Photo of a man inspecting his leg (PantherMedia / Marin Conic)

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.

It often goes unnoticed and dissolves on its own. But it may cause symptoms like pain and swelling. If someone is diagnosed with DVT, they will need treatment to avoid serious complications such as pulmonary embolism. This can occur if the blood clot breaks away from its original site and is carried to the lungs in the bloodstream.

The risk of deep vein thrombosis increases after more major operations such as knee or hip replacement surgery. Because of this, people who have had this kind of surgery are usually given medication to prevent blood clots from forming.


Typical symptoms of DVT include pain in a leg or hip, tenderness, tightness and red skin. The affected area may also swell and feel warm. But sometimes deep vein thrombosis doesn't cause any symptoms.

Causes and risk factors

Blood clots may form if the blood flow in a vein is too slow, if there is a blood clotting disorder, or if the wall of a blood vessel is damaged. This can happen in the following cases:

  • After longer periods of bed rest, for instance in hospital, after a bone fracture or injury. Staying in bed for a long time and not moving much can lead to poor blood circulation in your legs.
  • Blood clotting disorders: Some people are born with a disease that makes their blood clot too much.
  • More major operation: More major surgical procedures and serious injuries cause damage to blood vessels and activate the blood clotting (coagulation) system.

There are also various other factors that can increase someone's risk of developing DVT:

  • Previous DVT
  • Being older than 60 years
  • Family history (parents or siblings had a DVT)
  • Certain types of cancer
  • Heart failure
  • Being severely overweight (obese)
  • Taking the contraceptive pill
  • Hormone therapy for menopause
  • Very noticeable varicose veins
  • Smoking
  • Certain inflammatory diseases
  • Pregnancy
  • Regular long-haul journeys where you spend a lot of time sitting

Taken on their own, however, most of these risk factors only slightly increase the risk of DVT.


Studies have estimated that an average of about 1 out of 1,000 people develop noticeable (symptom-causing) deep vein thrombosis per year. The risk increases with age. It is somewhat more common in men than in women.


One common complication of DVT is known as post-thrombotic syndrome. People with this syndrome 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 the formation of chronic wounds.

Post-thrombotic syndrome occurs if the DVT has damaged the walls or valves of a vein, causing the blood to constantly build up in the vein. The syndrome often only develops several weeks or months after the DVT, sometimes even after one or two years.

Pulmonary embolism

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 in the blood 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
  • Rapid heartbeat
  • Coughing up blood (rare)

Pulmonary embolism needs to be treated quickly because the blocked blood vessel causes blood to build up between the heart and lungs. This can put too much strain on the heart and eventually lead to life-threatening heart failure.

If pulmonary embolism develops, then it usually does so within two weeks of the DVT. The risk of pulmonary embolism is greater if the thrombosis is in the upper part of your leg or in your pelvis than if you have DVT in your lower leg. This is also true for thrombosis that causes very noticeable symptoms.


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 and blood flow in the veins.

If there’s good reason to believe that someone has a DVT, an ultrasound scan is usually done immediately. Examinations involving a small procedure are only rarely needed. One example is a special x-ray examination of the blood vessels which helps to see if there are any narrow areas (angiography). This involves injecting a contrast medium into the vein.


There are different ways to try to prevent DVT. If someone has had surgery or was injured and has to stay in bed for a few days, it is important that they get up and start moving again as soon as possible. Even small movements like wiggling your feet are recommended in order to improve your circulation. If someone has an increased risk of DVT it can be a good idea for them to wear compression stockings or use medication too.

Compression stockings can lower the risk of deep vein thrombosis by applying pressure to the legs, which helps the blood flow back to the heart faster.

A third option is medication that reduces the clotting ability of the blood. This can be injected or swallowed. Medications that are injected into the skin or a vein already start working after a few hours. Examples include medications known as heparins and the drug fondaparinux.

There are two main kinds of oral medications, known as coumarins and direct oral anticoagulants (DOACs). DOACs are sometimes also called novel oral anticoagulants (NOACs). The best known coumarin is called phenprocoumon (known by many under the trade name Marcumar or Marcoumar). Examples of DOAC drugs include apixaban, dabigatran, edoxaban and rivaroxaban. Coumarins only start working after a few days, and direct oral anticoagulants already start working after a few hours.

Research summaries


Deep vein thrombosis is usually treated in the hospital over several days, using the same medications that are used to prevent thrombosis. The treatment is started with a medication that works quickly, such as heparin.

To make sure that the blood clot dissolves completely, patients are advised to take anticoagulant (anti-clotting) tablets for three months after having acute treatment. Sometimes it’s a good idea to take them for even longer too.

For people who can’t take anticoagulants, an alternative option is having a small metal filter (known as an “inferior vena cava filter” or “IVC filter”) implanted in the large vein above the kidney. The filter is meant to catch any blood clots that could otherwise travel from the leg to the lungs. It is implanted using a venous catheter, in a procedure similar to cardiac (heart) catheterization.

The risk of post-thrombotic syndrome can be lowered by wearing compression stockings for up to two years.