’Thrombosis’, or blood clotting, is a natural protective mechanism that prevents excessive bleeding when the body is injured. However, clotting becomes a problem when it obstructs blood flow. Clotting can occur in deep and superficial veins. Deep veins lie within muscles or body cavities, whereas superficial veins lie beneath the skin in the fat layer. Superficial veins connect with deep veins via ‘perforating veins’.

What is deep vein thrombosis?

Deep vein thrombosis (DVT) is the development of a blood clot in a deep vein. Although the deep veins of the legs are most commonly affected, a DVT can also affect the deep veins of the arms, pelvis or abdomen.

When clots form in superficial veins, this is called superficial venous thrombosis (SVT). If SVT leads to inflammation of the adjacent skin and fat, red tender lumps develop. This is called superficial thrombophlebitis (STP), or simply ‘phlebitis’.

While blood clots in superficial veins rarely cause serious problems, DVT requires immediate medical evaluation. Prompt treatment of DVT is vital to minimise immediate or long-term complications.

DVT can damage the affected deep vein, leading to chronic pain, swelling and skin changes of the affected limb. This is called post-thrombotic syndrome. DVT can also interfere with the return of blood to the heart, and result in swelling, pain and inflammation of the affected limb. If part of the blood clot breaks off and travels to the lungs, it creates a condition called pulmonary embolism (PE), which may be life threatening. Signs of PEcan include chest pain, palpitations, shortness of breath, coughing (with or without blood) and even sudden death.

Signs and symptoms

Symptoms of DVT can include swelling, tenderness and sometimes warmth of the affected limb. However, up to 50 per cent of patients with a DVT show no symptoms. These are called ‘silent’ or ‘asymptomatic’ DVTs. Silent DVTs may resolve by themselves, or can lead to complications such as PE.

Other conditions may present with similar symptoms to a DVT. These include soft tissue injury (muscle strain), inflammation of superficial veins (phlebitis), infection (cellulitis), skin inflammation (dermatitis) or a ruptured cyst behind the knee (ruptured Baker’s cyst).

What causes DVT?

Many external factors contribute to the formation of a DVT. One of the most common risk factors is immobility caused by long distance travel or hospitalisation. Some people are naturally predisposed to DVT because of an inherited tendency to clotting.

Several risk factors need to be present for a DVT to develop. An example is a female who is taking an oral contraceptive (risk factor 1) who has a sprained ankle (risk factor 2), is travelling from Sydney to New York in a plane (risk factor 3) and carries an inherited tendency to clotting (risk factor 4).

DVT Risk Factors

Reduced mobility

Contraction of the calf muscles during walking results in blood flow in the veins towards the heart. With immobility, blood flow becomes sluggish and the blood is more likely to clot. This leads to an increased risk of DVT. Patients over 60 are generally at greater risk, although patients of any age with reduced mobility (such as during long distance travel, or through illness or hospitalisation) can be affected.

Long distance travel with prolonged sitting

‘Long distance’ is defined as any trip of more than five hours. This includes car, coach and plane travel. In addition, the lower oxygen levels and humidity in an aeroplane cabin may further increase the risk of blood clots in the lower leg.


Blood vessels may be damaged through trauma to the leg. Examples of such injuries include physical trauma, surgery or radiation therapy for cancer. Traumatic injuries damage the inner lining of veins, which can then trigger clot formation.


Obesity is a known risk factor for cardiovascular diseases (strokes and heart attacks). Obesity is usually associated with reduced mobility, which adds to the DVT risk.


Pregnancy increases the risk of DVT by several-fold. Most cases occur in the third trimester and immediately following delivery.

Oral contraceptives and hormone replacement therapy (HRT)

Hormonal preparations such as oral contraceptive pills or HRT increase the risk of clotting, particularly when combined with other risk factors like smoking, high blood pressure, obesity and inherited clotting disorders. Patients with a history of DVT should not take these preparations.

Cancer and other medical illnesses

Cancers may increase the blood’s tendency to clot. Cancers of the ovaries, pancreas, lymphatic system, liver, stomach, and colon are particularly likely to provoke DVT. Infections and medical conditions such as systemic lupus erythematosus (SLE), Crohn’s disease, rheumatoid arthritis, and glomerulonephritis may also stimulate the blood clotting process and promote DVT.

Inherited clotting disorders (Thromobophilia)

If blood clots occur in more than one person in a family, there may be an inherited clotting disorder. Genetic mutations can cause defective blood clotting factors. Factor V (five) Leiden and the prothrombin gene mutations are among the most commonly encountered gene mutations in the community. Individuals who inherit one of these mutations from either parent (heterozygous) may experience recurrent episodes of DVT, but the risk is highest if both parents were affected (homozygous).

