Excessive blood clotting is a condition in which blood clots form too easily or don't dissolve properly. Normally, blood clots form to seal small cuts or breaks on blood vessel walls and stop bleeding.
Slow blood flow in the blood vessels also can cause blood clots to form. For example, if a blood vessel narrows, blood may slow down as it moves through the vessel.
Excessive blood clotting has many causes. Problems with the blood, blood vessel defects, or other factors can cause the condition. Regardless of the cause, blood clots can limit or block blood flow. This can damage the body's organs and may even cause death.
Excessive blood clotting can be acquired or genetic. Acquired causes of excessive blood clotting are more common than genetic causes.
"Acquired" means that another disease, condition, or factor triggers the condition. For example, atherosclerosis (ath-er-o-skler-O-sis) can damage the blood vessels, which can cause blood clots to form. Atherosclerosis is a disease in which a fatty substance called plaque (plak) builds up inside the arteries.
If excessive blood clotting is genetic, it’s caused by a faulty gene. Most genetic defects that cause excessive blood clotting occur in the proteins needed for blood clotting. Defects also can occur with the substances that delay or dissolve blood clots.
Although the acquired and genetic causes of the condition aren't related, a person can have both. People at highest risk for excessive blood clotting have both causes.
The outlook and treatment for excessive blood clotting depend on the cause of the blood clots, how severe they are, and how well they can be controlled.
Life-threatening blood clots are treated as emergencies. Medicines that thin the blood are used as routine treatment for blood clotting problems. Some people must take these medicines for the rest of their lives.
With medicines and ongoing care, many people who have excessive blood clotting can successfully manage it.
To understand what causes excessive blood clotting, it helps to understand the body's normal blood clotting process.
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.
Blood clotting is a complex process, but it mainly involves:
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. (Arteries and veins are the blood vessels that carry blood to your heart and body.)
Certain diseases and conditions, genetic mutations, medicines, and other factors can cause excessive blood clotting.
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, also called APS, is an autoimmune disorder. If you have APS, your body makes antibodies (proteins) that attack phospholipids (fos-fo-LIP-ids)—a type of fat. Phospholipids are found in all living cells and cell membranes, including blood cells and the lining of blood vessels.
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.
APS is more common in women and people who have other autoimmune or rheumatic disorders, such as lupus. ("Rheumatic" refers to disorders that affect the joints, bones, or muscles.)
Some bone marrow disorders can cause your body to make too many blood cells that can lead to blood clots. Examples include polycythemia vera (POL-e-si-THE-me-ah VE-ra), or PV, and thrombocythemia (THROM-bo-si-THE-me-ah).
PV is a rare blood disease in which your body makes too many red blood cells. These extra red blood cells make your blood thicker than normal. This slows the flow of blood through your small blood vessels, which can cause blood clots to form.
Thrombocythemia is a condition in which your body makes too many platelets. The platelets can stick together to form blood clots.
Two rare, but serious conditions that can cause blood clots are thrombotic thrombocytopenic purpura (throm-BOT-ik throm-bo-cy-toe-PEE-nick PURR-purr-ah), or TTP, and disseminated intravascular coagulation (ko-ag-u-LA-shun), or DIC.
TTP causes blood clots to form in the body's small blood vessels, including vessels in the brain, kidneys, and heart.
DIC is a rare complication of pregnancy, severe infections, or severe trauma. DIC causes tiny blood clots to form suddenly throughout the body.
After a blood clot has done its job, the body normally breaks down the fibrin that holds the clot together.
Several rare genetic and acquired conditions affect the fibrin network that holds blood clots together. Thus, the clots don't break down properly, and they remain in the body longer than needed.
In one condition, for example, the body's fibrin is abnormal and resists being broken down. In another condition, the body has a decreased amount of plasmin. This protein helps break down fibrin.
Any condition that damages the smooth inner surface of the blood vessels can trigger blood clotting. Many of these conditions are acquired. However, some genetic problems also can damage the inner surface of the blood vessels.
Certain diseases, conditions, or factors can trigger excessive blood clotting mainly in the arteries and veins of the heart and brain.
Atherosclerosis. Atherosclerosis is a disease in which a waxy substance called plaque builds up inside your arteries. Over time, the plaque may rupture (break open). Platelets clump together to form blood clots at the site of the damage. Atherosclerosis is a major cause of damage to the blood vessel walls.
Vasculitis. Vasculitis (vas-kyu-LI-tis) 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. Vasculitis also is a major cause of damage to the blood vessel walls.
