This was published when the organisation was the Royal Pharmaceutical Society.

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PGx testing case studies

The case study examples included in this resource support and link to the PGx principles covered in domains one and two of this document. Details about whether testing has been carried out, or how test results are used in practice have been intentionally excluded due to differences in the availability of commissioned testing across the UK and the potential for these arrangements and clinical guidance to evolve over time.

Case study 1. Clopidogrel and CYP2C19 PGx testing (emerging practice)

A 65-year-old man experiences sudden left-sided weakness after an afternoon nap. He is admitted to the local A&E and is diagnosed with a suspected acute stroke and transferred to the acute stroke unit. Following examination, the prescriber intends to start treatment with clopidogrel in 14 days’ time as per national stroke guidelines.¹

The prescriber:

• Identifies the need for CYP2C19 PGx testing before clopidogrel initiation,^2, 3 to determine if the patient carries a variant which could impact their ability to metabolise clopidogrel effectively, and to guide optimal antiplatelet selection. Published evidence describes that approximately 30% of the UK population are believed to carry a CYP2C19 variant which will likely predict a lack of response to clopidogrel treatment⁴
• Assesses the patient’s eligibility for PGx testing, ensuring patient understands its purpose, interpretation and limitations using validated resources to support clinical interpretation
• Establishes if there has been any prior PGx testing found in the patient’s medical record or within their family history and/or questions the patient on whether they or any family member has had previous testing performed
• Engages with the patient and their relatives to explain the purpose, benefits and risks of the test before requesting PGx testing
• Considers local guidelines and clinical need for interim antiplatelet management in the absence of PGx results
• Refers to relevant resources when interpreting the test result to inform the treatment decision^3, 5
• Explains the genetic result and its implications for clopidogrel when making a shared decision about antiplatelet prescribing
• Explains the relevance of the genetic result for any other medications and explains that unlike other clinical tests, genetic results remain constant, although the interpretation of results may evolve and may need revisiting over time
• Prescribes an appropriate antiplatelet for the patient
• Documents the event and treatment decisions within medical records, shares the results within the GP discharge letter and ensures result is available within electronic records where the infrastructure is available
• Documents within the patient record that other drugs metabolised by the same enzyme (e.g., citalopram) may be affected if prescribed, informing current and future treatment choices
• Reviews treatment plans when new medicines (which could potentially interact, such as enzyme inhibitors or inducers) are added, as these may affect PGx findings. Drug–drug–gene interactions and adjusted review periods may be needed to maintain safe, effective therapy and retain appropriate clinical context over time.

Competencies shown: 1.2, 1.6, 1.9, 1.10, 1.11, 2.4, 2.7, 2.8, 3.1, 3.3, 4.1, 4.12, 4.14, 4.15, 5.2, 6.1, 8.2, 10.1, 10.2.

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Case study 2. Gentamicin and MT-RNR1 PGx testing (emerging practice)

A baby is born by C-section at the local hospital maternity ward at 32 weeks’ gestation weighing 5lbs 12oz. After three days, the baby starts to show clinical signs of poor feeding, lethargy and a spiking temperature. The baby has a suspected diagnosis of early-onset neonatal sepsis and is transferred to the neonatal intensive care unit. After a thorough examination, the prescriber decides to start empirical treatment with gentamicin in combination with another antibiotic aligned with trust guidance.

The prescriber:

• Identifies the need for a MT-RNR1 POCT prior to prescribing the gentamicin as per reference sources,^6-8 as the presence of a MT-RNR1 gene variant can increase the risk of aminoglycoside- (gentamicin) induced hearing loss
• Uses the result to inform the selection of antibiotic for the baby, within the window for effective antibiotic prescribing. A risk–benefit decision on prescribing should be made in the event of delayed or unavailable results
• Documents the result of PGx testing within the patient record
• Explains the purpose, benefits and limitations of the test and implications of the result to the baby’s parents. Ensures they have appropriate understanding of the need for the test and familial implications, while keeping communication appropriate and patient centred
• Engages the baby’s parents in a discussion about the potential family history of hearing loss and how it could affect the future prescribing of aminoglycosides
• Discusses the need for the patients’ parents to share information with family members if potentially relevant for them guided by clinical judgement
• Knows how to contact specialist genetics services, if appropriate
• Documents the event thoroughly within medical records and ensures the PGx test result is included in the discharge letter to the GP, with a recommendation to record it in the primary care system, ensuring continuity of care across settings and multidisciplinary working.

