Glossary of Terms

World Thrombosis Day Key Terms (Updated 2022)

Know the Key Terms

The following are medical terms related to thrombosis – and specifically blood clots in the leg or lung – that you should know and that may be useful.

Anticoagulant medication – Sometimes called blood thinners; anticoagulants are used to stop the formation of blood clots, thereby reducing the risk of clots in the leg or lungs, strokes and other dangerous events. Examples include: heparin, warfarin, enoxaparin and newer drugs such as dabigatran, rivaroxaban, apixaban and edoxaban.

Antiphospholipid Syndrome (APS or APLS) – Antiphospholipid antibody syndrome (APS) is when the body’s immune system mistakenly creates antibodies that make your blood much more likely to clot. APS can lead to many health problems including VTE. Very rarely, some people who have APS develop many blood clots within weeks or months. This condition is called catastrophic antiphospholipid syndrome (CAPS).

Antithrombin Deficiency – Antithrombin is an anticoagulant found in the body that limits the blood’s ability to clot and keeps the blood thin. Antithrombin deficiency is when the body does not make enough antithrombin and increases the likelihood of forming a blood clot.

Arterial thrombosis – A blood clot that develops in an artery. A clot in a coronary artery blocks blood flow to the heart and is the underlying cause of most heart attacks. A clot that blocks blood flow in an artery in the brain is a major cause of strokes.

Atherosclerosis – A disorder caused by a buildup of plaque (a waxy substance containing fat and cholesterol) on the inner walls of large arteries. This narrows the artery and slows the flow of blood. Atherosclerosis/plaques are the underlying process on which thrombosis can take place if ruptured.

Atrial fibrillation – An irregular and often rapid heartbeat that can lead to clot formation in a chamber of the heart. In atrial fibrillation, the heart’s upper chambers called the atria beat irregularly and out of synch with the lower chambers. Atrial fibrillation can cause a stroke if the clot breaks free and travels to the brain.

Blood clot – A thick mass of blood cells, platelets and fibrin. Clotting is a natural process to stem the flow of blood from damaged blood vessels.

Blood vessels – Include (1) arteries, which carry blood from the heart to the brain, limbs and organs; (2) veins, which carry blood from the limbs and body organs toward the heart; and (3) capillaries, very small vessels that connect the two.

Cardiovascular disease – Any disease affecting the heart or the circulatory system.

Clotting – The process in which liquid blood becomes a solid mass (called a thrombus). Clotting is also called coagulation. This process is important to prevent excessive bleeding when a blood vessel is injured (such as when you cut yourself). However, the process can be harmful when clots form inside the vessel and block the flow of blood.

Clotting factors – A group of proteins (sometimes called “factors”) in the blood that works together to cause blood clotting.

Deep vein thrombosis (DVT) – A blood clot that forms in the veins located deep within a limb, usually the lower leg or thigh. By blocking the flow of blood back to the heart, these clots are often characterized by pain and swelling of the leg. Clots in the leg can break off, travel to the lungs and lodge there as pulmonary embolism (PE). These can be fatal because they block the flow of blood from the lungs back into the heart.

D-dimer – A molecule released from the breakdown of clot; raised levels may indicate a deep vein thrombosis (DVT) or pulmonary embolism (PE), but levels are also raised in many other conditions. Measurement of D-dimer is useful to doctors in helping rule out a diagnosis of DVT or PE.

Embolus – A mass, usually a detached blood clot, that travels through the bloodstream and the heart and then lodges in an artery, blocking it.

Factor V deficiency – An inherited bleeding disorder in which the clotting factor V (five) is low. The disorder is very rare, occurring in only 1 in 1,000,000 people. This is not the same as factor V Leiden.

Factor V Leiden – Factor V Leiden is caused by a change in the gene for Factor V, which helps the blood to clot. When a person inherits Factor V Leiden, the Factor V molecule in the blood is more resistant to breaking down and the clotting process takes longer, which makes a person more prone to blood clots.

