Southern Cross Medical Library

Southern Cross Medical Library

A deep vein thrombosis (DVT) is a blood clot that forms in the major veins of the body – usually the legs. It is estimated one in every 1200 New Zealanders will develop a DVT each year. Treatment mainly involves medication. 

Life-threatening complications can arise from DVT when blood clots dislodge, travel in the bloodstream, and then lodge in other veins or arteries causing a blockage (this blockage is called an embolism). This can be life threatening, especially when the embolism occurs in the lungs, heart, or brain. Pulmonary embolism (a blood clot in the lung) is the most common of these serious DVT complications.

However, it is quite common for blood clots to form in the deep veins and not cause any symptoms (asymptomatic clots).


Veins are blood vessels that carry blood from the tissues of the body back to the heart. Veins that lie just beneath the skin surface are referred to as “superficial veins” while veins found deep inside the muscles are referred to as “deep veins”. Other veins connect the superficial and deep veins, allowing blood to flow between them. A blood clot (thrombus) is a gel-like clump of blood, which forms in response to an injury. Blood clots (thrombi) act as plugs to prevent excessive bleeding from injured blood vessels. However, sometimes clots form in blood vessels when there is no risk of excessive blood loss.

When a blood clot occurs in a vein it is referred to as a venous thrombosis. A DVT is a blood clot that forms inside the deep veins. DVTs can occur in any of the deep veins but most commonly occur in the leg veins. The clot will either partially or completely block the flow of blood through the affected vein.

A DVT is usually more serious than a blood clot in one of the superficial veins, as there is a much greater risk with a DVT that part of the clot may dislodge and circulate to the legs.

Three main factors contribute to the development of DVT: injury to the vein’s lining; increased tendency for blood to clot; and slowing of blood flow.

Injury to veins

Veins can sustain damage as a result of surgery, physical trauma to an arm or leg, inflammation, and certain medical conditions, leading to an increased risk of DVT. Also, a clot can injure a vein making the formation of a second clot more likely.

Slow blood flow

A DVT is also more likely to occur when the blood flow through the deep veins is sluggish. Immobility is one such factor; blood flow is slowed when a person remains immobile for long periods of time as a result of such things as:

  • Paralysis, eg: following a stroke or injury
  • Being bedridden, eg: following surgery or due to illness
  • Having a leg in a plaster cast or splint
  • Sitting for long periods of time while travelling (for 3 hours or more), eg: in a bus, train, plane, or car.

Other factors that can lead to sluggish blood flow include:

  • Injury to a vein, eg: as a result of a broken bone or severe muscle injury
  • Surgery – particularly orthopaedic and cancer surgery
  • Heart disease – particularly heart failure (where a weakened heart does not pump blood as well as it should)
  • Varicose veins.

Increased clotting tendency

A DVT is also more likely to occur where there is some factor that makes the blood more likely to clot, including:

Risk factors

General factors that indicate an increased risk of developing a DVT include:

  • Obesity
  • Smoking
  • Being older than 40 years (although DVT can occur at any age)
  • Having previously had a DVT or pulmonary embolism
  • Having a family member who has had a DVT or pulmonary embolism.

Signs and symptoms

A DVT does not always cause symptoms. If symptoms do occur, the first symptom is usually a cramp-like aching pain in the affected muscle. Symptoms of a DVT in the calf muscle may include:

  • Swelling of the lower leg
  • Tenderness of the calf muscle
  • Localised redness and warmth
  • A mild fever
  • Lower leg veins may become more prominent (darker and raised) and sometimes the skin becomes darker.

A DVT can also occur in the upper leg or arms and cause similar symptoms in those areas of the body.

If a pulmonary embolism (a blood clot in the lungs) occurs as a result of a DVT it may produce barely noticeable symptoms such as chest discomfort and mild breathlessness, or more noticeable symptoms such as sharp chest pain, a rapid heart rate, breathlessness, and coughing-up blood.


If a DVT is suspected – even if symptoms are mild – it is important to seek medical attention promptly. Accurate diagnosis and appropriate treatment of a DVT are necessary to reduce the risk of potentially life-threatening complications, such as pulmonary embolism.

