Deep vein thrombosis (DVT) occurs when a blood clot forms in a large vein, usually in a leg.
If the clot blocks the vein, blood builds up there.
This can lead to swelling and pain in the leg, as well as reddish skin.
Thrombosis needs to be treated quickly with medications that stop the blood from clotting.
Measures to prevent DVT include doing exercises, wearing compression stockings and taking medication.
Thrombosis is the medical term for the formation of a blood clot in a blood vessel. In deep vein thrombosis (DVT), the blood clot forms in one of the larger, deeper veins that run through the muscles. Deep vein thrombosis usually occurs in the lower leg.
Smaller clots often go unnoticed and then dissolve on their own. Larger clots can affect the flow of blood and cause pain and swelling. DVT needs to be treated quickly to prevent damage to the veins and complications such as a blood clot in the lungs (pulmonary embolism).
Certain factors can increase the risk of deep vein thrombosis either temporarily or permanently. For instance, it increases after major operations, such as knee or hip replacement surgery. Because of this, people are usually given medications to prevent blood clots after having this kind of surgery.
The typical signs of deep vein thrombosis include:
Pain in a leg or hip
Swelling and a tight feeling
Warm, reddish skin
Healthy leg (left) and lower leg with deep vein thrombosis (right)
The symptoms usually only occur in one leg because thrombosis typically only develops on one side of the body.
The symptoms also depend on where the clot is blocking the vein: near the calf, behind the knee, or in the thigh or pelvis. Thrombosis below the knee usually only causes symptoms in the lower leg. Thrombosis that is higher up or more extensive can also lead to symptoms in the thigh or pelvis.
Causes and risk factors
Blood clots may form if the blood flow in a vein is too slow, if the blood clots too easily or if the wall of a blood vessel is damaged. That may result from:
Longer periods of bed rest, for instance in the hospital, after a bone fracture or injury. Spending a lot of time in bed and not moving much can lead to a build-up of blood in a leg.
Blood clotting disorders: Some people are born with a disease that makes their blood clot too much.
Major surgery: Major surgical procedures and serious injuries cause damage to blood vessels. The body then increases your blood's clotting ability to stop the bleeding and close wounds. This involves the release of more coagulation factors (clotting factors).
There are also various other things that can increase someone's risk of developing thrombosis:
Being older than 60 years
Family history (parents or siblings had thrombosis)
Regular long-haul journeys where you spend a lot of time sitting
Taken on their own, though, most of these risk factors only slightly increase the risk of DVT.
One common complication of DVT is known as post-thrombotic syndrome (PTS). This condition develops if the DVT has damaged the walls or valves of a vein, causing the blood to constantly build up in the vein. It often occurs a few weeks or months after thrombosis.
People with PTS may have a swollen leg that feels heavy and hurts. The skin on their leg may also become discolored and itch, and a rash may develop. More severe cases of post-thrombotic syndrome can lead to chronic wounds. It is important to seek medical advice quickly if you have these symptoms.
A rarer but more serious complication of deep vein thrombosis is known as pulmonary embolism. This happens if the blood clot breaks away from its original site and is carried through the blood stream to the lungs, where it blocks a blood vessel. The symptoms of pulmonary embolism include:
Sudden shortness of breath
Chest pain (particularly when breathing in or coughing)
Dizziness, lightheadedness or unconsciousness
Coughing up blood
If pulmonary embolism develops, it usually does so within two weeks of the DVT developing. It then needs to be treated quickly because severe pulmonary embolism can lead to life-threatening heart failure.
Deep vein thrombosis can’t be diagnosed for sure based on typical symptoms alone. Symptoms such as pain, swelling and red skin can be caused by other things too, including an inflammation of superficial veins, varicose veins, erysipelas or narrow leg arteries (peripheral artery disease, or PAD).
To diagnose DVT, a blood test called a D-dimer test is usually done first. This test reacts to substances that are released when blood clots are broken down. The substances are called D-dimers. If the results of the test are normal, doctors can be quite sure that you don’t have DVT. If the results are abnormal, a special kind of ultrasound scan (a Doppler ultrasound) is done in order to confirm the diagnosis. Doppler ultrasounds provide information about the condition of the deep veins in the legs and blood flow in the veins.
If there’s good reason to believe that someone has a DVT from the start, an ultrasound scan is usually done immediately.
An x-ray of the blood vessels (angiography) is only rarely necessary. Angiography involves using a catheter to inject a contrast agent (a dye-like substance) into the vein so that doctors can see where the blood vessels are narrowed.
There are different ways to try to prevent DVT. People who are recovering from surgery or an injury usually need to spend a few days resting in bed. But it's still important to move if you can and to get up again as soon as possible. Small exercises like wiggling your toes are recommended to help with blood circulation.
Medications called anticoagulants can be used for a few days or weeks to help reduce blood clotting. Their suitability will depend on things like whether you have had surgery, how major it was and whether you have risk factors for thrombosis. These medications can be injected or swallowed.
Compression stockings can also be used. The stockings put pressure on the veins, which helps the blood to flow. If you're already using anticoagulants, you don't need to use compression stockings. But they are an option for people who can't use anticoagulants. That is rare, though.
Deep vein thrombosis is usually treated for a few days in the hospital, but it can also be treated in an outpatient setting. This involves using the same medications that are used to prevent thrombosis.
To make sure that the blood clot dissolves completely, patients are advised to take anticoagulant (anti-clotting) tablets for three to six months after having the acute treatment. Sometimes it’s a good idea to take them for even longer too. It is important to take anticoagulants as prescribed. That helps to prevent both another thrombosis and post-thrombotic syndrome. But anticoagulants can cause bleeding as a side effect.
To lower the risk of post-thrombotic syndrome, people are sometimes advised to wear compression stockings for up to two years. Studies on their effectiveness haven't shown any clear results, though. So it's best to talk with your doctor about whether preventive treatment with compression stockings is a good idea for you.
Some people aren't able to use anticoagulants – for example, because they tend to bleed more. Then doctors might consider using a catheter to implant a small metal filter (inferior vena cava filter, or IVC filter) into the large vein above the kidneys. The filter is meant to catch any blood clots that could otherwise travel from the leg to the lungs and cause a pulmonary embolism. There is not enough research on the pros and cons of IVC filters. Because of the possible risks, they are only used in exceptional cases. Some medical societies even advise against using them.
IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.
Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.
Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.
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