Blood Clots

Normally, if you get hurt, your body forms a blood clot to stop the bleeding. Some people get too many clots or their blood clots abnormally.

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Normally, if you get hurt, your body forms a blood clot to stop the bleeding. Some people get too many clots or their blood clots abnormally. Many conditions can cause the blood to clot too much or prevent blood clots from dissolving properly.


  • Certain genetic disorders
  • Atherosclerosis
  • Diabetes
  • Atrial fibrillation
  • Overweight, obesity, and metabolic syndrome
  • Some medicines
  • Smoking

Blood clots can form in, or travel to, the blood vessels in the brain, heart, kidneys, lungs, and limbs. A clot in the veins deep in the limbs is called deep vein thrombosis (DVT). DVT usually affects the deep veins of the legs. If a blood clot in a deep vein breaks off and travels through the bloodstream to the lungs and blocks blood flow, the condition is called pulmonary embolism. Other complications of blood clots include stroke, heart attack, kidney problems and kidney failure, and pregnancy-related problems. Treatments for blood clots include blood thinners and other medicines.


Risk Factors

People at highest risk for excessive blood clotting have both acquired and genetic risk factors. For example, if you smoke and have the Factor V Leiden mutation and atherosclerosis, you’re at higher risk than someone who has only one of these risk factors.


You’re more likely to have a genetic cause of excessive blood clotting if you have:

  • Family members who have had dangerous blood clots
  • A personal history of repeated blood clots before the age of 40
  • A personal history of unexplained miscarriages

Factor V Leiden is one of the most common genetic mutations that can alter the blood clotting process. This mutation accounts for 40–50 percent of genetic blood clotting disorders in Caucasians.


Another risk factor for excessive blood clotting is antiphospholipid antibody syndrome. APS is an autoimmune disorder that can trigger blood clots to form in the body’s arteries and veins. These blood clots can lead to many health problems, including frequent miscarriages.



Normally, blood clots form to seal small cuts or breaks on blood vessel walls and stop bleeding. After the bleeding has stopped and healing has occurred, the body breaks down and removes the clots.

Excessive blood clotting can occur if the body’s clotting process is altered or wrongly triggered. Blood clots can form in, or travel to, the arteries or veins in the brain, heart, kidneys, lungs, and limbs.



Many diseases and conditions can cause the blood to clot too much or prevent blood clots from dissolving properly. Certain diseases and conditions are more likely to cause clots to form in certain areas of the body.

This condition, called APS, is an autoimmune disorder. If you have APS, your body makes antibodies that attack phospholipids—a type of fat. Phospholipids are found in all living cells and cell membranes. In APS, the antibodies trigger blood clots to form in the body’s arteries and veins. These blood clots can lead to many health problems, including frequent miscarriages.

Some bone marrow disorders can cause your body to make too many blood cells that can lead to blood clots. Examples include polycythemia vera, or PV, and thrombocythemia.

Certain diseases, conditions, or factors can trigger excessive blood clotting mainly in the arteries and veins of the heart and brain.

Atherosclerosis is a disease in which a waxy substance called plaque builds up inside your arteries. Over time, the plaque may rupture. Atherosclerosis is a major cause of damage to the blood vessel walls.

Vasculitis is a disorder that causes the body’s blood vessels to become inflamed. Platelets may stick to areas where the blood vessels are damaged and form blood clots.

Diabetes increases the risk of plaque buildup in the arteries, which can cause dangerous blood clots.

Heart failure is a condition in which the heart is damaged or weakened. Heart failure slows blood flow, which can cause blood clots to form.

Atrial fibrillation, or AF, is the most common type of arrhythmia. AF can cause blood to pool in the upper chambers of the heart, which can cause blood clots to form.

Metabolic syndrome. Some research shows that people who have metabolic syndrome are at increased risk for excessive blood clotting. Metabolic syndrome is the name for a group of risk factors that increases your chance of having heart disease and other health problems.

Hospitalization for major surgery, a serious medical illness, trauma, or broken bones. If you’re not able to be active for long periods, blood flow in your veins slows down. As a result, blood clots may form in your legs.

“Coach class.” This term refers to a situation in which you must stay in one position for a long time, such as in a car or plane. Staying in one position can slow blood flow and lead to blood clots in the legs.


Genetic causes of excessive blood clotting most often are due to gene mutations.

Common mutations include Factor V Leiden and Prothrombin G20210A mutations. Fairly rare genetic mutations include proteins C and S deficiencies and antithrombin III deficiency.


Some medicines can disrupt the body’s normal blood clotting process. Medicines containing the female hormone estrogen are linked to an increased risk of blood clots. Examples of medicines that may contain estrogen include birth control pills and hormone therapy.


  • Smoking. Smoking raises the risk of unwanted blood clots and makes it more likely that platelets will stick together. Smoking also damages the lining of the blood vessels, which can cause blood clots to form.
  • Increased homocysteine levels. Homocysteine is an amino acid linked to a high risk of vascular disease. Increased levels of this substance may damage the inner lining of the arteries.
  • Pregnancy. Women are more likely to develop blood clots when they’re pregnant. This is because they have more platelets and clotting factors in their blood during pregnancy. Also, during pregnancy, the uterus compresses the veins. This slows blood flow, which can lead to blood clots.
  • HIV and HIV treatments. The risk of blood clots is highest in HIV patients who have infections, are taking certain medicines, have been hospitalized, or are older than 45.
  • Dehydration. This is a condition in which your body doesn’t have enough fluids. This condition causes your blood vessels to narrow and your blood to thicken, which raises your risk for blood clots.
  • Organ transplants and implanted devices, such as central venous catheters and dialysis shunts. Surgery or procedures done on blood vessels may injure the vessel walls. This can cause blood clots to form. Also, catheters and shunts have a man-made surface that may trigger blood clotting.



