Topic Overview

What is deep vein thrombosis?

Deep vein thrombosis (DVT) is a blood clot (thrombus) in a deep vein, usually in the legs.

Clots can form in superficial veins and in deep veins. Blood clots with inflammation in superficial veins (called superficial thrombophlebitis or phlebitis) rarely cause serious problems. But clots in deep veins (deep vein thrombosis) require immediate medical care. See pictures of a developing blood clot and the leg veins.

These clots are dangerous because they can break loose, travel through the bloodstream to the lungs, and block blood flow in the lungs (pulmonary embolism). A pulmonary embolism is often life-threatening. DVT can also lead to long-lasting problems. DVT may damage the vein and cause the leg to ache, swell, and change color. It can also lead to leg sores after years of having a DVT.

Blood clots most often develop in the calf and thigh veins, and less often in the arm veins or pelvic veins. This topic focuses on blood clots in the deep veins of the legs, but diagnosis and treatment of DVT in other parts of the body are similar.

Each year in the United States, between 350,000 and 600,000 people get a blood clot in the legs or in the lungs.¹

What causes deep vein clots to form?

Blood clots can form in veins when you are inactive. For example, clots can form if you are paralyzed or bedridden or must sit while on a long flight or car trip. Surgery or an injury can damage your blood vessels and cause a clot to form. Cancer can also cause deep vein thrombosis. Some people have blood that clots too easily, a problem that may run in families.

What are the symptoms?

Symptoms of DVT include swelling of the affected leg. Also, the leg may feel warm and look redder than the other leg. The calf or thigh may ache or feel tender when you touch or squeeze it or when you stand or move. Pain may get worse and last longer or become constant.

If a blood clot is small, it may not cause symptoms. In some cases, pulmonary embolism is the first sign that you have DVT.

How is deep vein thrombosis diagnosed?

If your doctor suspects that you have DVT, you probably will have an ultrasound test to measure the blood flow through your veins and help find any clots that might be blocking the flow. Other tests, such as a venogram, are sometimes used if ultrasound results are unclear. A venogram is an X-ray test that takes pictures of the blood flow through the veins.