Factor Xa – Factor Xa converts prothrombin to thrombin, which then converts fibrinogen to fibrin, a blood clot. Anticoagulant drugs known as Xa inhibitors act by inhibiting Factor Xa and preventing the formation of thrombin.

Fibrin – The protein substance in blood clots; fibrin creates a web-like structure that binds together platelets and red and white blood cells at the site of injury.

Hemostasis (or Haemostasis) – Hemostasis is a physiological process that maintains blood in a fluid state normally and prevents excessive bleeding from damaged vessels. Because hemostasis has to keep blood fluid in the vessel and form clots when the vessel is damaged – two opposing roles – it is very complicated. Hemostasis is responsible for the balance between bleeding and thrombosis.

Hematologist – Physician who specializes in the treatment of blood diseases and disorders. Many combine hematology with oncology (cancer specialist) and treat cancer and blood diseases.

Heterozygous – Having one abnormal gene. If you are heterozygote for factor V Leiden, you have inherited the trait from one parent.

Homozygous – Having two abnormal genes. If you are homozygote for factor V Leiden, you inherited an abnormal gene from both parents.

Hypercoaguable – An abnormally increased tendency to form blood clots, due to an inherited or acquired disorder.

INR (International Normalized Ratio) – Blood test that monitors whether the therapeutic or beneficial effect of anticoagulation is within normal range, usually between 2.0 and 3.0. It is calculated from the prothrombin time (PT), or the time it takes for blood to clot in a test tube. INR can be monitored by a lab, or done by selected patients at home with a self-testing device.

Ischemia – Insufficient oxygen supply due to a blockage or constriction of a blood vessel.

Low Molecular Weight Heparin (LMWH) – A form of heparin (“blood thinner”) that is injected right below the skin. LMWHs’ effects last longer and are more predictable, require less monitoring, and generally have fewer side effects than standard heparin. LMWHs are often used as an alternative to heparin or as “bridging” therapy for patients on oral anticoagulants.

May Thurner Syndrome – May-Thurner syndrome is an often underdiagnosed condition in which the right iliac artery (in the pelvis) compresses and pushes the left iliac vein up against the lumbar spine. This compression slows the blood flow and increases the chances of developing a blood clot.

MTHFR- Stands for Methylene-Tetra-Hydro-Folate-Reductase – Some individuals with the homozygous MTHFR mutation have elevated homocysteine levels. Elevated homocysteine levels are a risk factor for blood clots. The individuals with MTHFR mutations who have normal homocysteine levels are not at increased risk for clots. Thus, the MTHFR mutation by itself is not a clotting disorder. MTHFR mutation has been associated with an increased risk for hyperhomocysteinemia.

Myocardial infarction (MI) – Commonly known as a heart attack, this event is triggered when plaque ruptures in a heart artery. This triggers the formation of a blood clot that blocks the flow of blood and deprives the heart muscle of oxygen. Unless the artery is opened rapidly, an area of the heart is deprived of oxygen, known as ischemia, and the cells can die, which can trigger life threatening abnormal heart rhythms or impair the heart’s pumping activity and lead to heart failure.

Plasma – The liquid portion of blood that contains the clotting factors.

Plasminogen – A substance naturally produced by the body that helps break down blood clots.

Platelet – A small blood particle, which when activated, will clump together with other platelets (and other cells) and contribute to blood clot formation.

Platelet aggregation – One of the first steps in the formation of a blood clot to stem the flow of blood; platelets clump together to form “aggregates” or masses that plug the hole in the vessel.

Post-thrombotic syndrome – A complication of deep vein thrombosis (DVT) caused by damage to the veins. The condition usually causes swelling and heaviness in the legs that becomes worse with standing and is relieved with leg elevation. In severe cases, ulcers can develop around the ankles.

Protein C Deficiency – Protein C is a natural anticoagulant that is found in the body that assists in thinning the blood. When individuals have Protein C deficiency, there is a lack of protein C, which results in the body being more likely to clot.

