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Interpretive Handbook

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Test 60334 :
Cytochrome P450 2D6 Genotype, Saliva

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

Primary metabolism of many drugs is performed by cytochrome P450 (CYP), a group of oxidative/dealkylating enzymes localized in the microsomes of many tissues including the intestines and liver. One of these CYP enzymes, CYP2D6, is wholly or partially responsible for the hydroxylation or dealkylation of many commonly prescribed drugs such as analgesics, anticonvulsants, antidepressants, antiemetics, antihypertensives, antiestrogens, antineoplastics, antipsychotics, antiretrovirals, antitussives, beta-blockers, cardioactive drugs, H-2 blockers, stimulants, and sympathomimetics. The current clinical application of this test is focused on the treatment of depression with selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs). Amitriptyline, clomipramine, desipramine, imipramine, fluoxetine, fluvoxamine, paroxetine, sertraline, and venlafaxine are metabolized by CYP2D6.


CYP2D6-mediated drug metabolism is highly variable. Some individuals have altered CYP2D6 gene sequences that result in synthesis of enzyme devoid of catalytic activity, or in enzyme with diminished catalytic activity. These individuals metabolize SSRIs and TCAs poorly. Duplication of the functional CYP2D6 gene has been observed, which may result in ultrarapid metabolism of SSRIs and other drugs. Up to 13 copies of CYP2D6 have been reported.


Dosing of SSRIs and TCAs that are metabolized through CYP2D6 may require adjustment based on the individual patient's genotype. Patients who are poor metabolizers may require lower than usual doses to achieve optimal response. Patients who are ultrarapid metabolizers may benefit from increased doses. Patients with either ultrarapid or poor metabolism also may benefit by conversion to other comparable drugs that are not primarily metabolized by CYP2D6 or by therapeutic drug monitoring where applicable.


A number of specific polymorphisms have been found in the CYP2D6 gene that result in enzymatic deficiencies. The frequency of these polymorphisms varies within the major ethnic groups. CYP2D6 polymorphisms that produce poor metabolizers are found with frequencies of 7% to 10% in Caucasians, 2% in Africans and African Americans, and 1% in Asians. Individuals without inactivating polymorphisms, deletions, or duplications have the phenotype of an extensive drug metabolizer (normal) and are designated as CYP2D6*1/*1.


All of the identified polymorphisms associated with CYP2D6 are autosomal recessive. Consequently, only individuals who are homozygous or compound heterozygous for these polymorphisms are poor metabolizers. Individuals who are heterozygous, with 1 normal gene and 1 polymorphic gene, will have metabolism intermediate between the extensive (normal) and poor metabolizers.


The following information outlines the relationship between the polymorphisms detected in this assay and the effect on the activity of the enzyme produced by that allele:


CYP2D6 Allele

Nucleotide Change

Effect on Enzyme Metabolism


None (wild type)

Extensive metabolism (normal)



Decreased activity


2850C->T and -1584C->G

Increased activity



No activity



No activity


Gene deletion

No activity



No activity



No activity



No activity



Decreased activity



Decreased activity



No activity



No activity


100C->T and 1758G->A

No activity



Decreased activity



No activity



Decreased activity

Gene duplication

Depends on the allele

duplicated (increased/no effect)

A complicating factor in correlating CYP2D6 genotype with phenotype is that many drugs or their metabolites are inhibitors of CYP2D6 catalytic activity. SSRIs, as well as some TCAs and other xenobiotics, may reduce or increase CYP2D6 catalytic activity. Consequently, an individual may require a dosing decrease greater than predicted based upon genotype alone. It is important to interpret the results of testing in the context of other coadministered drugs.


Cytochrome P450 Patient Education Brochure (Supply T526) is available upon request.

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

Identifying patients who are poor or extensive metabolizers of antidepressant drugs metabolized by CYP2D6


Adjusting dosages for antidepressant drugs that are metabolized by CYP2D6


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


Predicting anticoagulation response to clopidogrel


Genotyping patients who prefer not to have venipuncture done

Interpretation Provides information to assist in interpretation of the test results

An interpretive report will be provided.


Based on the test sensitivity and currently available CYP2D6 polymorphism carrier frequencies, persons of Caucasian descent who tested negative for the above polymorphisms would be estimated to have a <1.4% residual risk for carrying 1 or more copies of an undetected poor metabolizer allele. This residual risk may be higher or lower in other ethnic groups. The frequency of polymorphisms causing poor metabolism is highest in the Caucasian population and lower in African-Americans and Asians. Patients with an extensive (normal) or intermediate metabolizer genotype may have CYP2D6 enzyme activity inhibited by a variety of medications, or their metabolites. The following is a partial listing of drugs known to affect CYP2D6 activity as of the date of this report.


