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From the American Heart Association
Factor V Leiden | Circulation
What Is Factor V Leiden?
People with factor V Leiden have a mutation in the gene for factor V. Factor V Leiden is an abnormal version of factor V that is resistant to the action of APC. Thus, APC cannot easily stop factor V Leiden from making more fibrin. Once the coagulation process is turned on in people with factor V Leiden, it turns off more slowly than in people with normal factor V. Having factor V Leiden, therefore, results in a condition known as APC resistance.
How Did I Get Factor V Leiden?
You inherit all your genes, including factor V Leiden, from your parents. You have two copies of every gene, one from your mother and one from your father. You may have inherited one copy of the factor V Leiden gene from one parent and one copy of the normal factor V gene from the other parent, making you heterozygous for the factor V Leiden gene mutation. This means that you have about 50% of normal factor V and about 50% of abnormal factor V Leiden in your blood. Sometimes both parents pass factor V Leiden to their offspring, making it possible to have two abnormal genes. If this applies to you, then you are homozygous for factor V Leiden, and 100% of your factor V is the abnormal Leiden variant.
How Is the Diagnosis Made?
The diagnosis of factor V Leiden is made by a blood test. Initially, a screening test may be done to determine if you have APC resistance. Because factor V Leiden is only one of several causes of APC resistance, you may have a DNA test (also done on the blood) to see if you have factor V Leiden if your blood shows APC resistance. This test will also determine whether you have the heterozygous or homozygous form. Sometimes the initial screening test for APC resistance is omitted and only the DNA test is performed.
What Are the Implications of Having Factor V Leiden?
Heterozygous factor V Leiden is found in about 5% of the white population and is most common in people of Northern European descent and in some Middle Eastern populations, whereas the homozygous form is found in fewer than 1%. Factor V Leiden is less common in the Hispanic populations and is rare in Asian, African, and Native American populations.
Factor V Leiden is associated with an increased risk of developing an episode of DVT (with or without a PE). Approximately 1 in every 1000 people will develop a DVT or PE each year, and this increases from about 1 in 10 000 for those in their twenties to about 5 in 1000 for those in their seventies. Heterozygous factor V Leiden increases the risk of developing a first DVT by 5- to 7-fold (or 5 to 7 in 1000 people each year). Thus, even though the relative risk of developing a DVT seems high, the absolute risk of having a DVT is still quite low with factor V Leiden. Factor V Leiden is thus a weak risk factor for developing blood clots; in fact, most people who have heterozygous factor V Leiden never develop blood clots. Homozygous factor V Leiden increases the risk of developing clots to a greater degree, about 25- to 50-fold. If you have the heterozygous form of factor V Leiden, the lifetime risk of developing a DVT is 10% or less, but may be higher if you have close family members who have had a DVT. Very often, people with factor V Leiden have additional risk factors that contributed to the development of blood clots (Table). Having factor V Leiden alone does not appear to increase the risk of developing arterial thrombosis, that is, heart attacks and strokes.
Risk Factors for Developing a DVT or PE
How Is Factor V Leiden Treated?
If you have had a DVT or PE, then you were most likely treated with blood thinners, or anticoagulants. Anticoagulants such as warfarin are given for varying amounts of time depending on your 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 there are additional risk factors present (Table). Having had a DVT or PE in the past increases your risk for developing another one in the future, but having factor V Leiden does not seem to add to the risk of having a second clot. If you have factor V Leiden but have never had a blood clot, then you will not routinely be treated with an anticoagulant. Rather, you should be counseled about reducing or eliminating other factors that may add to your risk of developing a clot in the future. In addition, you may require temporary treatment with an anticoagulant during periods of particularly high risk, such as major surgery.
Factor V Leiden | Circulation
What Is Factor V Leiden?
People with factor V Leiden have a mutation in the gene for factor V. Factor V Leiden is an abnormal version of factor V that is resistant to the action of APC. Thus, APC cannot easily stop factor V Leiden from making more fibrin. Once the coagulation process is turned on in people with factor V Leiden, it turns off more slowly than in people with normal factor V. Having factor V Leiden, therefore, results in a condition known as APC resistance.
How Did I Get Factor V Leiden?
You inherit all your genes, including factor V Leiden, from your parents. You have two copies of every gene, one from your mother and one from your father. You may have inherited one copy of the factor V Leiden gene from one parent and one copy of the normal factor V gene from the other parent, making you heterozygous for the factor V Leiden gene mutation. This means that you have about 50% of normal factor V and about 50% of abnormal factor V Leiden in your blood. Sometimes both parents pass factor V Leiden to their offspring, making it possible to have two abnormal genes. If this applies to you, then you are homozygous for factor V Leiden, and 100% of your factor V is the abnormal Leiden variant.
How Is the Diagnosis Made?
The diagnosis of factor V Leiden is made by a blood test. Initially, a screening test may be done to determine if you have APC resistance. Because factor V Leiden is only one of several causes of APC resistance, you may have a DNA test (also done on the blood) to see if you have factor V Leiden if your blood shows APC resistance. This test will also determine whether you have the heterozygous or homozygous form. Sometimes the initial screening test for APC resistance is omitted and only the DNA test is performed.
What Are the Implications of Having Factor V Leiden?
Heterozygous factor V Leiden is found in about 5% of the white population and is most common in people of Northern European descent and in some Middle Eastern populations, whereas the homozygous form is found in fewer than 1%. Factor V Leiden is less common in the Hispanic populations and is rare in Asian, African, and Native American populations.
Factor V Leiden is associated with an increased risk of developing an episode of DVT (with or without a PE). Approximately 1 in every 1000 people will develop a DVT or PE each year, and this increases from about 1 in 10 000 for those in their twenties to about 5 in 1000 for those in their seventies. Heterozygous factor V Leiden increases the risk of developing a first DVT by 5- to 7-fold (or 5 to 7 in 1000 people each year). Thus, even though the relative risk of developing a DVT seems high, the absolute risk of having a DVT is still quite low with factor V Leiden. Factor V Leiden is thus a weak risk factor for developing blood clots; in fact, most people who have heterozygous factor V Leiden never develop blood clots. Homozygous factor V Leiden increases the risk of developing clots to a greater degree, about 25- to 50-fold. If you have the heterozygous form of factor V Leiden, the lifetime risk of developing a DVT is 10% or less, but may be higher if you have close family members who have had a DVT. Very often, people with factor V Leiden have additional risk factors that contributed to the development of blood clots (Table). Having factor V Leiden alone does not appear to increase the risk of developing arterial thrombosis, that is, heart attacks and strokes.
Risk Factors for Developing a DVT or PE
How Is Factor V Leiden Treated?
If you have had a DVT or PE, then you were most likely treated with blood thinners, or anticoagulants. Anticoagulants such as warfarin are given for varying amounts of time depending on your 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 there are additional risk factors present (Table). Having had a DVT or PE in the past increases your risk for developing another one in the future, but having factor V Leiden does not seem to add to the risk of having a second clot. If you have factor V Leiden but have never had a blood clot, then you will not routinely be treated with an anticoagulant. Rather, you should be counseled about reducing or eliminating other factors that may add to your risk of developing a clot in the future. In addition, you may require temporary treatment with an anticoagulant during periods of particularly high risk, such as major surgery.
