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Test ID: WARSQ Warfarin Response Genotype, Varies

Ordering Guidance

If patient is using medications other than warfarin, the preferred test is 2C9QT / Cytochrome P450 2C9 Genotype, Varies, which tests for only the CYP2C9 gene.


Testing is available as the single gene assay (this test) or as a part of a focused pharmacogenomics panel, which includes testing for the following genes: CYPs 1A2, 2C9, 2C19, 2D6, 3A4, 3A5, 4F2, SLCO1B1, and VKORC1. Order PGXQP / Focused Pharmacogenomics Panel, Varies if multiple pharmacogenomic genotype testing is desired.

Specimen Required

Multiple genotype tests can be performed on a single specimen after a single extraction. See Multiple Genotype Test List in Special Instructions for a list of tests that can be ordered together.


Submit only 1 of the following specimens:


Specimen Type: Whole blood

Container/Tube: Lavender top (EDTA)

Specimen Volume: 3 mL

Collection Instructions:

1. Invert several times to mix blood.

2. Send specimen in original tube.

Specimen Stability Information: Ambient (preferred) 9 days/Refrigerated 30 days


Specimen Type: Saliva

Patient Preparation: Patient should not eat, drink, smoke, or chew gum 30 minutes prior to collection.

Supplies: Saliva Swab Collection Kit (T786)

Specimen Volume: 1 swab

Collection Instructions: Collect and send specimen per kit instructions.

Specimen Stability Information: Ambient 30 days


Specimen Type: Extracted DNA

Container/Tube: 2 mL screw top tube

Specimen Volume: 100 mcL (microliters)

Collection Instructions:

1. The preferred volume is 100 mcL at a concentration of 50 ng/mcL.

2. Include concentration and volume on tube.

Specimen Stability Information: Frozen (preferred)/Ambient/Refrigerated


1. New York Clients-Informed consent is required. Document on the request form or electronic order that a copy is on file. The following documents are available:

-Informed Consent for Genetic Testing (T576)

-Informed Consent for Genetic Testing-Spanish (T826)

2. If not ordering electronically, complete, print, and send 1 of the following forms with the specimen:

-Neurology Specialty Testing Client Test Request (T732)

-Therapeutics Test Request (T831)

-Cardiovascular Test Request (T724)

Useful For

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

Method Name

Real-Time Polymerase Chain Reaction (PCR) with Allelic Discrimination Analysis

Reporting Name

Warfarin Response Genotype, V

Specimen Type


Specimen Minimum Volume

Blood: 0.4 mL
Saliva: 1 swab

Specimen Stability Information

Specimen Type Temperature Time Special Container
Varies Varies

Clinical Information

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 (CYP) 2C9 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 overmedicating, driving the INR above the therapeutic target.


The second gene, VKORC1 encodes vitamin K epoxide reductase complex subunit-1, 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 2C9QT / Cytochrome P450 2C9 Genotype, Varies.


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



Reduced activity



Normal activity



Reduced activity



Reduced activity



No activity



Reduced activity



No activity



Reduced activity



Reduced activity



No activity



No activity



Reduced activity



Reduced activity



Reduced activity



No activity



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.-1639G>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

(VKORC1 NM_024006.5; CYP4F2 NM_001082.4)

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


* 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

An interpretive report will be provided.


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.

Clinical Reference

1. Oldenburg J, Bevans CG, Muller CR, Watzka M: Vitamin K epoxide reductase complex subunit 1 (VKORC1): the key protein of the vitamin K cycle. Antioxid Redox Signal. 2006;8(3-4):347-353. doi: 10.1089/ars.2006.8.347

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. doi: 10.1111/j.1538-7836.2010.04095.x

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(13):1745-1751. doi: 10.1093/hmg/ddi180

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(7):2329-2333. doi: 10.1182/blood-2005-03-1108

5. PharmVar: Pharmacogene Variation Consortium. Updated March 3, 2021. Accessed March 22, 2021.  Available at

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;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(9894):790-796. doi: 10.1016/S0140-6736(13)60681-9

8. McDonald MG, Rieder MJ, Nakano M, Hsia CK, Rettie AE: CYP4F2 is a vitamin K1 oxidase: An explanation for altered warfarin dose in carriers of the V433M variant. Mol Pharmacol. 2009;75(6):1337-1346. doi: 10.1124/mol.109.054833

9. Washington University; Accessed October 14, 2020. Available at

10. U.S National Library of Medicine: DailyMed. National Institutes of Health; Accessed October 14, 2020. Available at

Day(s) Performed

Monday through Friday

Report Available

3 to 10 days

Test Classification

This test was developed, and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
WARSQ Warfarin Response Genotype, V 93196-4


Result ID Test Result Name Result LOINC Value
610175 Warfarin CYP2C9 Genotype 46724-1
610176 Warfarin VKORC1 Promoter Genotype 50722-8
610560 Warfarin CYP2C9 and VKORC1 Promoter Phenotype 54451-0
610177 Warfarin Resistance Variants 50722-8
614410 Warfarin VKORC1 Resistance Genotype 50722-8
610178 Warfarin CYP4F2 *3 Genotype 93197-2
610179 Warfarin rs12777823 Genotype 93198-0
610180 Interpretation 69047-9
610181 Additional Information 48767-8
610182 Method 85069-3
610183 Disclaimer 62364-5
610184 Reviewed by 18771-6
Mayo Clinic Laboratories | Genetics and Pharmacogenomics Catalog Additional Information: