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Medically reviewed by C.H. Weaver M.D. Medical Editor 9/2020

A blood clot is a mass of thickened blood. Clotting is a mechanism used by the body to stop bleeding. A blood clot becomes harmful when it blocks an artery or vein and stops blood flow. Doctors refer to a blood clot occurring in a vein as a venous thromboembolism. Cancer is recognized as an independent and major risk factor for the development of venous thromboembolism (VTE).

Having cancer can increase the risk of VTE. In a retrospective observational study of 2119 patients, 20% of patients with VTE were cancer patients. Not only does cancer increase the risk for VTE, but blood clots are often the actual cause of death for people with cancer.

If blood clots are diagnosed or you are considered to be at risk of forming blood clots, drugs that prevent clotting, referred to as anti-coagulants, may be administered to decrease your risk of clotting.

  • What is a blood clot?
  • When can a blood clot become harmful?
  • What are the signs and symptoms of a blood clot?
  • Who is at increased risk for harmful blood clots?
  • How are blood clots diagnosed?
  • How are blood clots treated?

What is a blood clot

Clotting is a mechanism used by the body to stop bleeding. The first step in clotting is adhesion of platelets, which are fragments of blood cells that circulate in the blood, to the cut edges of a damaged blood vessel. In this way, a platelet plug is formed and external bleeding stops. Next, small molecules, called clotting factors, cause strands of materials, called fibrin, to stick together and seal the inside of the wound. Eventually, the cut blood vessel heals and the blood clot dissolves after a few days.

When can a blood clot become harmful

A blood clot becomes harmful when it blocks an artery or vein and stops blood flow. The blood clot is then called a thrombus. Although a thrombus may occur in any blood vessel, it most commonly develops in the veins of the leg and can travel through the major blood vessels of the pelvis and lung where it can be fatal. A thrombus in the leg or pelvic vein is called a deep vein thrombosis (DVT).

A thrombus that breaks free and travels through the bloodstream it is called an embolus. As it travels, it compromises blood flow and may become lodged in a smaller blood vessel, causing blockage. For example, if an embolus blocks an artery in the lung, it is called a pulmonary embolism.

What are the signs and symptoms of a blood clot

Common signs of a DVT include:

  • Pain in the calf or leg muscle
  • Swelling
  • Tenderness
  • Discoloration
  • Prominent veins

Common symptoms of a pulmonary embolus include:

  • Coughing up blood
  • Shortness of breath
  • Chest pain

Less common symptoms may include pain in the back, shoulder or upper abdomen; dizziness; fainting; painful respiration; new onset of wheezing; and any new heart arrhythmia.

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An embolus may also travel to the heart, brain, or eye. An embolus in the brain can cause a stroke to occur. An embolus blocking an artery in the heart can cause a heart attack. An embolus may cause sudden blindness in one eye. An embolus that blocks an artery is a life- threatening condition. For this reason, symptoms of a DVT or an embolus require immediate care.

Who is at increased risk of harmful blood clots

Several factors can increase the risk of blood clots. Patients with cancer are at a higher risk. Some other factors include:

  • Decreased mobility, bed rest
  • Obesity
  • Elderly
  • Chemotherapy
  • History of deep vein thrombosis
  • Fractures
  • Arteriosclerosis
  • Oral contraceptives (birth control pills)
  • Smoking

How are blood clots diagnosed

Ultrasound: The most common test used to diagnose a DVT is called an ultrasound. This technique uses sound waves to evaluate the flow of blood in your veins. A gel is put on the skin of your leg. A handheld device is placed on the leg and passed back and forth over the affected area. This device sends sound waves from the leg to an ultrasound machine. A computer then creates a visual image based on the different frequency sound waves that are recorded. The picture is displayed on a monitor, allowing your doctor to see the blood flow in your leg.

Venography: If the ultrasound does not provide a clear diagnosis, venography may be performed. A venogram is an x-ray used to examine veins. A dye is injected into a vein and then an x-ray is taken of the leg. The dye makes the vein visible on the x-ray. If the blood flow in the vein is impaired, it will show on the x-ray.

CT scan: Pulmonary embolism and DVT may also be diagnosed with computed tomography or “CAT” scans. A CT scan is similar to a venogram in that is uses x-ray equipment. However, CT provides a 3-dimensional image through the use of special x-ray equipment that is constructed in a ring. When a CT scan is used to diagnose an embolism, a dye is injected into the patient’s bloodstream through the arm. The scanner then rotates around the table, scanning the lungs and the veins of the legs from many different directions. Blood clots are highlighted by the dye.

How are blood clots treated

If blood clots are diagnosed or you are considered to be at risk of forming blood clots, anti-coagulant drugs that prevent clotting will be administered. A commonly used anti-coagulant drug is warfarin, which is administered in a pill.

Although treatment with warfarin is usually effective in most people with blood clots, it may be less effective in cancer patients. Also, treatment with warfarin may be complicated because anti-clotting is affected by poor diet and liver malfunction, which may be common problems in cancer patients. Heparin is another anti-coagulant drug. It is administered intravenously or subcutaneously and immediately blocks the clotting process.

Heparin is often administered for the first few days of anti-coagulant treatment since warfarin takes several days to begin working. For example, if blood clots are found in your legs or lungs, you will typically remain in the hospital for a few days and receive heparin to “thin” your blood. Once your risk of clotting has been decreased with heparin, you will stop receiving this drug, and warfarin will be prescribed to keep your blood “thin”.

There are several other available "anti-coagulants" that include:

  • apixaban (Eliquis)
  • dabigatran (Pradaxa)
  • edoxaban (Lixiana)
  • rivaroxaban (Xarelto)


  1. Goldhaber SZ, Bounameaux H. Pulmonary embolism and deep vein thrombosis. Lancet 2012;379:1835-1846
  2. Khorana AA, Francis CW, Culakova E, Kuderer N, Lyman H, et al. Thromboembolism in hospitalized cancer patients. J Clin Oncol. 2006;24:484–90.
  3. Sallah S, Wan JY, Nguyen NP. Venous thrombosis in patients with solid tumors: determination of frequency and characteristics. Thromb Haemost. 2002;87:575–9.
  4. Davide Imberti, et al. Clinical characteristics and management of cancer-associated acute venous thromboembolism: findings from the MASTER Registry; haematologica | 2008; 93(2)
  5. Mahajan A, Burnson A, White R, Wun T. The epidemiology of cancer-associated venous thromboembolilsm; an update. Semin Thromb Hemost 2019;45:321-25.