Diabetes. Diabetes increases the risk of plaque buildup in the arteries, which can cause dangerous blood clots. Nearly 80 percent of people who have diabetes will eventually die of clot-related causes. Many of these deaths are related to complications with the heart and blood vessels.
Heart failure. Heart failure is a condition in which the heart is damaged or weakened. As a result, it can't pump enough blood to meet the body's needs. Heart failure slows blood flow, which can cause blood clots to form.
Atrial fibrillation. Atrial fibrillation (A-tre-al fih-brih-LA-shun), or AF, is the most common type of arrhythmia (ah-RITH-me-ah). An arrhythmia is a problem with the rate or rhythm of the heartbeat. AF can cause blood to pool in the upper chambers of the heart. This can cause blood clots to form.
Overweight and obesity. Overweight and obesity refer to body weight that's greater than what is considered healthy for a certain height. These conditions can lead to atherosclerosis, which increases the risk of blood clots.
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.
Blood clots can form in the veins deep in the limbs. This condition is called deep vein thrombosis (DVT). DVT usually affects the deep veins of the legs.
A blood clot in a deep vein can break off and travel through the bloodstream. If the clot travels to the lungs and blocks blood flow, the condition is called pulmonary embolism (PULL-mun-ary EM-bo-lizm), or PE.
Certain diseases, conditions, or factors can trigger excessive blood clotting mainly in the deep veins of the limbs. Examples include:
Genetic causes of excessive blood clotting most often are due to gene mutations (changes to a normal gene).
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.
Heparin is a medicine commonly used to prevent blood clots. But, in some people, the medicine can cause blood clots and a low platelet count. This condition is called heparin-induced thrombocytopenia (HIT).
HIT rarely occurs outside of a hospital because heparin usually is given in hospitals. In HIT, the body's immune system attacks the heparin and a protein on the surface of the platelets. This attack prompts the platelets to start forming blood clots.
Many other factors also can alter the clotting process and lead to excessive blood clotting. Common examples include:
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.
For more information about the diseases, conditions, and other factors that can lead to excessive blood clotting, go to "What Causes Excessive Blood Clotting?"
You're more likely to have a genetic cause of excessive blood clotting if you have:
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.
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 (an inability to move), 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.
Blood clots can form in, or travel to, the arteries or veins in the brain, heart, kidneys, lungs, and limbs. Blood clots can limit or block blood flow. This can damage the body's organs and cause many problems. Sometimes blood clots can be fatal.
A stroke can occur if blood flow to your brain is cut off. If blood flow is cut off for more than a few minutes, the cells in your brain start to die. This impairs the parts of the body that the brain cells control.
A stroke can cause lasting brain damage, long-term disability, paralysis (an inability to move), or death.
A blood clot in a coronary artery can lead to a heart attack. A heart attack occurs if blood flow to a section of heart muscle becomes blocked. If blood flow isn't restored quickly, the section of heart muscle becomes damaged from lack of oxygen and begins to die.
For more information, go to the Health Topics Heart Attack article.
A blood clot in the kidneys can lead to kidney problems or kidney failure. Kidney failure occurs if the kidneys can no longer remove fluids and waste from your body. This causes a buildup of these fluids and waste in your body, high blood pressure, and other health problems.
If a blood clot travels from a deep vein in the body to the lungs, it's called a pulmonary embolism, or PE. PE is a serious condition that can damage your lungs and other organs and cause low oxygen levels in your blood.
For more information, go to the Health Topics Pulmonary Embolism article.
A blood clot in a vein deep in your arm or leg can cause pain, swelling, redness, or increased warmth in the affected limb. This type of clot is called deep vein thrombosis, or DVT. Deep vein clots can break off, travel to the lungs, and cause PE.
For more information, go to the Health Topics Deep Vein Thrombosis article.
Blood clots can cause miscarriages, stillbirths, and other pregnancy-related problems, such as preeclampsia (pre-e-KLAMP-se-ah). Preeclampsia is high blood pressure that occurs during pregnancy.
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 primary care doctor may refer you to a hematologist. This is a doctor who specializes in diagnosing and treating blood diseases and disorders.
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:
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.
In this situation, your doctor will want to know the number of platelets in your blood. Platelets are blood cell fragments that stick together to form clots.
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.
Clotting proteins or factors react with each other along two pathways called the intrinsic and extrinsic pathways. (A pathway is a string of chemical reactions that always occur in a certain order.) The two pathways join in a common pathway to make a fibrin network that holds blood clots together.
