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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

Method Name

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

(PCR is utilized pursuant to a license agreement with Roche Molecular Systems, Inc.)

Reporting Name

CYP2C19 Genotype, B

Specimen Type

Whole Blood EDTA

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.

Forms:

1. New York Clients-Informed consent is required. Please document on the request form or electronic order that a copy is on file. An Informed Consent for Genetic Testing (T576) is available in Special Instructions.

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

Neurology Test Request Form-General (T732) (http://www.mayomedicallaboratories.com/it-mmfiles/neurology-request-form.pdf)

Cardiovascular Test Request Form (T724) (http://www.mayomedicallaboratories.com/it-mmfiles/cardiovascular-request-form.pdf)

Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time
Whole Blood EDTA Ambient (preferred)
  Refrigerated 

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

*1

None (wild type)

Extensive metabolizer (normal)

*2

c.681G>A

No activity

*3

c.636G>A

No activity

*4

c.1A>G

No activity

*5

c.1297C>T

No activity

*6

c.395G>A

No activity

*7

c.819+2T>A

No activity

*8

c.358T>C

Severely decreased

*9

c.431G>A

Decreased

*10

c.680C>T

Severely decreased

*17

c.-806C>T

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.

Cautions

Rare genetic variants, if present, could lead to false-negative or false-positive results. If results obtained do not match the clinical findings (phenotype), additional testing should be considered.

 

Blood 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 pretransplant DNA specimen is recommended for testing.

 

CYP2C19 genetic test results in patients who have undergone liver transplantation may not accurately reflect the patient's CYP2C19 status.

 

This method may not detect all variants that result in altered CYP2C19 activity. Therefore, absence of a detectable gene variant does not rule out the possibility that a patient has an altered CYP2C19 metabolism due to other CYP2C19 variants that cannot be detected with this method. Furthermore, when 2 or more gene variants are identified, the cis-/trans- status (whether the variants are on the same or opposite chromosomes) is not always known.

 

This test is designed to detect only the variants specified above. Other variants in the primer binding regions can affect the testing and, ultimately, the genotype and phenotype predictions made.

Day(s) Performed

Monday through Friday; 8 a.m.

Report Available

1 day (not reported Saturday or Sunday)

Performing Laboratory

Mayo Medical Laboratories in Rochester

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)

NY State Approved

Conditional