Competencies shown: 1.2, 1.6, 1.10, 1.11, 1.14, 2.3, 2.4, 2.5, 2.6, 2.10, 3.1, 4.1, 4.2, 4.13, 4.14, 10.1.

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Case study 3. Codeine and PGx testing (illustrative example)

A 29-year-old, 34-week pregnant patient visits the GP practice with severe lower back pain. Despite taking paracetamol regularly, she is still suffering pain which is affecting her daily life. The local prescribing guidelines recommend codeine in this scenario. The prescriber is aware that CYP2D6 metaboliser status can impact codeine treatment response and checks to see if they can access testing.^9-12 PGx testing is not available locally for this gene, but the prescriber takes a comprehensive medical history from the patient, including allergies and personal and family history of adverse drug reactions before proceeding with the prescription.

After 24 hours after starting codeine treatment, the patient develops excessive sedation, nausea and respiratory depression. She is admitted to the maternity unit at her local hospital and receives foetal monitoring which shows reduced movements potentially due to increased exposure of codeine. The codeine treatment is stopped immediately, and she is closely monitored for any changes in her or the baby’s signs or symptoms. The patient makes a full recovery and returns to the GP practice following the birth of her baby for further back pain control. She also asks the prescriber’s opinion of direct-to-consumer testing (as she has read about this online) and believes her DNA may have caused this reaction to occur.

The prescriber:

• Refers to guidance/resources around PGx testing and the prescribing of codeine treatment
• Is aware that certain health conditions, symptoms or adverse drug reactions may be linked to specific PGx variants in the patient
• Reviews the patient including hospital discharge letters to ensure knowledge of the patient’s management to date including information about her possible ADRs to codeine and any testing done in hospital
• Checks that opioid risk has been recorded in the patient’s (electronic) health record
• Discusses an alternative option to manage the pain with the patient, including both pharmacological and non-pharmacological options in the absence of PGx testing
• Advises the patient to avoid the use of codeine in the future and to inform other HCPs
• Submits a Yellow Card report to the MHRA for further exploration of this ADR
• Discusses considerations of DTC testing with the patient, including benefits and limitations of testing and access to treatment pathways.

Competencies shown: 1.6, 1.8, 1.9, 1.10, 2.1, 2.4, 2.5, 2.6, 3.3, 4.1, 4.2, 4.3, 4.10, 6.4, 8.5.

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Case study 4. Capecitabine and DPYD PGx testing (established UK clinical practice)

A 62-year-old woman has been referred to the cancer centre for adjuvant chemotherapy following surgery for stage III colorectal cancer. After assessment, the prescriber plans to start capecitabine chemotherapy as a first line treatment. The prescriber is aware of the MHRA safety alert,¹³ recommending the use of DPYD gene tests to predict the likelihood of the patient experiencing adverse drug reactions from fluoropyrimidine drugs (e.g., capecitabine, 5-FU).^{14, 15}

The prescriber:

• Accesses the patient record to review whether DPYD genetic testing has already been requested and interprets any relevant history and results
• Decides that the patient will need a DPYD genetic test before they commence the capecitabine prescription
• Explains the purpose, benefits and limitations of the test to the patient in the context of their chemotherapy treatment, including possible impact on treatment plan and start date, considering their level of genomics understanding
• Requests the DPYD test along with other relevant investigations to inform treatment options
• Explains to a new staff member why the patient needs to come back to clinic next week and is not being started on the capecitabine treatment today
• Interprets the DPYD result and discusses this with other members of the multi-disciplinary team who have more experience in using PGx reports, before making a decision about prescribing¹⁶
• Ensures the DPYD result has been recorded in the patient’s (electronic) health record
• Educates the patient on adverse effects that they may experience regardless of the outcome of the test result and establishes a plan for monitoring and patient reporting of their condition and potential unwanted effects. The prescriber should be aware that the results are not always definitive and clinical judgement is essential, as genetic data should be interpreted alongside other patient factors.

