A blood clot behind the knee is a type of venous thromboembolism or deep vein thrombosis (DVT). Symptoms include pain, swelling, warmth, and discoloration of the skin. Prompt treatment can lower the risk of life threatening complications, such as a pulmonary embolism.

The popliteal vein runs behind the knee and transports blood back to the heart. When a blood clot forms in this vein, doctors call it a popliteal vein thrombosis.

The symptoms include pain, swelling, and inflammation in the leg and knee area. Popliteal vein thrombosis can occur due to poor blood flow, damage to a blood vessel, or an external injury.

This article explains what popliteal vein thrombosis is and discusses its causes, risk factors, and symptoms. It also covers diagnosis, treatment, complications, and prevention.

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Popliteal vein thrombosis is a condition in which a blood clot develops in the popliteal vein. The popliteal vein runs behind the knee and is one of several blood vessels that carry blood from the leg into the inferior vena cava — a large vein that carries blood from the lower body to the heart.

Popliteal vein thrombosis is a type of venous thromboembolism (VTE), which is also called deep vein thrombosis (DVT). The popliteal vein is one of the most common locations for DVT to develop.

It is potentially life threatening because part of the thrombus can sometimes break free and travel through the heart to the lungs. A thrombus that travels to the lungs is known as a pulmonary embolism.

According to the National Blood Clot Alliance, the signs and symptoms of blood clots include:

  • swelling in the knee or leg
  • leg pain or tenderness that may feel similar to a cramp
  • reddish skin discoloration
  • the leg being warm to the touch

Anyone who suspects they have a DVT should consult a doctor right away.

If a person develops symptoms of DVT in their leg, they should not delay seeking medical treatment. DVT can result in more serious complications without treatment.

According to the National Institutes of Health (NIH), DVT may not cause symptoms, meaning that it can go unnoticed in approximately 30–40% of cases. Some people may not realize they have a DVT until it leads to a more serious condition, such as a pulmonary embolism. If a pulmonary embolism occurs, this usually happens weeks of the DVT developing.

In some cases, the body may dissolve the clot without treatment. However, even if this happens, it can cause permanent damage to the popliteal vein.

Read more about the difference between a thrombosis and embolism.

Sometimes, there is no obvious cause of a blood clot behind the knee, but various factors can increase a person’s risk of developing one.

In particular, anything that can affect or reduce blood flow in this area can increase the risk of a blood clot. When blood does not circulate properly, it can pool in the vein, forming a blood clot.

According to the American Society of Hematology (ASH), risk factors that increase a person’s risk of developing a blood clot include:

  • having obesity
  • being pregnant
  • smoking
  • being immobile
  • prolonged inactivity, such as on long plane or car journeys

Medical conditions that can increase the risk of blood clotting include:

Trauma, such as damage to a vein from surgery or a significant injury that affects the leg, can sometimes lead to blood clots forming behind the knee.

Birth control pills, hormone replacement therapy, and other medications that contain estrogen can also increase the risk of blood clots.

Increasing age is another risk factor for blood clots. A person’s risk increases once they pass the age of 60.

Learn about the safety of flying with a blood clot.

To diagnose a blood clot behind the knee, a doctor will physically examine the affected area and check the person’s heart rate. They will ask the person about their symptoms and medical history, including any risk factors for blood clotting.

The doctor may then order one or more of the following tests:

  • Duplex ultrasonography: This uses sound waves to check blood flow in the veins.
  • D-dimer blood test: This measures a substance present in the blood once a clot has broken up.
  • Contrast venography: This is a type of X-ray. A doctor will inject a dye into the large vein near the foot and ankle.
  • MRI or CT scan: These imaging tests can provide visual evidence of the blood clots. Although doctors do not generally use them to diagnose DVT, these tests can help diagnose a PE.

The ASH states that it can take 3 months to treat a deep vein blood clot in the lower leg. In some cases, treatment can last for 3–6 months.

There are various treatment options for a blood clot behind the knee, including:


Doctors most commonly prescribe anticoagulant medication, known as blood thinners, for people with DVT. These drugs work by:

  • reducing the blood’s ability to clot
  • preventing the clot from getting larger
  • helping prevent new clots from forming

Anticoagulant medications include:

Anticoagulants can cause side effects, which may include bleeding. People who experience side effects or other problems while taking these medications should speak with a doctor.

Vena cava filters

Doctors may recommend a vena cava filter for people who are unable to take anticoagulant medications and have a high risk of a blood clot moving to the lungs.

A vena cava filter is a cone-shaped device. A surgeon implants this filter into a person’s inferior vena cava — a large vein in the abdomen that carries blood from the lower body to the heart. The filter catches blood clots and stops them from traveling to the lungs, which reduces the risk of a pulmonary embolism.

Thrombolytic therapy

Thrombolytic therapy may be necessary if a person has a very large blood clot or if anticoagulant medications are not working effectively.

This type of therapy involves injecting a medication that breaks down the blood clot.

Doctors usually only recommend thrombolytic therapy for severe blood clots. In most cases, they will thoroughly examine the individual first to ensure it is safe for them to undergo this type of therapy.

Thrombectomy or embolectomy

In rare cases, a person may undergo surgery to remove the clot.

A thrombectomy is a procedure to remove the clot in people with DVT. An embolectomy is a procedure to remove the blockage resulting from the clot in those who have a pulmonary embolism.

Compression stockings

Compression stockings are elastic socks with a special design that can help improve blood flow in the legs.

Doctors generally only recommend compression stockings for people who have previously experienced a blood clot or have an increased risk rather than to treat a current clot.

These stockings can also help with post-thrombotic syndrome, which is a complication that can occur after DVT. Possible symptoms include:

  • pain and swelling
  • aching or heaviness in the legs
  • cramp

People with a blood clot behind the knee are at risk of having a pulmonary embolism. A pulmonary embolism can block blood flow to the lungs, which can be life threatening.

The symptoms of a pulmonary embolism can include:

  • shortness of breath or difficulty breathing
  • pain in the chest
  • rapid heart rate
  • coughing, including coughing up bloody mucus
  • feeling feverish or faint

Anyone with symptoms of a pulmonary embolism should go straight to the emergency room or call 911 or their local emergency number immediately.

Doctors usually prescribe anticoagulant medication to people at high risk of blood clots, such as individuals recovering from certain types of surgery or those who have previously had DVT.

People on anticoagulant medication should take it according to the prescription.

A person can also reduce their risk of DVT by:

  • wearing compression stockings
  • reaching and maintaining a moderate body mass index (BMI)
  • exercising regularly
  • getting up and moving around every hour or so, where possible
  • changing positions or flexing the feet from time to time when sitting
  • drinking plenty of water to stay hydrated
  • avoiding crossing the legs for extended periods
  • stopping for breaks or stretching and walking if traveling by car, train, bus, or airplane
  • stopping smoking, if applicable

Doctors refer to a blood clot behind the knee as popliteal vein thrombosis, which is a type of DVT. People with DVT are at risk of a pulmonary embolism, which is a potentially life threatening condition.

Anyone with symptoms of DVT needs to contact a doctor as soon as possible. People with symptoms of a pulmonary embolism should seek immediate medical attention.

The treatment options for DVT include anticoagulant medication, vena cava filters, and thrombolytic therapy.