What is factor V Leiden (FVL)?

Factor V Leiden (FVL), or factor “5” Leiden, is a genetic mutation (change) that makes the blood more prone to abnormal clotting. Factor V Leiden is the most common genetic predisposition to blood clots. Individuals born with FVL are more likely to develop vein clots (deep vein thrombosis or DVT) and pulmonary embolism (PE), but not heart attacks, strokes or blood clots in the arteries of the legs.

FVL results from a mutation (change) of the gene that makes one of the proteins of our coagulation system, called factor V (or “factor 5”). Because each protein in our body is made by two different genes (one inherited from Mom, one inherited from Dad), there are two types of FVL: “heterozygous” (in which only one mutated copy of the F5 gene was inherited, either from the mother or the father), and “homozygous” (in which two mutated copies were inherited, one from each parent). The “heterozygous” FVL is by far the most common mutation. If you have FVL, the risk of developing abnormal blood clots may depend on whether you have the heterozygous or the homozygous mutation.

People who are born with FVL are at higher risk of developing DVT and PE. While DVT usually involves the veins of the legs or arms, it can more rarely occur in other veins of the body, such as veins of the liver, kidneys, intestines or brain. Pulmonary embolism occurs when pieces or fragments of a blood clot – usually from a DVT of the leg - break off and travel to the vessels in the lungs.

The mutation is named factor V “Leiden” because it was originally discovered at the University of Leiden in the Netherlands, in 1994.

Who is likely to have factor V Leiden (FVL)?

FVL can only be inherited from a parent who has the mutation, which is more common among individuals of Northern European ancestry. Children from a parent with heterozygous FVL mutation have a 25% chance of having inherited it from the parent who has the mutation.

How common is factor V Leiden (FVL)?

In the U.S., FVL is present in approximately 5% of the general population. It is less common in Native Americans and African-Americans than in those of Northern European ancestry.

In some countries of Northern Europe, FVL is found in up to 10-15% of the population. It is less common in South America, Africa and Asia, where it can be found in less than 1-3% of the population.

What causes factor V Leiden (FVL)?

FVL is caused by a genetic mutation to the Factor V (or “factor 5”) gene. This gene helps our body make the coagulation factor V protein, which is one of the many proteins in our coagulation system that help our blood clot after an injury. The FVL mutation makes coagulation factor V work in “overdrive,” which increases the tendency of the blood to clot. As a result, people with FVL are at higher risk of developing abnormal blood clots.

What are the symptoms of factor V Leiden (FVL)?

The FVL mutation in itself does not cause any symptoms. Most people who inherited FVL from their mother and/or father will never develop abnormal blood clots, so they may not even know that they have it. Some will have a family history of deep vein thrombosis/pulmonary embolism (DVT/PE) but will never have an abnormal blood clot in their lifetime.

People born with the FVL may have a personal or a family history of:

  • DVT or PE before age 60
  • Recurring DVT or PE
  • DVT or PE during or immediately following pregnancy
  • DVT or PE soon after beginning to take birth control pills or other hormonal treatments

When people who have FVL develop any symptoms, these will be related to DVT and PE, so it is important to recognize the symptoms of DVT or PE. Both of these conditions are medical emergencies.

If you have a DVT in a vein of the leg or arm, your symptoms in the affected area may include:

  • Swelling
  • Pain, tenderness
  • Purple or “bluish” discoloration of the skin
  • Skin that is warm to the touch, sometimes with a vivid red discoloration

Symptoms of PE may include:

  • Sudden shortness of breath
  • Sharp chest pain that gets worse by taking deep breaths, coughing or sneezing
  • Rapid heartbeat and palpitations (tachycardia)
  • Fainting or near-fainting
  • Coughing up blood

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