Factor V Leiden thrombophilia - Not a rare disease

Other Names: Hereditary resistance to activated protein C; APC resistance, Leiden type Factor V Leiden thrombophilia is an inherited disorder of blood clotting. Factor V Leiden is the name of a specific gene mutation that results in thrombophilia, which is an increased tendency to form abnormal blood clots that can block blood vessels.

Epidemiology

Factor V Leiden is the most common inherited form of thrombophilia. Between 3 and 8 percent of people with European ancestry carry one copy of the factor V Leiden mutation in each cell, and about 1 in 5,000 people have two copies of the mutation. The mutation is less common in other populations.

Cause

A particular mutation in the F5 gene causes factor V Leiden thrombophilia. The F5 gene provides instructions for making a protein called coagulation factor V. This protein plays a critical role in the coagulation system, which is a series of chemical reactions that forms blood clots in response to injury.

The coagulation system is controlled by several proteins, including a protein called activated protein C (APC). APC normally inactivates coagulation factor V, which slows down the clotting process and prevents clots from growing too large. However, in people with factor V Leiden thrombophilia, coagulation factor V cannot be inactivated normally by APC. As a result, the clotting process remains active longer than usual, increasing the chance of developing abnormal blood clots.

Other factors also increase the risk of developing blood clots in people with factor V Leiden thrombophilia. These factors include increasing age, obesity, injury, surgery, smoking, pregnancy, and the use of oral contraceptives (birth control pills) or hormone replacement therapy. The risk of abnormal clots is also much higher in people who have a combination of the factor V Leiden mutation and another mutation in the F5 gene. Additionally, the risk is increased in people who have the factor V Leiden mutation together with a mutation in another gene involved in the coagulation system.

Inheritance

The chance of developing an abnormal blood clot depends on whether a person has one or two copies of the factor V Leiden mutation in each cell. People who inherit two copies of the mutation, one from each parent, have a higher risk of developing a clot than people who inherit one copy of the mutation. Considering that about 1 in 1,000 people per year in the general population will develop an abnormal blood clot, the presence of one copy of the factor V Leiden mutation increases that risk to 3 to 8 in 1,000, and having two copies of the mutation may raise the risk to as high as 80 in 1,000.

Symptoms

People with factor V Leiden thrombophilia have a higher risk for blood clots. However, the severity of factor V Leiden thrombophilia varies greatly from person to person. Only 5% of people with one factor V Leiden mutation develop a clot by age 65. The most common type of blood clots associated with factor V Leiden thrombophilia, are deep venous thrombosis or DVT and pulmonary embolism or PE. Signs and symptoms of DVT include leg pain, tenderness, swelling, increased warmth or redness in one leg. Signs and symptoms of pulmonary embolism usually include cough, chest pain, shortness of breath or rapid heartbeat or breathing. While less common, other possible sites of blood clots, include superficial veins of the leg, veins carrying blood from the digestive organs and spleen to the liver, veins carrying blood away from the liver, and veins supplying the brain. Factor V Leiden thrombophilia may contribute a small amount of risk toward a heart attack, stroke, or pregnancy complication.

Diagnosis

A diagnosis of factor V Leiden thrombophilia may be considered in people with a notable personal or family history of venous thromboembolism (VTE), such as having a VTE at an atypically young age, in an unusual location, or having multiple VTEs. A doctor may confirm the diagnosis by ordering a genetic or APC resistance test. Alternatively, it is becoming more common for people to learn they have a factor V Leiden gene mutation from an advertised genetic test they purchased directly.

The APC (activated protein C) resistance assay, a coagulation screening test, measures the anticoagulant response to APC. This screening test has a sensitivity and specificity for factor V Leiden approaching 100%. The sensitivity of a test is a measure of the test's ability to detect a positive result when someone has the condition, while the specificity is a measure of the test's ability to identify negative results.

Targeted mutation analysis (a type of DNA test) of the F5 gene for the Leiden mutation is considered definitive and has a mutation detection frequency of approximately 100%. This means that approximately all individuals who have the factor V Leiden mutation will be detected by this genetic test. It is generally recommended that individuals who test positive by another means should then have the DNA test both for confirmation and to distinguish heterozygotes (individuals with a mutation in one copy of the gene) from homozygotes (individuals with mutations in both copies of the gene).

Treatment

People with factor V Leiden thrombophilia who've had a deep venous thrombosis (DVT) or pulmonary embolism (PE) are usually treated with blood thinners, or anticoagulants (such as heparin and warfarin). Anticoagulants are given for varying amounts of time depending on the person's situation. It is not usually recommended that people with factor V Leiden be treated lifelong with anticoagulants if they have had only one DVT or PE, unless they have additional blood clot risk factors.

People who have factor V Leiden but have never had a blood clot are not routinely treated with an anticoagulant. Instead, they are counseled about reducing or eliminating other factors that add to their risk for clots. They may require temporary treatment with an anticoagulant during periods of particularly high risk, such as major surgery.

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