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A UK-first guideline has recommended that all patients should have pharmacogenetic testing to identify clinically relevant CYP2C19 genetic variants, regardless of indication.
The clinical guideline, published by the UK Centre of Excellence in Regulatory Science and Innovation in Pharmacogenomics (UK CERSI-PGx) on 4 December 2025, suggests that testing should be undertaken “where available” to optimise patients’ antiplatelet therapy.
Clopidogrel is widely used in the UK as an antiplatelet medication to prevent blood clots; however, it does not work uniformly in all patients owing to genetic differences.
The CYP2C19 gene influences how well a person can convert clopidogrel into its active form as it affects the level of CYP2C19 enzyme in the liver.
Those with lower enzyme levels will activate clopidogrel less efficiently than individuals with higher levels. The variability can be detected by a genetic test.
A significant proportion of the population have lower enzyme levels, while in white individuals it is 20–30%. However, the proportion can be as high as 50–60% in those of Asian descent.
“The availability of pharmacogenetic testing may vary in time, indication and in geography. In the absence of available relevant pharmacogenetic information or testing, the current best practice clinical guidelines should be followed,” the guideline states.
The UK CERSI-PGx is led by the University of Liverpool and is funded by Innovate UK and the Medical Research Council, in conjunction with the Medicines and Healthcare products Regulatory Agency (MHRA).
Munir Pirmohamed, centre lead at the University of Liverpool, commented: “With the new CERSI-PGx guideline for clopidogrel, we aim to ensure patients are prescribed the right treatment at the right dose, based on their genetics.
“By integrating CYP2C19 testing into routine pathways, we can improve efficacy, reduce adverse drug reactions, ease pressure on the NHS, and support cost-effective, precision prescribing.”
Sue Hill, chief scientific officer for England, said: “These guidelines are an important step in using genomics to inform treatment pathways and to prevent avoidable side effects.
“I am delighted that an inclusive and multidisciplinary approach was taken to support the development of these guidelines and that this approach will be used for other clinical use cases. The availability of UK clinical guidelines for pharmacogenomics will enable the systematic introduction into mainstream care.”
Alison Cave, chief safety officer at the MHRA, said: “By analysing a patient’s genetic information, pharmacogenomics ensures they receive the most appropriate medication and dosage for their specific genetic profile, minimising the risk of adverse drug reactions.
“Such approaches form part of MHRA’s long-term vision for more personalised medicine prescribing and will be a gamechanger for patient safety and wellbeing.”
In July 2024, the National Institute of Health and Care Excellence published guidance that recommended the use of CYP2C19 genotype testing to assess if clopidogrel is a suitable antiplatelet drug for people who have had an ischaemic stroke or a transient ischaemic attack.
The results of a study, published in March 2025, revealed that testing patients for just three genes — including CYP2C19 — could help prevent three-quarters (75%) of avoidable side effects of certain medicines.
At a roundtable hosted by The Pharmaceutical Journal in October 2025, Pirmohamed discussed the development of new pharmacogenomic tests that can be added to the national genomic directory and said he was open to working with others on developing areas that need a guideline.
