Test Catalog

Test ID: WARSV    
Warfarin Response Genotype, Varies

Useful For Suggests clinical disorders or settings where the test may be helpful

Identifying patients who may require warfarin dosing adjustments(3,4) including:

-Patients being started on a first prescription for warfarin

-Patients who have previously been prescribed warfarin and have required multiple dosing adjustments to maintain the international normalized ratio in the target range

-Patients with a history of thrombosis or bleeding when taking warfarin

Genetics Test Information Provides information that may help with selection of the correct genetic test or proper submission of the test request

This test is used for assessing CYP2C9, VKORC1, CYP4F2, and rs12777823 for variants affecting the metabolism of warfarin (Coumadin). This assay should be ordered on patients who are receiving warfarin for the first time or who are experiencing difficulties in maintaining the international normalized ratio (INR) in the therapeutic range.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Warfarin is a Coumarin-based drug commonly utilized in anticoagulation therapy to prevent thrombosis due to inherited and acquired hemostatic disorders. The drug is also used in a number of other medical conditions and treatments including atrial fibrillation and hip replacement surgery. Warfarin acts by interfering with the metabolism of vitamin K, which is necessary for production of key coagulation factors. Warfarin inhibits vitamin K recycling by blocking its metabolism at the vitamin K-epoxide intermediate; thereby decreasing the amount of available vitamin K. Warfarin has a narrow therapeutic window; undermedicating increases the risk for thrombosis and overmedicating increases the risk for cerebrovascular accidents. Warfarin therapy has one of the highest rates of severe adverse drug reactions.


Warfarin is dosed using nongenetic factors including gender, weight, and age, and is monitored by coagulation testing in order to maintain the international normalized ratio (INR) within specific limits. However, warfarin metabolism is highly variable and dependent upon genetic factors. Variants within 3 genes and 1 intragenic locus are known to affect the metabolism of warfarin and the dose needed to maintain the correct serum drug level and degree of anticoagulation.


The CYP2C9 gene encodes the cytochrome P450 2C9 (CYP2C9) enzyme that primarily metabolizes the more active isomer of warfarin (S-warfarin) to inactive products. Some CYP2C9 variants result in decreased enzymatic activity and may lead to increases in serum warfarin and over-medicating, driving the INR above the therapeutic target.


The second gene (VKORC1) encodes vitamin K epoxide reductase complex subunit-1 (VKORC1), a small transmembrane protein of the endoplasmic reticulum that is part of the vitamin K cycle and the target of warfarin therapy.(1) Vitamin K epoxide, a by-product of the carboxylation of blood coagulation factors, is reduced to vitamin K by VKORC1. A VKORC1 promoter variant leads to decreased expression of the gene, resulting in reduced availability of vitamin K. This may cause increases in serum warfarin and overmedicating, driving the INR above the therapeutic target. In addition, there are variations in VKORC1 that lead to warfarin resistance that are tested by this assay. These variations are rare.


CYP4F2 metabolizes reduced vitamin K to hydroxyl-vitamin K1, thus removing it from the pathways involved in the activation of clotting factors impacted by warfarin. In individuals who self-identify as being of non-African ancestry, carriers of the CYP4F2*3 (C.1297G->A; rs2108622) variant may need a small (5%-10%) warfarin dosage increase to achieve therapeutic goals.


The rs12777823G->A variant is located intragenic in the CYP2C locus on chromosome 10. The A allele has been associated with the need for a 10% to 15% decrease in dose in individuals who self-identify as being of African ancestry.



CYP2C9 metabolizes a wide variety of drugs including warfarin and phenytoin. (Note that if testing is desired for other CYP2C9 substrates, order 2C9GV / Cytochrome P450 2C9 Genotype.


A number of specific CYP2C9 variants result in enzymatic deficiencies. The following information outlines the relationship between the variants detected in this assay and their effect on the activity of the enzyme (Table 1):

Table 1:

CYP2C9 Allele

cDNA Nucleotide Change

Effect on Enzyme Metabolism


None (wild type)

Normal activity



Reduced activity



No activity



Reduced activity



Reduced activity



No activity



Substrate specific



Reduced activity



Reduced activity



Reduced activity



Minimal activity



Minimal activity



No activity



Minimal activity



Reduced activity



No activity



No activity



Minimal activity



Minimal activity



Minimal activity



Minimal activity


374G->T + 430C->T

No activity



The c.-1639 promoter variant is located in the second nucleotide of an E-Box (CANNTG) and its presence disrupts the consensus sequence, reducing promoter activity. In vitro experiments show a 44% higher transcription level of the G versus the A allele.(1) The c.-1639 G->A nucleotide change results in decreased gene expression and reduced enzyme activity. This test also determines the genotype for multiple other loci within VKORC1 that have been associated with warfarin resistance. The mechanism by which these variations cause warfarin resistance is not clearly understood.