Other inherited causes include deficiencies of certain blood components (antithrombin, protein C, and protein S) that ordinarily help dissolve blood clots.

Some genetic mutations can lead to increased amounts of homocysteine in the circulation. Homocysteine is a compound that increases the tendency for the blood to clot, as well increasing the risk of stroke and heart attack.


The role of smoking is controversial. In principle, smoking reduces the amount of oxygen in the blood stream and may damage vessel walls, potentially leading to clot formation.

How to reduce the risk of DVT

• If you are scheduled for surgery or other procedures, notify your doctor if you have suffered from a blood clot in the past.

• If driving long distances, break up the trip with frequent rest stops every two hours to walk around.

• If you are on a plane, perform calf stretching exercises at regular intervals and walk around when you can.

• Avoid sitting still for long periods. When watching TV or reading, get up and move around once an hour.

• When sitting, perform heel/toe lifts frequently. This causes the calf muscle to contract and move the blood in your veins, making it less likely to clot.

• If at risk, drink plenty of fluids, but avoid alcohol, coffee and tea, which can contribute to dehydration and increase the risk of clotting.

• If you have an inherited risk of clotting, do not use birth control pills.

• There are blood tests available to determine if you have an inherited problem. If there is a history of DVT in your family, discuss this with your doctor.

• If you have had a DVT in the past, are pregnant, or have varicose veins, it may be helpful to wear compression stockings. Discuss this with your doctor.

• Do not smoke.

Diagnosing DVT

If DVT is suspected, the following tests may be useful to confirm the diagnosis.

Duplex Ultrasound

This is a combination of ultrasound and Doppler (hence ‘duplex’) technology to assess the blood flow. The ultrasound waves travel through the tissue and back, enabling a computer to transform them into a moving image. Duplex ultrasound is the most popular method for diagnosing DVT; it is painless, non-invasive and relatively easy to perform.


Venography involves the injection of a dye into a large vein in the foot or ankle. An x-ray image is then taken to reveal the location of possible clots. Venography is one of the most accurate ways to identify deep vein thrombosis, but it may be uncomfortable. Occasionally it may cause phlebitis, an inflammation of the superficial veins. For these reasons, venography is no longer used routinely for DVT diagnosis.

Magnetic Resonance Imaging (MRI)

MRI uses a strong magnet to create a clear, high quality image of the body’s internal structures. It can be very effective in diagnosing DVT, especially in the thigh and pelvic areas. However, it is an expensive procedure and is only used in special circumstances.

Diagnosing Pulmonary Embolism

The symptoms of pulmonary embolism (PE) are not very specific, and can be present with many other conditions. Up to 75 per cent of patients suspected of having PEdo not actually have this condition. Special diagnostic tests, therefore, need to be performed. The three tests currently used to diagnose PE are pulmonary angiography, nuclear medicine lung scans and spiral CT scans.

Treating DVT

Once a DVT is diagnosed, treatment begins immediately to reduce the risk of the clot extending further or breaking off. Early treatment also reduces the risk of post thrombotic syndrome.

If you have been diagnosed with a DVT, your doctor may recommend treatment at home or in hospital. The type of treatment depends on the location of the clot and the likelihood of further complications. It is important that treatment be started as soon as possible. Patients who receive early treatment will significantly reduce their chances of developing PE.

To treat blood clots, doctors use various drugs, including anticoagulants. Standard anticoagulant medications include heparin and warfarin. Heparin is given either through a vein (intravenously) or as an injection under the skin. Warfarin is given orally as a tablet. Heparin acts immediately, while warfarin takes several days to become effective. Both are usually started at the same time, but heparin is discontinued after warfarin becomes effective. Regular blood tests are required to monitor the effectiveness of warfarin. It is important to take warfarin as directed and to record all dosages.

Two types of heparin are available for treatment of DVT. Unfractionated heparin (UH) is given in hospital, whereas low-molecular-weight heparin (LMWH) can be self-injected at home, which usually is more convenient and less expensive. LMWHhas the added benefit of not usually requiring periodic blood tests to monitor its effects. An example of low-molecular-weight heparin is enoxaparine.

Contrary to popular belief, anticoagulants do not actively dissolve the clot; they prevent new clots from forming and reduce the risk of PE.

