What is deep vein thrombosis (DVT)?

Deep vein thrombosis (DVT) is the formation of a blood clot (thrombus) within a deep vein, usually in the calf and thigh veins of the leg. The clot may cause serious injury, including a pulmonary embolism, if it dislodges from the vein and blocks the pulmonary artery, which transports blood from the heart to the lungs. It is most prevalent after major surgery, in pregnant women, the elderly and people sitting or lying down for extended periods of time.

Leg veins showing normal blood flow, the formation of a deep vein thrombosis and a blood clot that has become loose.Leg veins showing the flow of blood in healthy veins and veins affected by DVT. 

Pulmonary embolism

A blockage of an artery in the lungs usually by a blood clot, but can be by a fat globule, air bubble or another substance that has travelled from elsewhere in the body.

Signs and symptoms

The most common site for DVT is in the deep veins of the calf or thigh. Signs and symptoms may include:

  • Swelling of the calf and sometimes the thigh;
  • Warmth in the affected area;
  • Tenderness or pain in the affected area, and;
  • Change in the color of the skin, usually redness.

While DVT is more common in the lower legs, it can also affect the veins of the upper arm. These patients can present with a swollen and painful arm.


DVT may be caused by anything that slows and ultimately promotes blood clotting. Causes can be acquired or genetic, which include:

Injury and surgery

DVT may result from surgical procedures, particularly those involving the knee, hip or pelvis. A blood clot is formed when platelets and other clotting factors in our blood coagulate to form a mass. During injury or surgery, when blood vessels are cut or damaged, platelets gather at the affected location to start the healing process. They do this by creating a net or web to plug the injury site. This can give rise to DVT.

Blood clotting disorders

Polycythemia vera

Polycythemia vera is a blood disorder that causes the bone marrow to produce too many red blood cells. This can lead to increased blood clotting due to the blood being thicker and slower.

Essential thrombocythemia

Essential thrombocythemia is a blood disorder that causes too many platelets to form in the bone marrow. Platelets are essential for survival, but too many platelets can lead to the formation of a blood clot.

Sitting or lying down for extended periods of time

People sitting for prolonged periods of time, such as on long-haul plane flights, may experience sluggish blood flow and dehydration that can increase the risk of developing DVT. Lying down for an extended period of time also poses an increased risk.

Other clotting disorders

There are a number of inherited disorders that result in the blood being more likely to clot. These include, but are not limited to, factor V Leiden and deficiencies in proteins S and C (proteins that typically function to prevent blood clotting).

Bone marrow

The spongy, vascular or fatty tissue found inside bones, responsible for producing blood cells.


The state of insufficient hydration; excessive loss of water; requiring more water in order to function normally.

Factor V Leiden

A mutation in blood-clotting factor V. It increases a person's risk of spontaneously developing blood clots.

Red blood cells

Cells in the blood that transport oxygen from the lungs throughout the body and carbon dioxide from the body to the lungs.

Risk factors

Family history

If there is a family history of DVT, you may also have an increased risk.

Oral contraceptive pill

The oral contraceptive pill, particularly those with a high estrogen content, increases the risk of developing DVT.


It is estimated that women are 4-5 times more at risk of developing DVT during pregnancy. The risk persists for up to six weeks after delivery.


People who are obese have a greater risk of developing DVT.


Cigarette smoking can affect blood clotting and circulation, which can increase the risk of developing DVT.

Increasing age

DVT occurs more often in older people.

Methods for diagnosis

A DVT may be detected by various tests including:

Venous doppler ultrasound

A venous doppler ultrasound scan is a non-invasive, safe and inexpensive test for DVT. This ultrasound generates images of any abnormalities inside the veins using sound waves. A DVT is detected when a vein cannot be compressed under gentle probe pressure over the affected area.

D-dimer blood test

A D-dimer blood test is an investigation in patients with a low probability of having DVT. This measures the level of the D-dimer level in the blood, which rises when a blood clot is forming in the body. If the D-dimer level is normal, it is unlikely that you have DVT. However, D-dimer can be elevated by other medical conditions, so a raised D-dimer on its own cannot be used to diagnose DVT.

Types of treatment

If DVT is found, the treatment goals include stopping the clot from growing and preventing any new blood clots from forming. DVT may be treated by:


Anticoagulation drugs

Anticoagulation, or blood thinning, is the primary treatment for a DVT. The treatment prevents new blood clots from forming. Traditionally anticoagulants have been administered by an injection under the skin, but more recently a number of oral medications for the treatment of DVT have become available. Examples of blood thinners include low molecular weight heparins (LMWH), which is administered by injection, and warfarin, rivaroxaban and apixaban, which are administered orally.

Thrombolytic agents

Thrombolytic agents including alteplase or reteplase are known as 'clot busters'; these medications are used to dissolve blood clots. Unfortunately, they may also cause unwanted bleeding as they cannot differentiate between DVT and a clot involved in healing (e.g. the sealing of a stomach ulcer). Therefore, thrombolytics are only used to treat the life-threatening complications of DVT.

Blood-filtering devices

It is possible to place a filter in the large vein that drains blood from both legs, known as the inferior vena cava (IVC). This filter catches blood clots before they reach the heart or lungs and are then dissolved by your body's natural systems. An IVC filter is usually inserted if you are unable to take blood-thinning agents.

Potential complications

DVT may lead to a blood clot dislodging from a blood vessel, causing complications. These can include:

Pulmonary embolism

This is a condition in which the blood clot moves through the IVC to block the pulmonary artery that leads from the heart to the lungs. Depending on the size of the clot, it may block a small artery in the outer lung or, in the case of large clots, block the major arteries leading from the heart to the lungs. This is a medical emergency which needs immediate treatment.

A pulmonary embolism, where a blood clot has dislodged and travelled to the heart and lungs. A body showing a blood clot that has formed in a leg vein.A pulmonary embolism and the site of original blood clot in the leg vein. 

Post-thrombotic syndrome

This is when valves in your deep veins are damaged. As damaged valves may become leaky, blood can pool in the lower leg. Over time, this may lead to prominent superficial veins, pain, swelling and in serious cases, ulcers on your leg.

Pulmonary embolism

A blockage of an artery in the lungs usually by a blood clot, but can be by a fat globule, air bubble or another substance that has travelled from elsewhere in the body.


An open sore in the skin or mucous membranes such as those of the stomach lining, intestine or mouth.


Following treatment, DVT can disappear without any further problems. In some cases there may be residual pain, swelling or changes in skin color. Compression stockings may be worn to avert this problem.


Compression stockings

To reduce the risk of DVT, compression stockings may be worn. These stockings can enhance the flow of blood in the deep veins of the leg. Quitting smoking and avoiding excessive weight gain can also help.


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