vein thrombosis (DVT) develop in hospital, a member of your care team may recommend that you are tested for DVT as a result of your symptoms.

Blood clot in a vein

It may take a number of different tests to confirm a diagnosis of a blood clot in a vein. These are briefly described below.

D-dimer test

When blood clots to stop a cut or wound from bleeding, other proteins in your blood act to break the clot down again.

A D-dimer blood test can tell whether or not your blood has been working to break down a blood clot.

If the D-dimer blood test is positive, it only shows that your blood has been working to break down a clot. You will need to have another test to confirm the presence of a blood clot. This is because other conditions, such as such as bleeding, infection and inflammation, can result in D-dimers in the blood.

Ultrasound scan

An ultrasound scan can be used to detect blood clots in your veins. A special type of ultrasound, known as a Doppler ultrasound, shows how fast the blood is flowing through a blood vessel. This helps doctors to identify whether the blood flow is slowed or blocked, which could be the result of a blood clot.

Read more about ultrasound scans.


A venogram may be used if the results of a D-dimer test and an ultrasound scan cannot confirm a diagnosis of deep vein thrombosis (DVT).

During a venogram your doctor or nurse will inject a dye into a vein in your foot. The dye travels up the blood vessels of your leg.

An X-ray is taken to show the blood vessels in your leg, highlighted by the dye. If there is a blood clot in your leg, the dye will not be able to flow round it, and it will show up as a ‘gap’ in your blood vessel.

Ventilation perfusion scan

A ventilation perfusion is used to diagnose a pulmonary embolism (PE). The scan works by measuring the amount of air and the blood flow in your lungs. If a part of your lung has air in it but no blood supply, it may be due to a pulmonary embolism.

Computerised tomography pulmonary angiogram (CTPA)

A computerised tomography pulmonary angiogram (CTPA) is also used to diagnose a pulmonary embolism (PE). Dye is injected into the blood vessels of your lungs before a CT scan is taken. This works by taking detailed images of the inside of your body. It can show blood clots in your lungs. 

If there is a pulmonary embolism in one of your lungs, it will show up on a CT scan as a ‘gap’ in your blood supply.

Read more about CT scans.

Blood clot in an artery

The tests used to diagnose blood clots in the arteries (arterial thrombosis) will depend on what medical condition the blood clot has triggered.

Suspected cases of unstable angina and heart attacks are diagnosed using an electrocardiograph (ECG). An ECG measures the electrical activity in your heart. Every time your heart beats, it produces tiny electrical signals onto paper, allowing your doctor to assess how well your heart is functioning.

Blood may also be taken to measure levels of a protein called troponin, which is released when the heart muscle is damaged by a heart attack.

Suspected cases of stroke are diagnosed using CT scans or magnetic resonance imaging (MRI) scans to scan the brain.

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Treatment for thrombosis (a blood clot within a blood vessel) depends on whether your blood clot is in a vein (venous thromboembolism (VTE) or an artery (arterial thrombosis).

Blood clot in a vein

If your blood clot is in a vein, such as a deep vein thrombosis (DVT – a blood clot in one of the deep veins of your body) or a pulmonary embolism (a blood clot that has moved away from its original site and become lodged in one of your lungs), you will need to take a medicine called an anticoagulant.

Anticoagulant medicines

Anticoagulant medicines work in different ways. Some reduce the clotting factor levels in your blood. Others stop the clotting factors from working easily.

Two different types of anticoagulants are used to treat a blood clot in a vein:

  • heparin
  • warfarin

Heparin is given by injection and is usually prescribed first because it works immediately to prevent further clotting. Warfarin is taken as a tablet and can be taken for several days to reduce the stickiness of the blood. 

Read more about anticoagulant medicines.

Compression stockings

After having a DVT, compression stockings help to prevent long-term calf pain and swelling, and they lower the risk of ulcers developing. These are the symptoms of post-thrombotic syndrome, which is where the tissue of your calf is damaged by the increase in blood pressure that occurs when the vein is blocked by a clot.

Compression stockings should be worn every day for at least two years after having a DVT. This is to reduce the risk of post-thrombotic syndrome.

Compression stockings must be fitted professionally. They need to be worn all day but they can be taken off before you go to bed, or in the evening while you are resting with your leg raised. Below-knee stockings are easier to wear than full-length ones.

Blood clot in an artery

There are two main types of treatment for a blood clot that develops in an artery (arterial thrombosis):

  • medication, such as alteplase, which can help dissolve clots and restore the flow of the blood to the brain or heart 
  • surgery – where the affected artery is either unblocked or the blood flow is re-routed past the point of the blockage

The type of surgery that you have will depend on the location and severity of the arterial thrombosis.

