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Deep vein thrombosis - causes, symptoms, prevention

A deep vein thrombosis is a blood clot that forms in the major veins of the body – usually the legs. It can affect people of any age but the risk of developing a DVT increases after the age of 40 years.

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.

According to one estimate, approximately one in every 1200 New Zealanders will develop a DVT each year.


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.

When a blood clot occurs in a vein it is referred to as a venous thrombosis. A DVT is a blood clot that occurs in 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. When the blood clot is associated with inflammation of the vein it is referred to as thrombophlebitis.

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 through the body.

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

  • Obesity
  • Smoking
  • Having previously had a DVT
  • Having a family member who has had a DVT.

A DVT is also more likely to occur when the blood flow through the deep veins is slowed. 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, eg: in a bus, train, plane, or car.

Other factors that can slow 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
  • Varicose veins
  • Phlebitis (inflammation of the walls of the vein).

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

  • Hormone medications – some research studies have indicated that there may be a small increased risk of DVT associated with some types of oestrogen-containing oral contraceptive pills, as well as some menopausal hormone therapies (formerly known as hormone replacement therapy or HRT).
  • Inherited disorders – such as the deficiency of some blood clotting factors, eg: protein C; or defective blood clotting factors, eg: factor V Leiden
  • Inflammatory bowel diseases, eg: Crohn's disease     
  • Certain cancers
  • Pregnancy.

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 muscles. This pain might worsen when exercising but does not subside with rest. 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.   

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

If a pulmonary embolism 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, further tests to confirm the diagnosis are likely to be ordered.

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. The scan is painless and is able to detect up to 95% of DVTs.

If there is doubt about the diagnosis, venography may be recommended. This diagnostic test involves injecting a specialised dye that can be seen by x-ray as it flows through the veins, allowing them to be easily visualised.   

Other tests that may be used to assist with diagnosis include:

  • Blood tests to check for irregularities in the blood clotting system or for inherited disorders
  • Impedance plethysmography (IPG): This test measures the blood pressure at various places in the leg to identify the location of the blood clot.   

If a pulmonary embolism is suspected a range of additional tests may be used.


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:     


Anticoagulants: These medications "thin" the blood, reducing its ability to clot. Anticoagulant medications can be administered as a continuous infusion into a vein (intravenously), as an injection under the skin (subcutaneously), or in tablet form (orally). Blood tests to monitor their effectiveness are required regularly and dosage changes may be required. Anticoagulant treatment, in the form of tablets (e.g. warfarin) or subcutaneous injections (e.g. enoxaparin), 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, eg: alteplase, are given by intravenous injection 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 high-risk cases, where there have been recurrent or severe DVTs, surgery to insert a small filter into the main vein leading to the heart (the vena cava) may be recommended. This 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 include to quit smoking, maintain a healthy body weight, and take regular exercise. 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
  • 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

After a DVT some people may develop a chronic (long-term) condition called "post-phlebotic 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.

Recurrent 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.


Bates. S.M. et al. (2012). Diagnosis of DVT: Antithrombotic therapy and prevention of thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012;141(2 Suppl):e351S-418S.
Liao, S. et al. Incidence of venous thromboembolism in different ethnic groups: a regional direct comparison study. J Thromb Haemost 2014;12:214-9.
Mayo Clinic (2016). Deep vein thrombosis (DVT) [Web Page]. Rochester, IL: Mayo Foundation for Medical Education and Research. [Accessed: 10/07/16]
Ministry of Health (2015). Travel and blood clots. Wellington: Ministry of Health. [Accessed: 25/01/17]
New Zealand Medicines and Medical Devices Safety Authority (Medsafe) (2002). Oral contraceptives and blood clots. Wellington: Ministry of Health. [Accessed: 25/01/17]
O’Toole, M.T. (Ed.) (2013). Deep vein thrombosis (DVT). Mosby’s Dictionary of Medicine, Nursing & Health Professions (9th ed.). St Louis, MI: Elsevier Mosby.

Last Reviewed – January 2017    


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