Pharmacogenetic Tests: Purpose, Test Details & Results

Pharmacogenetic testing is revolutionizing the way we approach medication, offering a path toward more personalized and effective treatments. By uncovering how our genes influence drug response, these tests aim to reduce side effects, improve efficacy, and streamline medication choices. This article explores the purpose, details, and practical outcomes of pharmacogenetic testing, using the latest research to guide you through what these tests mean for patients and clinicians alike.

Purpose of Pharmacogenetic Tests

Pharmacogenetic tests bridge the gap between genetics and medication management. Their primary aim is to predict how an individual's genetic makeup influences their response to specific drugs—helping to avoid adverse reactions and increase treatment success. As healthcare shifts toward personalization, pharmacogenetics offers real-world benefits for patients with conditions ranging from heart disease to psychiatric disorders.

Table 1: Key Purposes and Benefits of Pharmacogenetic Testing

Understanding the Goals of Pharmacogenetic Testing

Pharmacogenetic testing is grounded in the quest for safer, more effective medication use.

• Reducing Adverse Drug Reactions (ADRs): ADRs are a significant cause of hospitalizations and healthcare costs. Genetic differences can make some people more susceptible to side effects. Tests can identify those at risk before medication is started, allowing clinicians to choose safer alternatives or adjust doses 1, 2.

• Increasing Treatment Efficacy: Not all patients respond to drugs the same way. For example, variations in CYP2C19 affect how well clopidogrel works after heart procedures, while CYP2D6 and CYP2C19 can impact antidepressant effectiveness 3, 12. Identifying these variations helps tailor therapies to the individual, boosting the chance of success.

• Personalizing Drug Dosing: Some medications have a narrow therapeutic window, where too much or too little can cause harm. Warfarin, a blood thinner, is a classic example: genes like CYP2C9 and VKORC1 are linked to dose requirements. Knowing a patient’s genetic profile can help find the right starting dose faster 3, 12.

• Supporting Cost-Effective Care: Economic evaluations show that preemptive pharmacogenetic testing can be cost-effective for certain drug-gene pairs, reducing overall healthcare spending by preventing costly ADRs and hospitalizations 2, 5.

Who Benefits Most?

Pharmacogenetic testing is especially useful for:

• Patients starting medications with known genetic interactions (e.g., certain anticoagulants, antidepressants, or antipsychotics)
• Individuals with a history of poor drug response or unexplained side effects
• Populations where high-risk genes are more common

Evolving Role in Medicine

While still emerging in many settings, pharmacogenetic testing is gaining traction as evidence accumulates and costs decrease. Its promise lies in moving medicine from a trial-and-error approach toward one that is proactive and precise 1, 4, 12.

Pharmacogenetic Tests Details

Pharmacogenetic testing comes in many forms—from single-gene assays to comprehensive multi-gene panels. Understanding what these tests look for, how they’re performed, and which drugs are impacted is crucial for both patients and healthcare providers.

Table 2: Types of Pharmacogenetic Tests and Their Clinical Scope

Types of Pharmacogenetic Tests

• Single-Gene Tests:
  Focus on specific gene-drug pairs with strong evidence. Examples:

  • CYP2C19 for clopidogrel response 12
  • HLA-B57:01* before abacavir (HIV therapy) 2
  • TPMT before thiopurine drugs 7

• Multi-Gene (Panel) Tests:
  Assess multiple genes simultaneously, covering a broad range of medications. Panels can include 12–16 genes and test for 50+ variants, making them more comprehensive for patients on complex regimens or with multiple conditions 1, 5, 6.

• Specialized Psychiatric Panels:
  Tailored for mental health medications, these panels test genes like CYP2D6, CYP2C19, HLA-A, and HLA-B—key for predicting antidepressant and antipsychotic response 3, 8, 10.

What Genes Are Commonly Tested?

• Cytochrome P450 Enzymes:
  • CYP2D6, CYP2C19, CYP2C9, CYP3A4, CYP3A5 influence metabolism of many drugs 1, 3, 5, 6.
• HLA Variants:
  • HLA-B57:01*, HLA-B15:02* linked to severe drug reactions (e.g., abacavir, carbamazepine) 2, 3.
• Other Genes:
  • VKORC1, TPMT, DPYD, SLCO1B1, COMT, and others, depending on the panel and drugs considered 5, 6.

How Are Pharmacogenetic Tests Performed?