If your doctor thinks your blood clotting condition is genetic, you may need other blood tests. For example, you might need tests to check:
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, called thrombolytics or "clot busters," are used to treat large clots that cause severe symptoms. 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.
Blood thinners are taken as either a pill, an injection under the skin, or through a needle or tube inserted into a vein (called intravenous, or IV, injection).
Warfarin and heparin are two types of blood thinners. Warfarin is given in pill form. (Coumadin® is a common brand name for warfarin.) Heparin is given as an injection or through an IV tube.
Your doctor may treat you with both heparin and warfarin at the same time. Heparin acts quickly. Warfarin takes 2–3 days before it starts to work. Once the warfarin starts to work, the heparin is stopped.
Pregnant women are treated with heparin because warfarin can harm the fetus.
Sometimes aspirin is used with warfarin. Other times, aspirin might be used alone. Aspirin also thins the blood and helps prevent blood clots.
Some people have to take blood thinners for the rest of their lives if their risk of forming blood clots remains high.
Side effects. The most common side effect of blood thinners is bleeding. This happens if the medicine thins your blood too much. This side effect can be life threatening. 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. They include:
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.
People who are treated with blood thinners usually need regular blood tests called PT and PTT tests to check how well their blood is clotting.
These tests also show whether you're taking the right amount of medicine. Your doctor will check to make sure that you're taking enough medicine to prevent clots, but not so much that it causes bleeding.
Short-term treatments may include treatment with antithrombin factor and protein C. Antithrombin factor might be helpful for people who have known antithrombin deficiencies. This treatment is used in special situations, such as before surgery if there's a very serious blood clot or repeat blood clots.
Some people who have antithrombin deficiencies may need this treatment because heparin doesn't work for them.
Protein C might be given to people who have protein C deficiencies before they receive warfarin. Protein C is given to protect them from skin necrosis—a serious side effect of warfarin. (Skin necrosis destroys and damages the skin cells.)
You can't prevent genetic causes of excessive blood clotting. However, you can take steps to control or avoid some acquired risk factors.
Your doctor may prescribe anticoagulants, or "blood thinners," before, during, and/or after surgery or medical procedures to prevent excessive blood clotting.
If you have excessive blood clotting, you should:
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.
Sexually active women who take warfarin should use birth control because warfarin can cause birth defects. Warfarin is a type of blood thinner.
If you need surgery, your doctor may adjust the amount of medicine you take before, during, and/or after the surgery to prevent bleeding. This also may happen for dental work, but it's less common.
If you take blood thinners, let everyone on your health care team know.
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. They include:
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.
For more information about how to quit smoking, go to the Health Topics Smoking and Your Heart article and the National Heart, Lung, and Blood Institute's (NHLBI's) "Your Guide to a Healthy Heart." Although these resources focus on heart health, they both include general information about how to quit smoking.
The National Heart, Lung, and Blood Institute (NHLBI) is strongly committed to supporting research aimed at preventing and treating heart, lung, and blood diseases and conditions and sleep disorders.
NHLBI-supported research has led to many advances in medical knowledge and care. Often, these advances depend on the willingness of volunteers to take part in clinical trials.
Clinical trials test new ways to prevent, diagnose, or treat various diseases and conditions. For example, new treatments for a disease or condition (such as medicines, medical devices, surgeries, or procedures) are tested in volunteers who have the illness. Testing shows whether a treatment is safe and effective in humans before it is made available for widespread use.
By taking part in a clinical trial, you may gain access to new treatments before they're widely available. You also will have the support of a team of health care providers, who will likely monitor your health closely. Even if you don't directly benefit from the results of a clinical trial, the information gathered can help others and add to scientific knowledge.
If you volunteer for a clinical trial, the research will be explained to you in detail. You'll learn about treatments and tests you may receive, and the benefits and risks they may pose. You'll also be given a chance to ask questions about the research. This process is called informed consent.
If you agree to take part in the trial, you'll be asked to sign an informed consent form. This form is not a contract. You have the right to withdraw from a study at any time, for any reason. Also, you have the right to learn about new risks or findings that emerge during the trial.
For more information about clinical trials related to excessive blood clotting, talk with your doctor. You also can visit the following Web sites to learn more about clinical research and to search for clinical trials:
For more information about clinical trials for children, visit the NHLBI's Children and Clinical Studies Web page.
The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.