Protein S Deficiency – Protein S deficiency is a disorder that causes abnormal blood clotting. When Protein S is missing or deficient, clotting may not be regulated normally and affected individuals have an increased risk of forming a blood clot.

Prophylactic therapy – Treatment intended to prevent a medical condition from taking place.

Prothombin 20210 (Factor II) – Individuals have a protein in the blood called prothrombin that that is involved in the clotting process. If an individual makes too much of the prothrombin protein, it increases the tendency for clots to form. If a person has Prothrombin 20210, his or her blood will tend to be thicker and more likely to clot.

Pulmonary embolism (PE) – A blockage of arteries in the lung by a blood clot that has travelled through the bloodstream from elsewhere in the body. Telltale signs include shortness of breath, chest pain, unexplained cough and rapid heart rate. Generally, a PE is caused by a blood clot in a deep vein in the leg (deep vein thrombosis), which breaks off and travels up to the lungs. A pulmonary embolism can be fatal. Therefore, patients with pulmonary embolism require immediate medical attention.

Pulmonary infarction – The death of a small area of lung resulting from pulmonary embolism. It occurs in a small, dead-end pulmonary artery and often causes pain in the back or chest.

Rudolf Virchow – The German physician and pathologist who formulated the major causes of thrombosis in the mid 19th century.

Stasis – The slowing of blood flow through arteries or veins.

Stenosis – Narrowing or constriction of a blood vessel.

Stroke – The interruption of blood flow to the brain as a result of either a blockage in a blood vessel in the brain (ischemic stroke) or bleeding from a ruptured blood vessel in the brain (hemorrhagic stroke). This results in death of brain cells in the affected area.

Thrombin – An enzyme in the blood that converts the soluble protein fibrinogen into insoluble fibrin; the protein that gives blood clots their structure and sets the stage for subsequent wound healing.

Thrombus – Also known as a blood clot, a thrombus is a mass of coagulated blood formed by platelets, and red and white blood cells bound together by fibrin within a blood vessel. If large enough, a thrombus can block the flow of blood in a blood vessel.

Thrombocytopenia – A low platelet count.

Thrombosis – The medical term for a blood clot that forms within a blood vessel can be either a venous or arterial thrombosis.

Thrombophilia – The medical term for genetic or acquired blood abnormalities that increase the risk of abnormal blood clotting.

Transient Ischemic Attack (TIA) – Often labeled mini-stroke, it is a temporary blockage of blood flow in the brain. It is a very serious warning that the individual may be at risk for a full stroke. Approximately one-third of people who experience a TIA will have a stroke within a year if not given preventive treatment.

Veins – The vessels that carry blood low in oxygen back to the heart from the legs and arms, the organs in the abdomen and the brain.

Venous thromboembolism (VTE) – Deep vein thrombosis and pulmonary embolism are collectively known as VTE.

Venography/Venogram – An X-ray that takes pictures of blood flow through the veins in a certain area of the body. During the test, a special dye (contrast material) is put into the veins so they can be seen clearly on an X-ray picture.

Von Willebrand Disease – Von Willebrand is a protein in the body that helps the blood to clot. People with von Willebrand disease (VWD) either have low levels of von Willebrand factor, or the protein does not work the way it should, leading to clotting problems. Many people who have VWD also have low levels of Factor VIII, which is another protein that assists in clotting.

Virchow’s Triad – The three factors involved in thrombosis, which include injury to the vessel wall (such as from trauma or surgery), abnormality of blood clotting, called “hypercoagulablity” of blood, which increases the risk of clots in blood vessels (caused by many factors including admission to hospital with an illness or for surgery, cancer, estrogen therapy, pregnancy, obesity, etc.) and slow blood flow (caused by immobility or paralysis).

VTE protocol – A standardized VTE risk assessment that is linked to a menu of appropriate prevention options, including pharmacologic and mechanical prophylaxis.