Factor V Leiden Mutation and PT 20210 Mutation
If you have a strong family history of thrombosis associated with these mutations; if you have unexplained thrombosis that requires further investigation
A blood sample taken from a vein in your arm
No test preparation is needed.
Factor V and prothrombin are proteins that are produced in the liver and belong to a group of proteins collectively known as coagulation factors. The coagulation factors are activated in a step-by-step process called the coagulation cascade when a blood vessel is injured. The result of the coagulation cascade is formation of a blood clot which creates a barrier at the injury site, protecting it until it heals. The coagulation cascade involves many pro-coagulant proteins (such as factor V and prothrombin) as well as many anti-coagulant proteins (such as activated protein C and protein S), which act in a coordinated manner to keep the process of blood coagulation tightly regulated.
Factor V Leiden is a variant form of the factor V protein that arises due to the presence of a common genetic point mutation, a change in one of the nucleotides on the gene that codes for the production of factor V protein. This altered Factor V Leiden protein activates normally to participate in stimulating blood coagulation, but resists being degraded by the anticoagulant activated protein C (APC) and thus remains active for longer than the wild type Factor V protein. The result of this resistance is an increased level of thrombin in the blood which is associated with a higher risk of venous thromboembolism (VTE).
The prothrombin (PT) G20210A is a variant form of prothrombin, also caused by a common genetic point mutation. PT G20210A is also associated with an increased risk of VTE. This variant does not alter the activity or the functional properties of the prothrombin protein, but is responsible for the production of higher levels of prothrombin in individuals carrying the variant.
Factor V Leiden is found at a high prevalence in the British Causcasian population, up to 1 in 10 individuals will carry the variant; its incidence varies among other ethnicities. An individual may carry one gene with the variant nucleotide alteration and one normal copy and they are described as ‘heterozygous for Factor V Leiden’ or they may, rarely, carry two copies of the variant gene and will be ‘homozygous for Factor V Leiden’. The risk of a person who is heterozygous for Factor V Leiden having a venous thromboembolism is increased by 5-10 fold and the risk of a person who is homozygous is increased around 80 fold. However, venous thromboembolism and its causes are complex and multiple and just because a person carries Factor V Leiden does not necessarily meant that they will have a venous thromboembolism.
Similarly, an individual with a single variant gene copy of the PT G20210A variant is heterozygous and those who carry two copies of the variant gene are described as homozygous. The affected person will have a mild to moderate increase in their thrombin production, raising their risk of developing a VTE. PT G20210A is less common in the U.K. than factor V Leiden (about 1-2% of the general population). As with Factor V Leiden, the PT G20210A variant is more prevalent in Caucasians than in those of other ethnic backgrounds. Patients who are heterozygous for PT G20210A have a 2-3 fold increased risk of venous thromboembolism. The risks of having a venous thromboembolism in those with homozygous PT G20210A have not been established due to the rarity of this occurrence.
Factor V Leiden and PT G20210A are independent mutations that are inherited separately and thus they are tested for as separate events. The testing of each is intended to identify whether or not the specific mutation is present and to determine whether the person is heterozygous or homozygous for that mutation.
How is the sample collected for testing?
The test is performed on a blood sample collected from a vein in the arm by a needle. This is a process which may be referred to as ‘venepuncture’.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Factor V Leiden mutation and prothrombin G20210A mutation tests are used, along with other tests related to thrombophilia, to help screen for the underlying causes of venous thromboembolism (VTE). These tests are used as part of a panel of investigations which can provide important information to medics regarding risk of clotting now and in the future.
Different laboratories may use different screening techniques to detect these variants. Some laboratories perform an initial screen for factor V Leiden using the activated protein C (APC) resistance test. Activated protein C helps to regulate coagulation by inactivating factor Va, which slows the clotting process in plasma. If a person is found to have a resistance to APC through testing, they may be at an increased risk for thrombosis. About 95% of the time, APC resistance is found to be due to a factor V Leiden mutation. If resistance is present, then a test for the factor V Leiden gene mutation can be done, both to confirm the diagnosis and to determine whether the patient is heterozygous or homozygous for the mutation. In the UK, some laboratories proceed straight to gene mutation analysis without first using APC resistance screening.
