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Thrombosis (throm-BO-sis) refers to the formation of a blood clot in a blood vessel. While clots can form in an artery or a vein, this article focuses only on clots that occur in veins, called venous thrombosis.

The clot, or "thrombus," blocks or impairs blood flow in the vein, leading to symptoms and secondary complications.

Thrombosis can happen for no apparent reason, but it is often associated with an inherited predisposition to blood clots, surgical procedures, immobility, oral contraceptive use or underlying medical conditions. Medical conditions such as pregnancy, cancer, inflammatory bowel disease, certain rheumatologic disorders and obesity are known to increase your risk for thrombosis.

Thrombosis often occurs in the legs. When it affects the deep veins — rather than the superficial veins you can see under your skin — it is called a deep vein thrombosis (also called deep venous thrombosis), or DVT.

Embolism (EM-bo-lizm) refers to the sudden blockage of a blood vessel from a clot or other material. Unlike a thrombus that develops at the site of blockage, an embolism originates in one location in the body and travels to a second site where it causes the blockage. One of the more common types of embolism occurs in the lung, called a pulmonary embolism (PE). Pulmonary embolisms usually come from a deep vein thrombosis that formed in the leg or pelvic region, and they can be life threatening.

Our approach to venous thrombosis

The main treatment for deep venous thrombosis, or blood clots that form in the deep veins, is anticoagulation therapy. Anticoagulant medications can prevent clots from forming and stop an existing clot from getting bigger, as the body works to clear the clot on its own. A small percentage of patients also need a filter inserted into the large vein that leads from the legs to the heart, to block any future clots from reaching the lungs. UCSF's world-class hematologists and interventional radiologists work together to provide expert insertion and removal of these filters.

For mothers-to-be who are at particular risk for blood clots, our hematology team works closely with UCSF obstetricians who specialize in high-risk pregnancy. This partnership provides the best possible treatment before, during and after pregnancy.

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Signs & symptoms

A venous thrombosis can form anywhere in the body, and symptoms depend on the site of the clot.

The legs are one of the more common sites. Signs and symptoms include pain or stiffness, swelling and redness in the affected leg. The discomfort may start anywhere from the upper thigh to the ankle. All of these signs and symptoms do not need to be present if you have a blood clot in your leg. Often, patients experience only swelling and some discomfort. If the clot or part of the clot travels to the lungs — a pulmonary embolism — you can experience chest pain, shortness of breath or lightheadedness.

Some patients receive drugs or fluids via catheters placed into veins. Catheters can stimulate thrombus formation in the vein near and around the catheter, called a catheter-associated DVT. Because the catheters are usually located in an arm or in the chest, such a thrombus can lead to arm or neck swelling, with or without pain.

Patients with inflammatory bowel disease and patients with certain blood disorders, including myeloproliferative neoplasms and paroxysmal nocturnal hemoglobinuria, sometimes develop thrombosis in veins in the abdomen. This is called a mesenteric venous thrombosis. Mesenteric venous thrombosis sometimes causes abdominal discomfort, but may have no associated symptoms.


An ultrasound exam is the most common test used to diagnose a blood clot in the leg or arm. A CAT (or CT) scan, a sophisticated type of x-ray, is commonly used to diagnose a pulmonary embolism. A CAT scan is also the most common method used to diagnose a thrombosis in the abdomen, pelvis and head.

A thrombosis in the chest, abdomen or pelvis is sometimes discovered during a CAT scan performed for a different reason. The patient and doctor need to develop individualized treatment plans for these incidentally discovered clots.

In addition to finding the clot itself, the diagnostic evaluation sometimes involves blood tests that can help explain why the clot happened in the first place. Some of these tests look for specific genetic changes, others look for antibodies in your blood, and others test for the amount or activity of proteins that help you form and clear blood clots.


Anticoagulation is the cornerstone for treating deep venous thrombosis and pulmonary embolism. Anticoagulation medications reduce the body’s ability to form blood clots. Some people call anticoagulants "blood thinners," but they do not actually "thin" your blood, they impair its ability to clot.

Blood vessels get small defects as part of everyday life, and the clotting system patches those defects to prevent bleeding. The clotting system is also responsible for stopping bleeding when you cut yourself. The goal of anticoagulation therapy is to find a middle ground where you continue to clot when your body needs to clot, yet at the same time prevents your thrombosis or embolism from getting worse. That is, anticoagulation therapy seeks to prevent new clots and the growth of current clots, while your body’s natural system for removing clots — called the 'lysis' or 'lytic’ system — clears the thrombus or embolism.

Anticoagulation therapy traditionally involves heparin or heparin-like medicines given by vein or by a shot under the skin, followed by an oral medication called warfarin (Coumadin). Patients taking warfarin need regular monitoring with a blood test called an INR. New oral anticoagulants are becoming available that do not require regular blood test monitoring, and some of them do not require receiving anticoagulants by vein or under the skin. For now, however, most DVTs and PEs are treated by medicines given by vein or under the skin, followed by an oral anticoagulant, most commonly warfarin.

In some circumstances, patients need a physical device, a "filter," inserted into the large vein that leads from their legs to their heart, called the inferior vena cava. The filter serves to block any future emboli from reaching the lung. While only a small fraction of patients require such a device, UCSF has world-class interventional radiologists who work closely with our hematologists to provide expert insertion and removal of these filters when needed.

For obstetrical patients who have had or are predisposed to having a blood clot, UCSF hematologists work closely with the UCSF high-risk obstetricians to provide optimal treatment plans before, during and after pregnancy.

UCSF Health medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.

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