Competencies shown: 1.9, 1.10, 1.11, 2.3, 2.4, 2.5, 2.6, 3.1, 3.3, 5.2, 6.1, 7.1, 8.2, 8.3, 9.1, 9.2, 9.3, 9.4, 9.5, 10.3, 10.4.

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Case study 5. Carbamazepine and PGx testing (emerging practice)

A 42-year-old man, originally from China, now living in the UK, visits the dentist experiencing sudden, electric-shock-like pain in the right side of his face consistent with trigeminal neuralgia. After initial assessment, the prescriber considers carbamazepine as an effective first line treatment. But while referring to reference sources,^17, 18 the prescriber notices there is a caution listed regarding the increased risk of Steven Johnsons Syndrome (SJS) in patients of Han Chinese or Thai descent, due to the increased likelihood of carrying the HLA-B*15:02 allele. Patients who carry the HLA-B*31:01 allele are also at increased risk of experiencing hypersensitivity reactions with carbamazepine.^{17, 18}

The prescriber:

• Takes and documents the patient history including ethnicity, ancestry and relevant family history. Explains the relevance of this to the use of the medicine, and possibility of accessing PGx testing, in a culturally competent way while considering the patient’s level of genomics understanding and avoiding assumptions based on ethnicity alone
• Reviews all available PGx test results (NHS and non-NHS (with care of test quality assurance)), including relevant family history of testing
• Explores availability of testing within local services, prior to prescribing carbamazepine and the patient’s eligibility within resources (e.g., NHSE NTD)
• Discusses treatment options with the patient, including pharmacological and/or non-pharmacological approaches, based on the patient’s genetic information available
• Discusses with the patient the risk of starting the medicine without PGx testing due to the patient’s ethnicity and potential ancestry and the severity of the adverse effects that could be experienced as a result
• Prescribes an appropriate analgesic following shared decision making with the patient
• Documents aspects of the decision making within the patients’ health record.

Competencies shown: 1.6, 1.9, 1.10, 2.1, 2.3, 2.4, 2.5, 2.6, 3.1, 3.3, 4.1, 4.2, 4.13, 4.15, 8.2.

Further information

Useful resources and links to support development of PGx competence. This list is not exhaustive and other relevant resources are available.

Medication safety, governance and implementation of PGx

• Medicines safety and pharmacovigilance
  • MHRA guidance on PGx www.gov.uk/mhra
    UK regulatory information, safety updates and guidance on genomics and medicine
  • MHRA Yellow Card biobank yellowcard.mhra.gov.uk/biobank
    A developing collaboration between the MHRA and Genomics England to explore links between adverse drug reactions and genomic data.
• Regulation, licensing and commissioning of testing
  • Electronic Medicines Compendium www.medicines.org.uk/emc
    Provides access to SmPCs which can include pharmacogenomic-related information
  • NHS England National Test Directory www.england.nhs.uk/publication/national-genomic-test-directories
    A directory of the genomic tests commissioned in NHS England and due to a specialist service specification agreement, NHS Wales also aligns its commissioning to this directory
  • NHS Scotland Genomic Test Directory www.genomics.nhs.scot/test-directories
    A directory of the genomic tests commissioned in NHS Scotland.
• Professional and policy position statements
  • British Pharmacological Society/Royal College of Physicians
    Personalise Prescribing document: Using pharmacogenomics to improve patient outcomes www.bps.ac.uk/fileadmin/uploads/bps/Reports/Personalised-prescribing_main_report_2022.pdf
  • British Society for Genetic Medicine
    Direct to Consumer Genomic testing Position statement bsgm.org.uk/media/12844/direct-to-consumer-genomic-testing-joint-position-statement-2025.pdf
  • Royal College of General Practitioners Genomics
    CYP2C19 Position statement www.rcgp.org.uk/representing-you/policy-areas/genomic-position-statement
  • Royal College of Pharmacy
    The Role of Pharmacy in Pharmacogenomics (Position Statement)
    Pharmacy Professionals and Genomic Medicine (Position statement).