The PT G20210A variant is diagnosed with genetic testing, checking directly for the gene mutation and determining whether the patient is heterozygous or homozygous. Although this mutation is associated with an increase in circulating prothrombin levels, measurement of this is not an assay that is clinically useful in finding this mutation.
Current guidelines written by experts in the UK do not recommend screening the general population or testing asymptomatic family members. Large studies have identified that the difference in risk of clotting between carriers of these mutations and non-carriers are not significantly different within families. This is due to the complex pathogenesis of venous thromboembolism and the existence of multiple external factors that influence the risk of clotting (age, sex, hormones, malignancy, weight, trauma, etc). Although carrying these variants is associated with an increased risk over the lifetime of the individual, not all carriers will experience venous thromboembolism. With factor V Leiden, for example, only 10% of those heterozygous for the mutation will ever have a thrombotic episode.
When is it requested?
Factor V Leiden mutation and PT G20210A tests may be requested when a patient has a personal or family history of recurrent VTE, a first VTE related to oral contraceptive use, pregnancy or hormone replacement therapy, or when they are experiencing unexplained miscarriages, especially those occurring in the second or third trimester of the pregnancy, or less often in patients suffering other complications of pregnancy such as pre-eclampsia and intra-uterine growth retardation.
Once APC resistance testing, factor V Leiden mutation testing, and PT G20210A gene mutation testing have been done, they are usually not repeated unless there is a need for verification.
While a positive APC resistance assay is confirmed by the factor V Leiden mutation test, the factor V Leiden mutation and the PT G20210A mutation tests require no further confirmation.
What does the test result mean?
Resistance to activated protein C is due to factor V Leiden 95% of the time, but if resistance is present, the factor V Leiden mutation should be confirmed with genetic testing.
If genetic testing shows one factor V Leiden gene copy, then the patient is heterozygous; if there are two copies, then the patient is homozygous. If there is one PT G20210A gene copy, then the patient is heterozygous; if there are two copies, then the patient is homozygous.
The risk potential of the mutation(s) will be variable and individual. If the patient is asymptomatic, he or she may never have a VTE. Generally, patients with factor V Leiden or PT G20210A mutations are treated in exactly the same was as patients with normal genes if they develop venous thromboses. Patients homozygous for factor V Leiden are at high risk of recurrent thrombosis and may need lifelong anti-coagulant treatment. The implications of a positive thrombophilia screen for an affected individual is complicated and should be discussed with a specialist haematologist.
Is there anything else I should know?
The risks that are associated with factor V Leiden, PT G20210A, and other inherited and acquired factor deficiencies are independent. A patient can have more than one of them, and their associated risks are cumulative. Added to these inherited risks and acquired risks are controllable risk factors, such as oral contraceptive use, that may exacerbate the combined underlying risk factors.
Some studies have found an association between factor V Leiden mutation and recurrent miscarriages and other complications of pregnancy; however, the clinical picture in these situations can be complex and other factors must be taken into consideration. Each patient will be evaluated on a case-by-case basis by specialised medical staff who will take into account all available clinical information.
How is a VTE treated?
Regardless of the underlying cause, a VTE is usually treated with a short course of anticoagulants (often 3-6 months with a combination of heparin, warfarin, and low-molecular weight heparins). At the end of this time period, the patient's risk level is assessed to see if further treatment is necessary. The current UK guidelines do not advise testing for Factor V Leiden or prothrombin G201210A mutations after a VTE as presence or absence of these mutations does not influence treatment or help describe the risk of recurrent thrombosis.
Should someone with a Factor V Leiden mutation be on long-term anticoagulant therapy?
In general, no they should not be on long-term therapy. Long-term therapy may be considered for specific individuals, especially if they are homozygous for Factor V Leiden and/or have had recurrent thrombotic episodes, however, this would be after review of all medical information and a discussion on the risks and benefits of long-term anticoagulant-associated bleeding or off-treatment thrombosis. Each case would be reviewed and a decision that is best for the patient would be agreed with the individual.