In rare cases, clot dissolving (thrombolytic) agents are administered. For patients who are unable to tolerate medications, surgical removal of the clot (thrombectomy) may be performed, as well as insertion of a filter in a major vein that acts like a clot sieve.

To minimise any pain, discomfort or complications associated with the treatment of a DVT, you should:

• elevate the affected leg whenever possible

• wear graduated compression stockings

• avoid long periods of immobility.

How long do patients need to be treated for a DVT?

Depending on the underlying condition that caused the DVT, treatment may be needed for as little as four weeks, or for the rest of the patient’s life. In patients with an obviously reversible cause of a DVT (for example, trauma, after orthopaedic surgery or prolonged bed rest), four to six weeks of therapy are usually adequate. The risk of a recurrent DVT in these patients is very low. 

In patients with an identified cause of DVT that cannot be reversed (for example, cancer, inflammatory bowel disease or inherited conditions), therapy should continue for at least six to 12 months, and may need to be continued indefinitely.

Patients with no identifiable cause of their DVT have been shown to experience a high incidence of recurrent blood clots if they are treated for three months or less. Most physicians, therefore, recommend at least six months of anticoagulant therapy for these patients.

With treatment, the clot stabilises and attaches firmly to the wall of the vein. The risk of the clot breaking off and travelling to the lungs, therefore, decreases over time.

Long-term consequences of DVT & PE

Very early in the development of a DVT, the body actually produces blood factors that help dissolve the clot. If the clot is in a superficial vein of the leg, or even in a calf vein, the clot frequently will dissolve totally. However, almost 90 per cent of patients with a large DVT in the thigh or groin will be left with some abnormality in the deep veins, even after adequate treatment.

In about 25 per cent of cases, DVT damages the affected vein and leads to long-lasting post-thrombotic syndrome (PTS). This presents as brownish discoloration of the skin, itching, swelling, slow-healing sores and pain. The affected skin becomes scarred and fragile, and can easily break down into an ulcer. It can also increase the risk of additional blood clots.

PTS almost always occurs within two years of the initial diagnosis of DVT. The damage may be partially prevented, however, if the patient wears graduated compression stockings as part of their long-term care for DVT.

A small number of patients suffer from recurrent pulmonary emboli, which can lead to high blood pressure in the lungs. This condition, in turn, can cause problems with the functioning of the heart and may require further medical or surgical treatment.

Frequently Asked Questions

1. Can DVT affect young and healthy people?

DVT mainly affects the older age group (over 60), but all ages are affected. Be aware of the DVT risk factors and adopt strategies to reduce this risk. Individuals carrying the abnormal clotting genes may suffer multiple blood clots at a young age.

2. Is the oral contraceptive pill a risk factor for DVT?

Yes. However, the vast majority of women on the pill do not suffer from blood clots. Other risk factors, such as prolonged immobility, should be avoided when taking the contraceptive pill.

3. Does being overweight increase the risk of DVT?

Obesity is a known risk factor for cardiovascular diseases, including the development of DVT. Obesity is usually associated with reduced mobility, which compounds the DVT risk.

4. If I develop DVT, do I have to be on treatment forever?

No, not usually. In patients with a clear DVT trigger that is reversible, treatment usually takes six to 12 weeks. For cases in which the risk of developing new thromboses remains high (such as in patients with certain cancers or genetic abnormalities), therapy may need to be continued for months to years. Some patients remain on oral medication for life.

5. What can I do to help prevent DVT on a long flight?

Wear comfortable, loose-fitting clothes for travel. Drink plenty of water or juice, but avoid drinking alcohol. When seated, shift the position of your body periodically. You can also do simple exercises, or walk around the plane when safely permitted. Wear compression stockings, and avoid sleeping tablets. Sometimes, a preventative anticoagulation is used, which can be self-injected by the patient

6. Can sclerotherapy treatment on leg veins cause DVT?

The DVT risk associated with sclerotherapy is exceptionally low, making it one of the safest procedures for vein treatment. Post-treatment compression and walking are important factors that help prevent DVT.

7. Does taking aspirin prevent DVT?

Although commonly taken and prescribed, there is no evidence that aspirin reduces the likelihood of DVT.

8. Can surgery on leg veins cause DVT?

The DVT risk associated with surgery can be quite significant. Reports of five per cent DVT after varicose vein operation on one leg, and 15 per cent after varicose vein operation on both legs, have appeared in scientific literature. The risk varies depending on patient factors such as age, weight, gender and degree of mobility after surgery. Most patients are discharged on the same day and are mobile fairly quickly, which is important in prevention of DVT.