Clinical guidance resources

• CERSI-PGx guidelines – Clopidogrel CYP2C19 testing cersi-pgx.org/new-cersi-pgx-clinical-guidelines-universal-cyp2c19-genotyping-for-clopidogrel-prescribing
  UK guidelines to help prescribers integrate PGx testing into standard care
• Clinical Pharmacogenetics Implementation Consortium (CPIC) cpicpgx.org
  International (USA led) peer-reviewed, evide nce-based guidelines for implement
• ClinPGx webpage www.clinpgx.org
  A comprehensive (USA-led) clinical PGx resource created to support and expand PGx knowledge, implementation and education
• Dutch Pharmacogenetics Working Group (DPWG) www.knmp.nl
  Internationally recognised PGx dosing guidance based on genotype.

PGx and genomic educational resources available to all prescribers

• CERSI-PGx education resource portal.bpsassessment.com/cersi-pgx-elearning-portal
  Provides targeted, bite-sized learning in a modern, user-friendly digital environment supporting the real-world implementation of PGx in clinical practice
• FutureLearn education packages www.futurelearn.com
  A collection of genomic and personalised education virtual courses suitable for prescribers
• GeNotes resource www.genomicseducation.hee.nhs.uk/about-us/genotes-genomic-notes-for-clinicians
  A ‘just in time’ educational resource for HCPs, designed to be used in a clinical setting
• NHSE Genomic Education Programme (GEP) www.genomicseducation.hee.nhs.uk
  Guidance and educational resources on genomic medicine aspects
• NHSE Specialist Pharmacy Service (SPS) www.sps.nhs.uk/articles/pharmacogenomics-resources-to-support-answering-questions
  A resource hub to support HCPs in answering questions about PGx
• NHS Scotland www.hcstraining.nhs.scot/cpd-resources/genomics-cpd-and-education-resources
  An NHS Scotland webpage to support Scottish HCPs (access to NHS Scotland employees)
• NHS Wales Health Education and Improvement Wales (HEIW) heiw.nhs.wales/our-work/genomics
  An NHS Wales webpage to support HCPs with introductory module resources.

Organisations

This list is not exhaustive and some include restrictions related to certain professions or geographical areas.

• Centre of Pharmacy Postgraduate Education (CPPE) www.cppe.ac.uk
• Royal College of Pharmacy (RCPharm) www.rcpharm.org/blogs/pharmacogenomics-in-prescribing-building-competence-for-safer-personalised-care
• Royal College of General Practitioners (RCGP) www.rcgp.org.uk/your-career/gp-extended-roles/clinical-genetics-genomics-resources
• Royal College of Physicians (RCP) www.rcp.ac.uk/improving-care/resources/pharmacogenomics-in-the-nhs-a-users-guide
• NHS Scotland www.hcstraining.nhs.scot/cpd-resources/genomics-cpd-and-education-resources
• UK Clinical Pharmacy Association (UKCPA) ukclinicalpharmacy.org.

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References

¹ Intercollegiate Stroke Working Party (2023) National Clinical Guideline for Stroke for the UK and Ireland. Available from: www.strokeguideline.org. [Accessed: 14 February 2026]

² NICE CYP2C19 genotype testing to guide clopidogrel use after ischaemic stroke or transient ischaemic attack (HealthTech guidance HTG724) (July 2024). Available from: www.nice.org.uk/guidance/htg724 [Accessed 14 February 2026]

³ Dello Russo C, et al. CYP2C19 genotype testing for clopidogrel: A guideline developed by the UK Centre of Excellence in Regulatory Science and Innovation in Pharmacogenomics (CERSI-PGx). Br J Clin Pharmacol. 2026;92(2):329–347.