Drugs known to increase CYP2D6 activity:



Coadministration of these drugs will increase the rate of excretion of CYP2D6 metabolized drugs, reducing that drug's effectiveness.


Drugs known to decrease CYP2D6 activity:





































Coadministration will decrease the rate of metabolism of CYP2D6-metabolized drugs, increasing the possibility of toxicity.


Drugs that undergo metabolism by CYP2D6:


























































Coadministration may decrease the rate of elimination of other drugs metabolized by of CYP2D6.


Drug-drug interactions and drug-metabolite inhibition or activation must be considered when dealing with heterozygous individuals. Drug-metabolite inhibition occurs frequently with selective serotonin reuptake inhibitors and tricyclic antidepressants, resulting in inhibition of residual functional CYP2D6 catalytic activity. Each report will include a list of commonly prescribed drugs, by drug class, that are known to alter CYP2D6 activity. This list includes only those drugs for which established, peer-reviewed literature substantiates the effect. The list provided is not all-inclusive.


CYP2D6 activity also is dependent upon hepatic and renal function status, as well as age. Patients also may develop toxicity if hepatic or renal function is decreased. Drug metabolism also is known to decrease with age. It is important to interpret the results of testing and dose adjustments in the context of renal and hepatic function and age.

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

Note that in patients who have received heterologous blood transfusions before a saliva sample was acquired, the saliva samples may contain donor DNA. Return to recipient genotype usually occurs after 6 weeks. Similarly, saliva samples obtained from patients after allogeneic blood or marrow transplantation can contain donor DNA. In both cases, this may result in genotyping results that reflect the genotype of the recipient, the donor, or a blend of the donor and recipient. Results obtained under these circumstances may not accurately reflect the recipient’s genotype.


CYP2D6 genetic test results in patients who have undergone liver transplantation may not accurately reflect the patient's CYP2D6 status.


Direct DNA testing will not detect all the known mutations that result in decreased or inactive CYP2D6. Absence of a detectable gene mutation or polymorphism does not rule out the possibility that a patient has an intermediate or poor metabolizer phenotype.


This test does not detect polymorphisms other than those listed. Gene duplications may occur by other mechanisms and may not be detected. Other polymorphisms in the primer binding regions can affect the testing, and ultimately, the genotyping assessments made. Testing may reflex to DNA sequencing to resolve difficult genotypes or to confirm interpretations.


Patients with an extensive or intermediate metabolizer genotype may have CYP2D6 enzyme activity inhibited by a variety of medications or their metabolites, including many tricyclic antidepressants, selective serotonin reuptake inhibitors, many histamine H-2 receptor antagonists, amiodarone, celecoxib, cimetidine, cocaine, methadone, quinidine, and ritonavir, as well as several other drugs. Treatment with drugs that are inhibitors of CYP2D6, or produce inhibitors through metabolism, may generate a poor metabolizer phenotype in an individual who has an extensive or intermediate metabolizer genotype.


CYP2D6 alleles with "reduced function" may metabolize different drugs at different rates, ranging from near normal to poor, but the literature on this is incomplete at this time.


The drug application that we currently support for testing and interpretation is for the treatment of depression and other psychiatric disorders.


This test is not for use in assessing for autoimmune hepatitis. Autoantibodies for CYP2D6 enzyme are found in many cases of autoimmune hepatitis. Order LKM / Liver/Kidney Microsome Type 1 Antibodies, Serum for autoimmune hepatitis assessment.


If considering treatment with tamoxifen, order 2D6TO / Cytochrome P450 2D6 Genotyping for Tamoxifen Hormonal Therapy, Saliva.

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.

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Bertilsson L, Dahl ML, Dalen P, Al-Shurbaji A: Molecular genetics of CYP2D6: clinical relevance with focus on psychotropic drugs. Br J Clin Pharmacol 2002 Feb;53(2):111-122

2. Lundqvist E, Johansson I, Ingelman-Sundberg M: Genetic mechanisms for duplication and multiduplication of the human CYP2D6 gene and methods for detection of duplicated CYP2D6 genes. Gene 1999 Jan 21;226(2):327-338

3. Kirchheiner J, Brosen K, Dahl ML, et al: CYP2D6 and CYPSC19 genotype-based dose recommendations for antidepressants: a first step towards subpopulation-specific dosages. Acta Psych Scand 2001 Sept;104(3):173-192

4. Lam YWF, Gaedigk A, Ereshefsy L, et al: CYP2D6 inhibition by selective serotonin reuptake inhibitors: analysis of achievable steady-state plasma concentrations and the effect of ultrarapid metabolism at CYP2D6. Pharmacotherapy 2002;22:1001-1006