What Causes Excessive Blood Clotting?

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:

• The inner layer of the cells lining the blood vessels. These cells play a major role in causing blood clots to form.
• Clotting factors. These proteins help create a network of fibrin—another type of protein. Fibrin acts as glue to hold a blood clot together.
• Platelets (PLATE-lets). These small blood cell fragments can stick together to form a clot.

Excessive blood clotting may 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.

Certain diseases and conditions, genetic mutations, medicines, or other factors can cause excessive blood clotting.

Diseases and Conditions

A number of 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.

Antiphospholipid Antibody Syndrome

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—the vessels that carry blood to your heart and body. These blood clots can lead to a number of health problems, including frequent miscarriages.

APS is more common in women and people who have other autoimmune or rheumatic disorders, such as lupus.

Bone Marrow Disorders

Some bone marrow disorders can cause your body to make too many blood cells that can lead to blood clots. Examples of such disorders 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 and can lead to blood clots.

Thrombocythemia is a condition in which your body makes too many platelets. The platelets can stick together to form blood clots.

Thrombotic Thrombocytopenic Purpura and Disseminated Intravascular Coagulation

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 clotting, or DIC.

TTP causes blood clots to form in the body's small blood vessels, including vessels in the brains, 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.

Problems With Blood Clot Breakdown

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 is needed to break down fibrin.

Excessive Blood Clotting That Mainly Affects the Heart and Brain

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 blood vessel walls.

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

Atherosclerosis. Atherosclerosis is a disease in which a fatty material called plaque (plak) builds up on the insides of your arteries. Over time, the plaque may crack. Platelets clump together to form blood clots where the cracks are. Atherosclerosis is a major case 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 where the arteries are damaged, and blood clots can form. Vasculitis also is a major cause of damage to the blood vessel walls.

Diabetes. Diabetes increases the risk for plaque buildup in the arteries. Nearly 80 percent of people who have diabetes die from dangerous blood clots. (Seventy-five percent of these deaths are due to complications with the heart and blood vessels.)

Heart failure. Heart failure is a condition in which the heart is damaged or weakened and can't pump enough blood to meet the body's needs. This condition slows blood flow, which can cause blood clots to form.

Atrial fibrillation. Atrial fibrillation (A-tre-al fi-bri-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 a person's overall body weight and where the extra weight comes from. Overweight is having extra body weight from muscle, bone, fat, and/or water. Obesity is having a high amount of extra body fat. These conditions can lead to atherosclerosis, which increases the risk of blood clots.

Excessive Blood Clotting That Mainly Affects the Limbs

Blood clots can form in the veins deep in the limbs. This condition is called deep vein thrombosis (DVT). It mostly affects the deep veins of the legs.

A blood clot in a deep vein can break off and travel through the bloodstream. When 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 may trigger excessive blood clotting mainly in the deep veins of the limbs. These include:

• Hospitalization for major surgery or serious medical illness. This causes nearly half of all cases of DVT and PE. If you’re not able to be active for long periods, blood flow in the veins is slowed and blood clots may form in the legs.
• Hospitalization for serious trauma and broken bones. This is the cause of about 12 percent of all blood clots that form in the deep veins of the 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 on a plane. This can slow blood flow and lead to blood clots in the legs.
• Cancer growth and cancer treatment. This accounts for about 20 percent of blood clots that form in the deep veins of the legs.

Genetic Mutations

Genetic causes of excessive blood clotting are most often 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.

Medicines

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 replacement 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.

Other Factors That Can Alter the Blood Clotting Process

A number of other factors also can alter the clotting process and lead to excessive blood clotting. Common examples include:

• Smoking. Smoking greatly raises the risk of unwanted blood clots. It 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 six times 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 treatment. 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. This increases the chance that clots will form.
• 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.