⁴ The ClinPGx database. Gene-specific Information Tables for CYP2C19. Available from: www.clinpgx.org/page/ cyp2c19RefMaterials [Accessed: 9 February 2026]

⁵ emc (2024) Dr. Reddy’s Laboratories (UK) Ltd Clopidogrel 75 mg film-coated tablets summary of product characteristics. Available from: www.medicines.org.uk/emc/product/15874/smpc [Accessed: 12 February 2026]

⁶ Medicines Healthcare Regulatory Agency (2021) Drug Safety Update: Aminoglycosides (gentamicin, amikacin, tobramycin, and neomycin) increased risk of deafness in patients with mitochondrial mutations. Available from: www.gov.uk/drug-safety-update/aminoglycosides-gentamicin-amikacin-tobramycin-and-neomycin-increased-risk-of-deafness-in-patients-with-mitochondrial-mutations [Accessed: 14 February 2026]

⁷ NICE. Genedrive MT-RNR1 ID Kit for detecting a genetic variant to guide antibiotic use and prevent hearing loss in babies: Early Value Assessment [Health technology evaluation HTE6] (2023). Available from: www.nice.org.uk/guidance/hte6 [Accessed: 14 February 2026]

⁸ emc (2024) Braun Medical Gentamicin 1 mg/ml solution for infusion summary of product characteristics. Available from: www.medicines.org.uk/emc/product/15144/smpc [Accessed: 14 February 2026]

⁹ Crews KR, et al. Clinical Pharmacogenetics Implementation Consortium. Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines for codeine therapy in the context of cytochrome P450 2D6 (CYP2D6) genotype. Clin Pharmacol Ther. 2012;91(2):321–326.

¹⁰ Medicines Healthcare Regulatory Agency (2014) Drug Safety Update: Codeine for analgesia: restricted use in children because of reports of morphine toxicity. Available from: www.gov.uk/drug-safety-update/codeine-for-analgesia-restricted-use-in-children-because-of-reports-of-morphine-toxicity [Accessed: 14 February 2026]

¹¹ emc (2024) Wockhardt UK Ltd. Codeine Phosphate 30mg Tablets summary of product characteristics. Available from: www.medicines.org.uk/emc/product/2375/smpc [Accessed: 14 February 2026]

¹² British National Formulary (2025) Codeine phosphate. Available from: bnf.nice.org.uk/drugs/codeine-phosphate/ [Accessed: 14 February 2026]

¹³ Medicines Healthcare Regulatory Agency (2020) Drug Safety Update: 5-fluorouracil (intravenous), capecitabine, tegafur: DPD testing recommended before initiation to identify patients at increased risk of severe and fatal toxicity. Available from: www.gov.uk/drug-safety-update/5-fluorouracil-intravenous-capecitabine-tegafur-dpd-testing-recommended-before-initiation-to-identify-patients-at-increased-risk-of-severe-and-fatal-toxicity [Accessed: 14 February 2026]

¹⁴ British National Formulary (2025) Capecitabine [Specialist drug]. Available from: bnf.nice.org.uk/drugs/capecitabine-specialist-drug/ [Accessed: 14 February 2026]

¹⁵ emc (2025) AstraZeneca Ltd Lynparza 100mg Film-Coated Tablets summary of product characteristics. Available from: www.medicines.org.uk/emc/product/9204/smpc [Accessed: 13 February 2026]

¹⁶ UK Systemic Anti-cancer Therapy Board (2024) Personalised Medicine Approach for Fluoropyrimidine-based Therapies. Available from: www.uksactboard.org/_files/ugd/638ee8_4d24d37a598c485d9ef4d1ba90abccd5.pdf [Accessed: 14 February 2026]

¹⁷ British National Formulary (2025) Carbamazepine. Available from: bnf.nice.org.uk/drugs/carbamazepine/ [Accessed: 14 February 2026]

¹⁸ emc (2025) Novartis Ltd Tegretol 200mg Prolonged Release Tablets summary of product characteristics. Available from: www.medicines.org.uk/emc/product/5932/smpc [Accessed: 14 February 2026]