Signs and symptoms of excessive blood clotting depend on where the clots form. For example, symptoms of a blood clot in the heart or lungs may include chest pain, shortness of breath, and upper body discomfort in the arms, back, neck, or jaw. These symptoms may suggest a heart attack or pulmonary embolism (PE).

Signs and symptoms of a blood clot in the deep veins of the leg may include pain, redness, warmth, and swelling in the lower leg. These signs and symptoms may suggest deep vein thrombosis (DVT).

Signs and symptoms of a blood clot in the brain may include headaches, speech changes, paralysis, dizziness, and trouble speaking or understanding speech. These signs and symptoms may suggest a stroke.

If you have signs or symptoms of a heart attack, PE, or stroke, call 9–1–1 right away. If you have signs or symptoms of DVT, call your doctor right away. The cause of the blood clot needs to be found and treated as soon as possible.



If your doctor thinks that you have excessive blood clotting based on your signs and symptoms, he or she will look for the cause of the condition.

Your doctor will ask about your medical and family histories, do a physical exam, and review the results from tests.


Your doctor may ask you detailed questions about your medical history and your family’s medical history. He or she may ask whether you or a blood relative:

  • Has a history of repeated blood clots before age 40
  • Has had blood clots during pregnancy or while on birth control pills
  • Has had unexplained miscarriages
  • Has a history of excessive or unusual blood clots (such as clots in the veins in the liver or kidneys)


Your doctor will do a physical exam to see how severe your blood clotting problem is and to look for a possible cause.


Your doctor may recommend blood tests to look at your blood cells and the clotting process. If he or she thinks you have a genetic condition, you may need more blood tests.

Tests to find the cause of excessive blood clotting might be delayed for weeks or even months while you receive treatment for a blood clot.

Initial blood tests will include a complete blood count and a platelet count. These tests measure the number of red blood cells, white blood cells, and platelets in your blood.

You also may need blood tests that look at the proteins active in the blood clotting process and how long it takes them to form a blood clot.

If your doctor thinks your blood clotting condition is genetic, you may need other blood tests. For example, you might need tests to check:

  • For gene mutations that can cause excessive blood clotting
  • For antibodies related to antiphospholipid antibody syndrome—a cause of excessive blood clotting
  • Your homocysteine levels (if you’re at risk for vascular disease)



Excessive blood clotting is treated with medicines. Depending on the size and location of the clot(s), you may need emergency treatment or routine treatment.


Blood clots can be dangerous. They can damage the body and lead to serious problems, such as stroke, heart attack, kidney failure, deep vein thrombosis, or pulmonary embolism. Blood clots also can cause miscarriages, stillbirths, or pregnancy-related problems.

Emergency treatment to prevent these problems often involves medicines that can quickly break up clots. These medicines can cause sudden bleeding. Thus, they’re used only in life-threatening situations.


Anticoagulants, or “blood thinners,” are used as routine treatment for excessive blood clotting. These medicines prevent blood clots from forming. They also keep existing blood clots from getting larger.

Some people have to take blood thinners for the rest of their lives if their risk of forming blood clots remains high.



If you have excessive blood clotting, you should:

  • Know the signs and symptoms of problem blood clots.
  • Take all your medicines as prescribed.
  • Get ongoing medical care.


Knowing the signs and symptoms of problem blood clots is important. You should seek care right away if you have any of these signs or symptoms. The cause of the blood clots needs to be found and treated as soon as possible. You may need emergency care.


Your doctor may prescribe anticoagulants, or “blood thinners,” to prevent blood clots or to keep them from getting larger. You should take these medicines exactly as your doctor instructs.

Tell your doctor about all other medicines you’re taking, including over-the-counter medicines. Some medicines can thin your blood. Taking two medicines that thin your blood may increase your risk of bleeding.


Have blood tests done as your doctor recommends. These tests help track how well your blood is clotting.

The medicines used to treat excessive blood clotting may cause bleeding. Bleeding can occur inside your body (internal bleeding) or underneath your skin or from the surface of your skin (external bleeding). Know the warning signs of bleeding, so you can get help right away.


  • Unexplained bleeding from the gums and nose
  • Increased menstrual flow
  • Bright red vomit or vomit that looks like coffee grounds
  • Bright red blood in your stools or black, tarry stools
  • Pain in your abdomen or severe pain in your head
  • Sudden changes in vision
  • Sudden loss of movement in your arms and legs
  • Memory loss or confusion

A lot of bleeding after a fall or injury or easy bruising or bleeding also might mean that your blood is too thin. Call your doctor right away if you have any of these signs.


Talk with your doctor about lifestyle changes that can help you stay healthy. Ask him or her whether the foods you eat might interfere with your medicines. If you take warfarin, some foods or drinks can increase or decrease the effect of the medicine.

Discuss with your doctor what amount of alcohol is safe for you to drink if you’re taking medicine. If you smoke, ask your doctor about programs and products that can help you quit. Also, try to avoid secondhand smoke.

If you have trouble quitting smoking on your own, consider joining a support group. Many hospitals, workplaces, and community groups offer classes to help people quit smoking.


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