Deep Vein Thrombosis (DVT)

Deep Vein thrombosis (DVT) is a blood clot in one of the deep veins in the body. 

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 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).

Who is at risk 

Each year, 1 in every 1,000 people in the UK is affected by DVT. 

Anyone can develop DVT, but it becomes more common with age. As well as age, risk factors include:

• previous venous thromboembolism 
• a family history of blood clots 
• medical conditions such as cancer and heart failure 
• inactivity – for example, after an operation 
• being overweight or obese

Warning signs

In some cases of DVT, there may be no symptoms, but it is important to be aware of the signs and risk factors of thrombosis.

See your GP as soon as possible if you think you may have a blood clot, DVT can cause pain, swelling and a heavy ache in your leg.

Treating DVT

Treatment for DVT usually involves taking anticoagulant medicines, which help reduce the ability of the blood to clot.

You will also be prescribed compression stockings to wear every day, as these help prevent complications and improve symptoms.

Avoiding DVT

There are several things you can do to help prevent DVT occurring, such as stopping smoking, losing weight if you are overweight, and walking regularly to improve the circulation in your legs.

There is no evidence that supports taking aspirin to reduce your risk of developing DVT.

Why do blood clots form in leg veins

Blood normally flows quickly through veins, and does not usually 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 for over 1-1.5 hours is the most common cause of a DVT. The 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, for a number of reasons, but mainly because they are very ill and secondly 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 vasculitis (inflammation of the vein wall) and some drugs (for example, some chemotherapy drugs) 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 an intravenous catheter is inserted into a vein). 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 separate leaflets called Nephrotic Syndrome and Thrombophilia for more information.) Some rare inherited conditions can also cause the blood to clot more easily than normal. An example of an inherited blood disorder than 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.
• The contraceptive pill and hormone replacement therapy (HRT) which contain oestrogen can cause the blood to clot slightly more easily. Women taking the 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 you 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 increases the risk. 
• Being male. Men tend to develop a DVT more often than women.
• Dehydration can make a DVT more likely to happen. Effectively, the blood becomes more sticky and liable to clot.