Factor V Leiden

Factor V represents an important control step in the blood clotting process.

What is Factor V Leiden?

Blood clotting is a complex process involving many steps and substances. One of these substances is a protein called Factor V (pronounced Factor Five). Factor V represents an important control step in the clotting process. Another protein, activated Protein C, turns off the clotting cascade by deactivating Factor V. Children born with Factor V Leiden produce a mutated form of Factor V that does not respond well to activated Protein C. This may lead to abnormal clot formation.

Signs & Symptoms

Factor V Leiden is a genetic disorder. An abnormality in the affected individual's DNA results in the production of an abnormal form of Factor V. The amount of abnormal Factor V, and the severity of disease, depends on the presence of one or two copies of the mutated gene. Those with one copy are 10 times as likely to have a significant abnormal clot in their lifetime compared to those born without the abnormal gene. Those with two copies are 60 times more likely.

Approximately 5% of the Caucasian population carries at least one Factor V Leiden gene. It is less common in other ethnic groups. Having one or two copies of the Factor V Leiden gene does not guarantee abnormal clots will form because other circumstances are involved. The lifetime incidence of an abnormal clot in the general population is 1 in every 100,000 people. This risk increases to 1 in every 10,000 people if one copy of the abnormal gene is present. It increases to 6 in every 10,000 people if two copies are present. This means most people with Factor V Leiden will never experience an abnormal clot in the course of their lives.

The vast majority of those affected with Factor V Leiden will never encounter a problem and most will never be aware they are affected. However, when abnormal clots do form, the symptoms can be severe. The exact set of symptoms depends on the location of the abnormal clot. Common locations include leg veins (especially in adults) and at the site of indwelling plastic catheters (IVs, central lines), which are common in children with underlying chronic health conditions. Clots at these locations may cause dysfunction of the associated catheter and painful swelling of the affected extremity. If a piece of this abnormal clot breaks off, it can float downstream and ultimately become trapped in the lung. This is called a pulmonary embolism. If the piece of clot is small, it may not cause a problem. However, if the clot piece is large, a pulmonary embolism may be life-threatening. Abnormal clots in the brain can also cause significant and potentially life-threatening problems.

The clotting process involves many steps and substances. While Factor V Leiden is one possible cause of abnormal clot formation, there are many others. Examples include abnormalities in Protein C, Protein S, Antithrombin, and Prothrombin 20210. Another disorder which leads to increased clotting is the Anti-Phospholipid Syndrome. This is an autoimmune disorder in which one's own immune system attacks normal anti-clotting mechanisms.

How is Factor V Leiden Diagnosed? 

The presence of Factor V Leiden is easily diagnosed with a laboratory test. This test looks at an individual's DNA to see if they are making normal Factor V or the abnormal Factor V Leiden. It also identifies if a patient has one or two copies of the Factor V Leiden gene.

What is the Treatment for Factor V Leiden ?

Most people with Factor V Leiden do not form abnormal clots and require no treatment. If abnormal clots become an issue, "blood-thinning" medications, like Coumadin, can be used to impair the clotting system. This will halt growth of the clot and reduce the risk of future clots.


The most serious complications occur when clots block blood flow in the lungs and brain. In the lung, large clots impede the normal exchange of oxygen and carbon dioxide. In the brain, they may cause swelling and prevent oxygen and nutrients from reaching delicate nervous system tissue. In these situations, abnormal blood clots can become life-threatening.

"Blood thinning" medications, like Coumadin, impair the clotting process, which can lead to easy bleeding. This may result in prolonged and dangerous bleeding during surgery and when injuries occur. It's important for patients taking these medications to follow-up regularly with their doctor to ensure the dose remains just right.


Since Factor V Leiden is an inherited condition, there is no way to prevent it. However, there are certain lifestyle changes one can make to reduce the risk of abnormal clot formation. Regular physical activity and avoidance of staying in a single position for a long period of time can reduce the risk of abnormal clots. Those on long car trips and airplane flights should stand up and move about on a regular basis. Estrogen-containing birth control medications and cigarette smoking are other risk factors that should be avoided.