Press "Enter" to skip to content

Medical News Today: What to know about a blood clot behind the knee

A blood clot behind the knee is a type of venous thromboembolism. It is a serious condition that can lead to life-threatening complications, such as a pulmonary embolism.

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

Symptoms include pain, swelling, and redness 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.

In this article, we explain what popliteal vein thrombosis is and discuss its causes, risk factors, and symptoms. We also cover diagnosis, treatment, complications, and prevention.

What is popliteal vein thrombosis?

A blood clot behind the knee can cause pain, swelling, and redness.

When blood hardens into a solid lump, it forms a blood clot. Blood clotting occurs in response to injuries that cause bleeding. The blot clot closes the wound and stops the bleeding, preventing further blood loss and starting the healing process.

When a blood clot forms inside a vein or artery, doctors refer to it as a thrombus. A thrombus can develop due to poor blood flow, damage to a blood vessel, or external injury. It is a serious condition because it can cause a blockage that completely stops the flow of blood.

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

Popliteal vein thrombosis is a type of venous thromboembolism (VTE), which is also called deep vein thrombosis (DVT). It is potentially life-threatening because 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 (PE).


The symptoms of a blood clot behind the knee or any type of VTE in the leg can include:

  • redness in the knee or calf area
  • swelling in the knee or leg
  • a warm area behind the knee or in the leg
  • pain in the knee or leg, which may feel similar to a cramp

Anyone who suspects that they have a blood clot should see a doctor right away. It is essential to seek immediate medical attention if the following symptoms occur alongside a potential clot:

  • shortness of breath
  • chest pain
  • coughing up blood

Causes and risk factors

blood clot behind knee desk sitting
Sitting still for long periods can reduce blood flow to the knee.

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.

Factors that can reduce blood flow include:

  • sitting still for prolonged periods
  • being immobile or bedridden
  • smoking cigarettes
  • obesity
  • pregnancy

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

Damage to a vein, potentially 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 clotting. The risk of VTE almost doubles every 10 years after the age of 40 years.


To diagnose a blood clot behind the knee, a doctor will carry out a physical examination of 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.

To help with their diagnosis, a doctor may order one or more of the following tests:


A doctor may use this test to examine the knee and leg area and check for signs of clotting. Ultrasound uses high-frequency sound waves to create images of the inside of the veins. The ultrasound operator may press on the vein to check whether blood is flowing correctly.

CT scan

This test takes pictures of the inside of the body. A doctor can use the images to check for blood clots in the legs. They may also check the chest for signs of a PE, which can happen when a blood clot travels to the lungs.

D-dimer test

A doctor performing this test will take a blood sample from the person to check their levels of D-dimer, which is a type of protein that blood clots release into the bloodstream.

High levels of D-dimer in the blood may indicate a clot. However, this test can sometimes give false positive results, particularly if the person has a previous history of VTE or has certain medical conditions, including:


There are several different treatment options for VTE, including:


Doctors commonly prescribe anticoagulant medication for people with VTE. Also known as blood thinners, anticoagulants stop existing blood clots from growing, help prevent new ones from forming, and reduce the risk of a PE.

Anticoagulant medications include:

  • heparin
  • warfarin
  • newer anticoagulants, such as rivaroxaban, apixaban, and dabigatran

A person will initially take an oral anticoagulant once or twice daily for between 5 and 21 days. Doctors may also recommend that a person takes these medications in the long term to prevent future blood clots. Treatment can last for 6 months or longer.

Anticoagulants can cause side effects, which may include bleeding. People who experience side effects or other problems while taking these medications should speak to their 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, which is a large vein 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 PE.

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 taking drugs to dissolve the blood clot or undergoing surgery to remove it. Doctors usually only recommend thrombolytic therapy for severe blood clots. In most cases, they will carry out a thorough examination of the individual first to ensure that it is safe for them to undergo this type of therapy.

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 of one, rather than to treat a current clot.

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

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


blood clot behind knee heart problems
A PE occurs when a blood clot travels to the lungs.

People with VTE are at risk of having a PE, which is when the blood clot travels to the lungs. It is possible for a PE to block blood flow to the lungs, which can be life-threatening.

Symptoms of a PE can include:

  • shortness of breath or difficulty breathing
  • pain in the chest
  • rapid heartbeat
  • coughing, including coughing up blood
  • feeling feverish or faint

Anyone with symptoms of a PE should go straight to the emergency room or call 911 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 VTE.

People on anticoagulant medication should take it according to their doctor’s instructions.

A person can also reduce the risk of VTE by:

  • wearing compression stockings
  • maintaining a healthy weight or losing weight if necessary
  • exercising regularly
  • getting up and moving around every 1–2 hours, 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 every hour or so if traveling by car, train, bus, or airplane
  • stopping smoking


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

It is, therefore, essential for anyone with symptoms of VTE to see a doctor as soon as possible. People with symptoms of a PE should seek immediate medical attention.

Treatment options for VTE include anticoagulant medication, vena cava filters, and thrombolytic therapy.