Because a DVT can occur without any obvious symptoms, diagnosis can sometimes be difficult. Initially, the doctor will examine the affected area and take a full medical history. The doctor may recommend a blood test called a D-Dimer, which measures a protein essential for blood clotting. If the D-Dimer test is positive, i.e., a higher-than-normal level of D-dimer in the blood, further tests to confirm a diagnosis of DVT are likely to be ordered. Further tests are necessary because a positive D-dimer test can be due to other reasons (e.g., pregnancy, heart disease, infection, recent surgery).

The most common diagnostic test for DVT is an ultrasound scan. The specific type of scan used is called a doppler ultrasound, which determines how fast blood is flowing through a blood vessel.

Blood tests may be done to check for irregularities in the blood clotting system or for inherited disorders.

If a pulmonary embolism is suspected a range of additional tests may be used, including a special CT (computerised tomography) scan of the lungs.


The immediate goal of treatment for a DVT is to limit the size and movement of the clot, and to prevent complications. Treatment will depend on the location and severity of the clot. Some small clots may resolve spontaneously without treatment but a DVT is generally treated intensively. Admission to hospital for treatment and observation for signs of complications may be required.

Treatment may include:



These medications "thin" the blood, reducing its ability to clot, while your own body will dissolve the original clot. They prevent an existing clot from getting bigger and reduce the risk of developing more clots.

There are many different types of anticoagulant medication, some are taken by mouth as a tablet or capsule and others are given by injection under the skin (subcutaneously).

Anticoagulant treatment is usually maintained for at least three months to be fully effective in treating a DVT. In some cases, it may be required on a long-term basis.

Thrombolytic agents:

In some cases, these medications are given by injection into a vein (intravenously) to help dissolve the clot. However, they can cause side effects, such as severe bleeding, so are usually used only in life-threatening situations, eg: the presence of a large pulmonary embolus.

Compression stockings

Elasticised compression stockings give support to the lower legs and encourage the return of blood to the heart and helps to reduce swelling. It is generally recommended that compression stockings are worn in situations where immobility is likely.


In rare high-risk cases, where there have been recurrent or severe DVTs, or where anticoagulant medication is not appropriate or has not worked, a small filter or sieve may be inserted into the main vein leading to the heart (the vena cava) may be recommended. The filter traps any blood clots travelling through the blood stream thus preventing the clot travelling to the heart and lungs.


General measures to help lower the risk of developing a DVT are to quit smoking, maintain a healthy body weight, and exercise regularly. When certain medical conditions or inherited disorders are present, long-term anticoagulant treatment to minimise the risk of DVT may be recommended.

Measures that can reduce the risk of DVT associated with long-distance travel include:

  • Compression stockings
  • Drinking plenty of non-alcoholic fluids to avoid dehydration
  • Leg and ankle exercises to encourage blood flow in the legs
  • People at high risk of DVT may be prescribed aspirin or anticoagulant tablets or injections whilst travelling.

Anyone at risk of DVT is advised to see their doctor prior to travelling to discuss preventative measures. Measures that can reduce the risk of DVT associated with being bedridden as a result of surgery or illness include:

  • Compression stockings
  • Anticoagulant medications
  • Specific leg and breathing exercises to promote blood flow.

Long-term complications

Pulmonary embolism is the most serious complication of DVT and can be potentially life-threatening. It occurs when a large blood clot in a leg or other body area breaks free and gets stuck in a blood vessel in a lung.

After a DVT some people may develop a long-term condition called "chronic venous insufficiency" or "post-thrombotic syndrome". This is due to damage and scarring to the veins and is characterised by swelling, discomfort, and skin pigmentation in the affected area. It can increase the likelihood of subsequent DVT.

Repeated pulmonary emboli can lead to a condition called pulmonary hypertension, which is where the blood pressure within the lungs is increased. This can cause serious problems with the functioning of the heart. Certain medications, compression stockings, and, in rare cases, surgery, may be recommended to help treat these long-term complications.


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Last reviewed: December 2022


The purpose of the Southern Cross Medical Library is to provide information of a general nature to help you better understand certain medical conditions. Always seek specific medical advice for treatment appropriate to you. This information is not intended to relate specifically to insurance or healthcare services provided by Southern Cross.