Clopidogrel Response (CYP2C19) Genotype
The CYP2C19 genotype test identifies inherited variants in cytochrome P450 2C19 that determine clopidogrel bioactivation. Carriers of loss-of-function alleles are intermediate or poor metabolisers with reduced antiplatelet response and higher stent thrombosis risk; guidelines recommend alternative P2Y12 inhibitors in these patients undergoing PCI.
1
Biomarkers
Buccal swab
Sample
7–10 business days
Turnaround
No
Fasting
What is the CYP2C19 Genotype test?
The Clopidogrel Response (CYP2C19) Genotype test identifies inherited variants in the gene encoding cytochrome P450 2C19, the principal enzyme responsible for converting clopidogrel to its active form. Practitioners order this test to guide antiplatelet therapy, particularly in patients undergoing percutaneous coronary intervention, because carriers of loss-of-function alleles derive less benefit from clopidogrel and face elevated thrombotic risk. Genotype-guided antiplatelet selection is supported by Clinical Pharmacogenetics Implementation Consortium and Dutch Pharmacogenetics Working Group guidelines, and by cardiovascular societies in several jurisdictions. The test is most commonly ordered preemptively before elective PCI or reactively when a patient experiences recurrent ischaemic events on clopidogrel; some centres incorporate it into routine pre-catheterisation pathways. Unlike on-treatment platelet-function assays, CYP2C19 genotyping provides a fixed, non-modifiable result that applies across the patient's lifetime and is unaffected by drug adherence or recent dosing.
What does the CYP2C19 Genotype test measure?
CYP2C19 is a hepatic cytochrome P450 enzyme that catalyses the bioactivation of clopidogrel, a prodrug that must be metabolised to its active thiol metabolite to inhibit the platelet P2Y12 receptor. Loss-of-function alleles (*2, *3, *4, *5, *6, *7, *8) encode reduced or absent enzyme activity, while gain-of-function alleles (*17) confer increased activity. An individual's diplotype (the pair of inherited alleles) determines their metaboliser phenotype and, consequently, the degree of antiplatelet effect achieved on standard-dose clopidogrel.
Carriers of CYP2C19 loss-of-function alleles produce less active metabolite, exhibit diminished platelet inhibition, and face higher rates of major adverse cardiovascular events — particularly stent thrombosis — after percutaneous coronary intervention. Guidelines from multiple cardiology and pharmacogenomics consortia recommend alternative P2Y12 inhibitors (prasugrel or ticagrelor) for intermediate and poor metabolisers undergoing PCI. Conversely, ultrarapid and rapid metabolisers may have increased bleeding risk, though the evidence is less robust. Genotype-guided antiplatelet therapy is an established application of pharmacogenomics in cardiovascular medicine.
Why is the CYP2C19 Genotype test ordered?
- Patients scheduled for elective or urgent percutaneous coronary intervention, to guide selection between clopidogrel, prasugrel, and ticagrelor
- Individuals with recurrent ischaemic events (stent thrombosis, myocardial infarction) while on clopidogrel therapy, to assess whether genetic poor metabolism is contributory
- Pre-emptive pharmacogenomic screening programmes in cardiovascular or surgical populations, where the result will be available when dual antiplatelet therapy is initiated
- Patients for whom prasugrel or ticagrelor are contraindicated (prior stroke, high bleeding risk) and clopidogrel is being considered, to confirm normal metabolism
- Individuals of East Asian or Oceanic ancestry, where loss-of-function allele frequencies are notably higher and genotype-guided therapy may have greater population impact
- Patients with a family history of early stent thrombosis or treatment failure on clopidogrel, though the test is ordered on the proband's indication rather than family history alone
Sample collection and turnaround
Sample type
Buccal swab
Fasting required
No
Collection method
At-home collection kit
Turnaround
7–10 business days
Collection notes
Patient self-collects buccal cells by rubbing the swab firmly against the inside of each cheek for the duration specified in the kit instructions (typically 30–60 seconds per cheek). Best collected before eating, drinking, or smoking, or at least 30 minutes afterward, to maximise DNA yield and avoid contamination. The swab is air-dried briefly, placed in the provided vial, and returned by prepaid post. Germline DNA is stable on the swab at ambient temperature for several weeks.