• Sample Collection:
  Usually via a blood draw or cheek swab—simple and minimally invasive 8.
• Laboratory Analysis:
  Tests are performed in specialized labs using genotyping or sequencing technologies. Quality control and standardization are critical to ensure results are accurate and reproducible 6, 9.
• Reporting:
  Results are typically returned as a report showing genotype, predicted phenotype (e.g., “poor metabolizer”), and clinical implications for specific drugs 9.

When Are Tests Used?

• Preemptive Testing:
  Conducted before starting any high-risk medication, with results stored for future use 1, 4.
• Point-of-Care Testing:
  Ordered when a specific drug is prescribed, often for drugs with known genetic risks 4, 5.

Limitations and Practical Considerations

• Panel Size vs. Utility:
  Larger panels can catch more actionable variants, but may include genes with less clear evidence 5.
• Clinical Guidelines:
  Standardized recommendations (e.g., from the CPIC or Dutch Pharmacogenetics Working Group) are used to interpret results and guide prescribing 1, 9.
• Access and Cost:
  While costs are decreasing, insurance coverage and access to testing can vary by country and health system 2, 5, 7.

Pharmacogenetic Tests Results & Follow-Up

Interpreting pharmacogenetic test results is a collaborative process involving patients, clinicians, and sometimes pharmacists or genetic counselors. Results can directly influence medication choices, but also require careful communication and ongoing follow-up to ensure the best outcomes.

Table 3: Pharmacogenetic Test Result Categories and Clinical Actions

How Are Results Reported and Interpreted?

• Standardized Terminology:
  Results are described using uniform terms for allele function (e.g., “normal function,” “decreased function,” “no function”) and predicted phenotype (“ultrarapid,” “normal,” “intermediate,” “poor metabolizer”) 9.
• Clinical Actionability:
  An “actionable” result means there is a clear guideline recommending a change to standard care—such as a different drug, altered dose, or additional monitoring 1, 3, 9.

Impact on Patient Care

• Medication Changes:
  Patients with actionable variants may have their medication switched or the dose adjusted. For example, a poor metabolizer for CYP2C19 may receive an alternative to clopidogrel, or a HLA-B58:01* carrier may avoid allopurinol due to risk of severe reactions 1, 2, 12.
• Improved Outcomes:
  Studies show that following pharmacogenetic recommendations leads to higher rates of treatment success and fewer adverse reactions, especially in psychiatry and cardiology 1, 10, 12.
• Patient Experience:
  Most patients find pharmacogenetic testing useful and are willing to undergo it again. However, clear communication between healthcare providers and patients is essential to ensure understanding and appropriate use of results 11.

Communication and Follow-Up

• Provider-Patient Discussion:
  Results should be discussed with patients, explaining what the findings mean and how they affect treatment. This step is critical for shared decision-making and medication adherence 11.
• Ongoing Monitoring:
  Even with genetic guidance, patients require follow-up to monitor for unexpected side effects or changes in drug response. Not all genetic effects are fully understood, and other factors (age, health status, other medications) also play a role 1, 4, 5.
• Integration into Medical Records:
  Storing pharmacogenetic data in electronic health records ensures the information is available for future prescribing decisions 9, 4.

Challenges and the Path Forward

• Standardization and Quality Control:
  Consistency in reporting and interpretation across labs and healthcare systems is key for broader adoption 6, 9.
• Clinical Utility for All Drugs:
  While evidence is robust for some drugs, further research is needed for others before widespread testing is justified 2, 7, 12.
• Healthcare System Integration:
  Interdisciplinary collaboration—including pharmacists, genetic counselors, and clinical decision support tools—enhances the utility of pharmacogenetic testing 5.

Conclusion

Pharmacogenetic testing is a powerful tool for making medication safer and more effective. As the evidence base grows and access expands, its role in routine care will likely increase—especially for high-risk drugs and patients with complex medication needs.

Key points:

• Pharmacogenetic tests aim to reduce adverse drug reactions, improve drug efficacy, personalize dosing, and support cost-effective healthcare 1, 2, 3, 5, 12.
• Testing options range from single-gene assays to comprehensive panels, with growing inclusion of key genes relevant to multiple medications 1, 3, 5, 6.
• Actionable results can lead to medication changes guided by clear clinical guidelines, improving patient outcomes and satisfaction 1, 10, 11, 12.
• Effective use of pharmacogenetic testing relies on standardized reporting, provider-patient communication, and integration into healthcare systems 6, 9, 11.
• Ongoing research and collaboration are needed to expand evidence and ensure equitable access to testing for all who might benefit 2, 4, 5, 7.

Pharmacogenetic testing is not just about the science of DNA—it’s about delivering the right medicine to the right person at the right time. As this field continues to evolve, it holds great promise for the future of personalized medicine.