Leiden factor - factor V/ APCR

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.