Table 2: Additional Variants Tested


cDNA Nucleotide Change

Effect on Enzyme Metabolism



Warfarin sensitivity



Warfarin resistance



Warfarin resistance



Warfarin resistance



Warfarin resistance



Warfarin resistance



Warfarin resistance



Warfarin resistance



Warfarin resistance



Warfarin resistance



Warfarin sensitivity

a. rs12777823G->A is an intergenic single nucleotide variant (SNV)


Warfarin dosing may require adjustment depending on the genotypes identified and the predicted phenotype. Patients who have high warfarin sensitivity may benefit from greatly reduced warfarin dosage or by transitioning to another comparable medication.(2) Similarly, in rare instances, individuals with VKORC1 warfarin resistance variants, may require a higher warfarin dose or may benefit from selection of an alternate medication.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

An interpretive report will be provided that includes assay information, genotype, and an interpretation indicating the patient's predicted warfarin response.


The CYP2C9 and CYP4F2 genotypes, with associated star alleles, are assigned using standard allelic nomenclature as published by the Pharmacogene Variation (PharmVar) Consortium.(5)


Individuals without a detectable alteration in CYP2C9 or CYP4F2 will be designated as CYP2C9*1/*1 or CYP4F2*1/*1


For additional information regarding pharmacogenomic genes and their associated drugs, see Pharmacogenomic Associations Tables in Special Instructions. This resource also includes information regarding enzyme inhibitors and inducers, as well as potential alternate drug choices.


Individuals who have variants in 1 or more gene tested by this assay may require more frequent monitoring of international normalized ratio (INR) to maintain the INR in the target range.


Drug-drug interactions and drug/metabolite inhibition must be considered when prescribing warfarin. Warfarin metabolism may be inhibited through drug-drug interactions, including amiodarone and some statins. It is important to interpret the results of testing and dose adjustments in the context of hepatic and renal function and patient age.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Samples may contain donor DNA if obtained from patients who received heterologous blood transfusions or allogeneic blood or marrow transplantation. Results from samples obtained under these circumstances may not accurately reflect the recipient's genotype. For individuals who have received blood transfusions, the genotype usually reverts to that of the recipient within 6 weeks. For individuals who have received allogeneic blood or marrow transplantation, a pre-transplant DNA specimen is recommended for testing.


CYP2C9, VKORC1, CYP4F2, and rs12777823 genetic test results in patients who have undergone liver transplantation may not accurately reflect the patient's CYP2C9 or VKORC1 status.


This method may not detect all variants that impact warfarin sensitivity or resistance. Therefore, absence of a detectable variant does not rule out the possibility that a patient has an altered CYP2C9 or VKORC1 metabolism due to other variants that cannot be detected with this method. Furthermore, when 2 or more variants are identified, the cis-/trans- status (whether the variants are on the same or opposite chromosomes) is not always known.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Oldenburg J, Bevens CG, Muller CR, Watzka M: Vitamin K epoxide reductase complex subunit I (VKORC1): the key protein of the vitamin K cycle. Antioxid Redox Signal 2006;8(3-4):347-353

2. Watzka M, Geisen C, Bevans CG, et al: Thirteen novel VKORC1 mutations associated with oral anticoagulant resistance: insights into improved patient diagnosis and treatment. J Thromb Haemost 2011;9(1):109-118

3. Yuan HY, Chen JJ, Lee MT, et al: A novel functional VKORC1 promoter polymorphism is associated with inter-individual and inter-ethnic differences in warfarin sensitivity. Hum Mol Genet 2005;14:1745-1751

4. Sconce EA, Khan TI, Wynne HA, et al: The impact of CYP2C9 and VKORC1 genetic polymorphism and patient characteristics upon warfarin dose requirements: proposal for a new dosing regimen. Blood 2005;106:2329-2333

5. Pharmacogene Variation Consortium database. Accessed 04/27/2018. Available at https://www.pharmvar.org/gene/CYP2C9

6. Johnson JA, Caudle KE, Gong L, et al: Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for Pharmacogenetics-Guided Warfarin Dosing: 2017 Update. Clin Pharmacol Ther 2017 Sep;102(3):397-404 doi:10.1002/cpt.668

7. Perera MA, Cavallari LH, Limdi NA, et al: Genetic variants associated with warfarin dose in African-American individuals: a genome-wide association study. Lancet 2013;382,790-796

8. McDonald MG, Rieder MJ, Nakano M, et al: CYP4F2 is a vitamin K1 oxidase: An explanation for altered warfarin dose in carriers of the V433M variant. Mol Pharmacol 2009;75,1337-1346

9. Warfarindosing.org website. Accessed June 2017. Available at http://www.warfarindosing.org/Source/Home.aspx

10. FDA approved drug label for warfarin. Accessed May 3, 2017.  Available at http://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=558b7a0d-5490-4c1b-802e- 3ab3f1efe760

Special Instructions Library of PDFs including pertinent information and forms related to the test