Heart surgery

You may need heart surgery if the blood clot is in an artery that supplies blood to your heart.

A coronary angioplasty is the most common type of surgery for a heart attack. A hollow metal tube called a stent is inserted to widen the artery and stop it from becoming blocked again. 

Read more about coronary angioplasties.

Occasionally, a coronary artery bypass graft may be carried out after a heart attack. This is where a blood vessel that is taken from another part of the body is used to bypass the point of the blockage. 

Read more about coronary artery bypass grafts.

Carotid endarterectomy

Another type of surgery for arterial thrombosis is called carotid endarterectomy, which you may have if you have had a blood clot in an artery in your neck (causing a stroke). The surgeon makes a cut in your neck to open up the artery and remove the fatty deposits.

Read more about carotid endarterectomies.

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It is not possible to prevent blood clots altogether, but there are a number of ways to minimise the risks of developing a blood clot in a vein or artery (thrombosis).

Whatever your risk of developing a blood clot is, you can help to reduce it by following the lifestyle advice below. As well as reducing your risk of thrombosis, you can improve your overall health by:

  • not smoking 
  • eating a healthy, balanced diet (low in fat and sugar, and high in fruit and vegetables) 
  • getting regular exercise  
  • maintaining a healthy weight (losing weight if you are overweight or obese)

If you smoke, the most important and positive thing that you can do for your health is to quit. Read more about quitting smoking.

To find out if you are overweight or obese, you can work out your body mass index (BMI) using the healthy weight calculator.

Increased risk of a blood clot

Some people have an increased risk of developing a blood clot in a vein or artery, and they may need to make changes to reduce it.

If you have a higher-than-normal risk of developing a blood clot within a vein (venous thromboembolism or VTE), you may be prescribed medication to prevent a blood clot from forming. 

Similarly, if you are at risk of developing a blood clot within an artery (arterial thrombosis), it is important that you take steps to reduce it, either to prevent a previous arterial blood clot from coming back, or to prevent one occurring in the first place.

In hospital

If you need to be admitted to hospital, your healthcare team will assess your risk of developing a blood clot while you are an inpatient. This risk is increased if you are having surgery, particularly if it is to the lower half of your body, or if you are confined to a bed on a medical ward.

Severe injuries or surgery itself may damage your blood vessels, and being inactive while in hospital reduces your blood flow – both of which can make a blood clot more likely to occur.

If you are considered at risk of having a blood clot, your healthcare team may recommend that you take an anticoagulant medicine while you are in hospital, and for a while after you go home.

Anticoagulant medicines, such as heparin or warfarin, make your blood less sticky, so it is less likely to form a clot within a vein. Read more about anticoagulant medicines.

You may also need to wear compression stockings and use compression devices during your stay in hospital, and for some time afterwards.

Compression stockings are worn around your feet, lower legs and thighs. They fit tightly to encourage your blood to flow more quickly around your body. Compression devices are inflatable, and work in the same way, inflating at regular intervals to squeeze your legs and encourage blood flow.

Once you are well enough, it is important to try to move around or do leg exercises as soon as possible. Avoid taking long journeys for four weeks after you come out of hospital.

Blood clot in an artery

If you have had a previous blood clot in an artery, you may need to take medicines to prevent it from happening again.

A number of medicines can be prescribed if your risk of developing a blood clot is thought to be high. These include:

  • statins, which are used to lower your blood cholesterol levels 
  • anticoagulant medicines, such as warfarin, or antiplatelet medicines, such as low-dose aspirin, which are used to prevent blood clots 
  • angiotensin-converting enzyme (ACE) inhibitors are used to treat high blood pressure (read more about treating high blood pressure)

In addition to these medicines, it is vital that you look after your health and take steps to improve your lifestyle.

Most cases of arterial thrombosis are associated with atherosclerosis (the ‘furring up’ of your arteries with fatty deposits). This can lead to a blood clot in an artery and cause a heart attack or stroke. Making healthy changes to your lifestyle can greatly reduce the effects of this condition, and others such as heart disease

To reduce your risk of developing heart disease and other associated conditions, such as arterial thrombosis, the National Service Framework (NSF) on coronary heart disease gives the following advice:

  • don't smoke 
  • reduce the amount of salt and fat that you eat (particularly saturated fat) 
  • eat at least five portions of fruit and vegetables a day 
  • eat at least two portions of fish (one oily) a week 
  • do a minimum of 30 minutes of moderate exercise, such as walking, cycling or energetic housework, at least five days a week
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