Deep venous thrombosis is a condition in which a blood clot forms in a vein that is deep inside the body.

Deep venous thrombosis (DVT) mainly affects the large veins in the lower leg and thigh. The clot can block blood flow. If the clot breaks off and moves through the bloodstream, it can get stuck in the brain, lungs, heart, or other area, leading to severe damage.

Risks for DVT include:

• Bedrest
• Cigarette smoking
• Fractures
• Giving birth within the last 6 months
• Medications such as estrogen and birth control pills
• Obesity
• Recent surgery (especially hip, knee, or female reproductive organ surgery)
• Sitting for a long time, such as on a long plane or car trip

You're also more likely to develop DVT if you have any of the following conditions:

• Blood that more likely to clot (hypercoagulability)
• Cancer
• Overproduction of red blood cells in bone marrow (polycythemia vera) or related conditions

DVTs are most common in adults over age 60, but can occur at any age.

• Changes in skin color (redness) in one leg
• Increased warmth in one leg
• Leg pain in one leg
• Leg tenderness in one leg
• Swelling (edema) of one leg

Your health care provider will perform a physical exam. The exam may show a red, swollen, or tender leg.

The following tests may be done:

• Doppler ultrasound exam of a limb
• D-dimer blood test
• Plethysmography of the legs
• X-rays to show veins in the legs (leg venography)

Blood tests may be done to check if there is increased blood clotting. Such tests include:

• Activated protein C resistance (checks for the Factor V Leiden mutation)
• Antithrombin III levels
• Genetic testing to look for mutations that make you more likely to develop blood clots, including the prothrombin G20210A mutation
• Lupus anticoagulant or anti-phospholipid antibodies
• Protein C and Protein S levels
• Screening for disseminated intravascular coagulation (DIC)

This list is not all-inclusive.

For years, the standard treatment has been a medication called heparin, which stops blood clots from forming. This type of medicine is called an anticoagulant (also known as a blood thinner).

If heparin is given continuously through a vein (IV), you must stay in the hospital. However, newer forms of heparin that can be given by injection once or twice a day can sometimes be used. You may not need to stay in the hospital as long, or at all, if you are prescribed this newer form of heparin.

The drug warfarin is usually prescribed along with heparin. Warfarin is taken by mouth and is usually used for long-term therapy. It takes several days to fully work. Heparin is continued until the warfarin has been fully effective for at least 24 hours. People usually take warfarin for a minimum of 3 months, but sometimes they must take it for the rest of their lives, depending on their risk for another clot.

You will need frequent lab tests to check the thickness of your blood when you first start taking warfarin. This lets your health care provider properly adjust your dose.

In rare cases, surgery may be needed if medicines do not work. Surgery may involve:

• Placement of a filter in the body's largest vein to prevent blood clots from traveling to the lungs
• Removal of a large blood clot from the vein or injection of clot-busting medicines

Many DVTs disappear without a problem, but they can return. Some people may have long-term pain and swelling in the leg known as post-phlebitic syndrome. Wearing tight (compression) stockings during and after the DVT may help prevent this problem.

A blood clot that breaks free in the leg and travel to the lungs (pulmonary embolus) can be life threatening. Rapid treatment of DVT helps prevent this problem.

Call your health care provider if you have symptoms of DVT.

Go to the emergency room or call the local emergency number (such as 911) if you have DVT and you develop chest pain, difficulty breathing, coughing blood, fainting, loss of consciousness, or other severe symptoms.

Doctors may prescribe blood thinners to help prevent DVT in people at high risk, or those who are undergoing high-risk surgery.

Sometimes patients in the hospital wear special soft boots that automatically (and gently) squeeze the calves periodically. This is called intermittent pneumatic compression. It helps keep blood moving and prevents blood clotting.

Moving your legs often during long plane trips, car trips, and other situations in which you are sitting or lying down for long periods of time can also help prevent DVT.

Brotman DJ, Jaffer AK. Prevention of venous thromboembolism in the geriatric patient.Cardiol Clin. 2008;26(2):221-34, vi.

Ginsburg J. Peripheral venous disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 81.

Snow V, Qaseem A, Barry P, et al. Management of venous thromboembolism: a clinical practice guideline from the American College of Physicians and the American Academy of Family Physicians. Ann Intern Med. 2007;146(3):204-10. Epub 2007 Jan 29.