Specimen requirements
Buccal swab with adequate epithelial-cell yield. If DNA quantity or quality is insufficient, the laboratory will request a repeat swab or, less commonly, a venous blood sample. Note recent transfusion or transplant history if present, though buccal DNA reflects the patient's own germline.
What can affect CYP2C19 Genotype results?
Insufficient DNA from the swab (recent eating, drinking, smoking, or poor collection technique) may require a repeat sample or venous draw; this is a pre-analytical rather than biological variable.
A buccal swab samples the patient's germline DNA, unaffected by transfusion, but a whole-blood sample in a recently transfused or transplanted patient could reflect donor genotype; confirm sample source if such history exists.
The genotype itself is fixed, but drugs that inhibit (proton-pump inhibitors, particularly omeprazole) or induce CYP2C19 can alter the phenotypic antiplatelet response; genotype and drug–drug interactions must both be considered in dosing.
Most panels interrogate the common alleles (*2, *3, *17); rare variants may be missed, and phenotype assignment algorithms differ slightly between laboratories, so results should specify which alleles were tested.
Diabetes, renal impairment, higher body weight, and low baseline platelet reactivity all influence on-treatment response independently of CYP2C19; genotype is one contributor among many.
What do CYP2C19 Genotype results mean?
Elevated
Not applicable. The test returns a genotype (diplotype) and derived phenotype classification, not a quantitative level. 'Elevated' in the functional sense would be an ultrarapid or rapid metaboliser phenotype (*1/*17 or *17/*17), who may have greater clopidogrel activation and, theoretically, higher bleeding risk, though evidence supporting dose reduction is limited and most guidelines do not recommend routine adjustment for gain-of-function alleles.
Normal
A normal (extensive) metaboliser phenotype (*1/*1) predicts standard clopidogrel bioactivation and on-treatment platelet inhibition, supporting use of clopidogrel 75 mg daily (after appropriate loading dose) as dual antiplatelet therapy. Even in normal metabolisers, other clinical and pharmacological factors influence response, so the genotype result is one input to the overall decision.
Reduced
Intermediate or poor metaboliser phenotypes (one or two loss-of-function alleles) indicate impaired clopidogrel bioactivation. CPIC guidelines recommend an alternative P2Y12 inhibitor (prasugrel 10 mg daily or ticagrelor 90 mg twice daily) in these patients undergoing PCI, because diminished platelet inhibition is associated with increased stent thrombosis and myocardial infarction. If an alternative is contraindicated, some data support higher clopidogrel doses (e.g. 150 mg loading, 150 mg maintenance), though this is less well validated.
Note: CYP2C19 genotype is a lifetime result that does not change and does not require repeat testing. It predicts drug metabolism, not clinical outcome in isolation, so it must be integrated with bleeding risk, contraindications to alternative agents, and the urgency of revascularisation. The test does not cover all rare alleles, and phenotype assignment may differ slightly between laboratories; specify the platform and alleles tested. Finally, genotype-guided therapy is best supported in the PCI population; evidence in other indications (stroke, peripheral artery disease) is less mature.
Frequently asked questions
CYP2C19 is a liver enzyme that converts clopidogrel from an inactive prodrug into its active form. Genetic variants that reduce enzyme function result in less active drug, weaker platelet inhibition, and higher rates of stent thrombosis and myocardial infarction.
How to access the CYP2C19 Genotype test
The CYP2C19 Genotype reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Scott SA, et al. Clinical Pharmacogenetics Implementation Consortium guidelines for CYP2C19 genotype and clopidogrel therapy: 2013 update. Clin Pharmacol Ther. (2013)
- Levine GN, et al. 2016 ACC/AHA guideline focused update on duration of dual antiplatelet therapy in patients with coronary artery disease. Circulation. (2016)
- Mega JL, et al. Cytochrome P450 genetic polymorphisms and the response to prasugrel: relationship to pharmacokinetic, pharmacodynamic, and clinical outcomes. Circulation. (2009)
- Swen JJ, et al. Pharmacogenetics: from bench to byte — an update of guidelines. Clin Pharmacol Ther. (2011)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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