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Test ID: 2C19B Cytochrome P450 2C19 Genotype, Blood

Useful For

Identifying patients who may be at risk for altered metabolism of drugs that are modified by CYP2C19


Predicting anticoagulation response to clopidogrel

Reporting Name

CYP2C19 Genotype, B

Specimen Type

Whole Blood EDTA

Specimen Required

Multiple whole blood EDTA genotype tests can be performed on a single specimen after a single extraction. See Multiple Whole Blood EDTA Genotype Tests in Special Instructions for a list of tests that can be ordered together.


Container/Tube: Lavender top (EDTA)

Specimen Volume: 3 mL

Collection Instructions: Send specimen in original tube.

Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time
Whole Blood EDTA Ambient (preferred)

Clinical Information

Primary metabolism of many drugs is performed by cytochrome P450 (CYP450) enzymes, a group of oxidative/dealkylating enzymes localized in the microsomes of many tissues including the intestines and liver. One of these CYP450 enzymes, CYP2C19, participates in the metabolism of a wide variety of drugs, including the activation of the anticoagulant clopidogrel, and the inactivation of citalopram.


CYP2C19 drug metabolism is variable among individuals. Some individuals have CYP2C19 genetic variants that lead to severely diminished or absent CYP2C19 catalytic activity (ie, poor metabolizers). The frequency of CYP2C19 variants (also referred to as polymorphisms) depends on ethnicity. CYP2C19 variants that produce poor metabolizers are found with frequencies of 2% to 5% in Caucasians, 4% in African Americans, 13% to 23% in Asians, and 38% to 79% in Polynesians and Micronesians.


The following table displays the CYP2C19 variants detected by this assay, the corresponding star allele, and the effect on CYP2C19 enzyme activity:


CYP2C19 Allele

Nucleotide Change

Effect on Enzyme Activity


None (wild type)

Extensive metabolizer (normal)



No activity



No activity



No activity



No activity



No activity



No activity



Severely decreased






Severely decreased



Enhanced activity


CYP2C19 drug metabolism is dependent on the specific genotype detected, and also on the number and type of drugs administered to the patient. Individuals without a detectable CYP2C19 variant will have the predicted phenotype of an extensive drug metabolizer and are designated as CYP2C19*1/*1. If an individual is homozygous or compound heterozygous for an alleles with no activity, the individual is predicted to be a poor metabolizer. If an individual is heterozygous for an allele with no activity, the individual is predicted to be an intermediate metabolizer. Individuals with the CYP2C19*17 allele (in the absence of any inactive or decreased activity alleles) may have enhanced metabolism of drugs. In some cases, a range of potential phenotypes may be given, depending on the combination of alleles identified.


Patients who are poor metabolizers may benefit from dose alteration or selection of a comparable drug that is not primarily metabolized by CYP2C19. It is important to interpret the results of testing in the context of other coadministered drugs.

Reference Values

An interpretive report will be provided.


A report will be provided that includes CYP2C19 genotype, predicted CYP2C19 phenotype, and assay information.


The genotype, with associated star alleles, is assigned using standard allelic nomenclature as published by the Human Cytochrome P450 (CYP) Allele Nomenclature Database Committee.(1)


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


Drug-drug interactions and drug-metabolite inhibition must be considered when treating intermediate metabolizers.


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. Human Cytochrome P450 (CYP) Allele Nomenclature Database. Accessed Sept 21, 2015. Available at

2. Blaisdell J, Mohrenweiser H, Jackson J, et al: Identification and functional characterization of new potentially defective alleles of human CYP2C19. Pharmacogenetics 2002 Dec;12(9):703-711

3. Hicks J, Bishop J, Sangkuhl K, et al: Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2D6 and CYP2C19 Genotypes and Dosing of Selective Serotonin Reuptake Inhibitors. Clin Pharmacol Ther 2015;98(2):27-34

4. Hicks J, Swen J, Thorn C, et al: Clinical Pharmacogenetics Implementation Consortium guideline for CYP2D6 and CYP2C19 genotypes and dosing of tricyclic antidepressants. Clin Pharmacol Ther 2013;93(5):402-408

5. Mega J, Close S, Wiviott D, et al: Cytochrome P-450 polymorphisms and response to clopidogrel. N Engl J Med 2009;360:354-362

Day(s) and Time(s) Performed

Monday through Friday; 8 a.m.

Analytic Time

1 day (not reported Saturday or Sunday)

Test Classification

This test has been modified from the manufacturer's instructions. Its performance characteristics were determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information

81225-CYP2C19 (cytochrome P450, family 2, subfamily C, polypeptide 19) (eg, drug metabolism), gene analysis, common variants (eg, *2-*10, *17)

LOINC Code Information

Test ID Test Order Name Order LOINC Value
2C19B CYP2C19 Genotype, B In Process


Result ID Test Result Name Result LOINC Value
36746 CYP2C19 Phenotype 79714-2
36747 CYP2C19 Star Alleles 57132-3
36372 CYP2C19 Interpretation 69047-9
36373 Reviewed by In Process

Method Name

Polymerase Chain Reaction (PCR) with Allele-Specific Primer Extension (ASPE)

Mayo Medical Laboratories | Genetics and Pharmacogenomics Catalog Additional Information: