Test Summary

This test detects the factor V Leiden variant, the most common cause of inherited thrombophilia; it may be used to evaluate individuals with a strong personal or family history of venous thromboembolism (VTE) and inform treatment or preventive decisions [1]. Testing the factor V R2 variant in factor V Leiden heterozygous patients helps assess additional risk of VTE.

Factor V Leiden refers to the c.1691G>A variant in the FV gene, which encodes coagulation factor V. This variant results in resistance to factor V protein degradation by activated protein C and increases the risk of VTE 6 to 8 fold in heterozygous carriers and 80 fold in homozygous carriers [1]. The mean age of symptom onset is 31 to 44 years, but some heterozygous carriers can be asymptomatic [2]. In the United States, Factor V Leiden is most prevalent in White individuals, with an estimated frequency of 5% [1].

Factor V Leiden testing may be indicated in clinical scenarios where results can help guide clinical decisions for the patient and family members. These clinical scenarios include first VTE developed under 50 years of age, VTE at an unusual site (eg, cerebral veins), recurrent VTE, a strong family history of VTE, and low activated protein C resistance activity [1].

Routine testing for factor V Leiden is not recommended for prenatal carrier screening, newborn screening, or individuals taking an oral contraceptive [2]. A negative result of this test does not rule out inherited thrombophilia. Other than factor V Leiden, variants in the genes that encode coagulation factor II, protein C, protein S, and antithrombin can also cause inherited thrombophilia.

If the factor V Leiden result is heterozygous, the R2 variant (c.3980A>G) in the FV gene will be tested. Although the R2 allele alone does not increase the risk of VTE, coinheritance of factor V R2 and factor V Leiden is associated with a 3- to 4-fold increase in risk of VTE [3].

The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.

References
1. Zhang S, et al. Genet Med. 2018;20(12):1489-1498.
2. Vnencak-Jones CL, et al. Genetics. In: Rifai R, et al, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier Inc; 2022.
3. Faioni EM, et al. Blood. 1999;94(9):3062-3066.

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