Atrial fibrillation requiring stroke prevention in a patient receiving carbamazepine — MSRA MCQ

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Hardall topics relevant for this examAtrial fibrillation requiring stroke prevention in a patient receiving carbamazepineMSRA

A 79-year-old man with permanent atrial fibrillation attends for an annual review. He has hypertension and had a transient ischaemic attack 3 years ago. He takes bisoprolol and carbamazepine modified-release 400 mg twice daily for focal epilepsy. He has been seizure-free for 8 years, and his neurologist advises that carbamazepine should not be changed. Renal and liver function are normal, body weight is 78 kg, and there is no history of major bleeding. After shared decision-making, he agrees to anticoagulation for stroke prevention. Which anticoagulation strategy is most appropriate?

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Correct answer: A — Initiate warfarin with INR monitoring through an anticoagulation service

He has a clear indication for long-term anticoagulation: age 75 years or over contributes 2 points, hypertension 1 point and previous TIA 2 points, giving a CHA2DS2-VASc score of 5. Normal renal and hepatic function would ordinarily make a DOAC an appropriate first-line option. However, carbamazepine is a strong inducer of CYP3A4 and P-glycoprotein. It can lower DOAC concentrations and reduce anticoagulant effect, which is particularly unacceptable in this patient because he has already had a TIA. This is not resolved simply by selecting a different DOAC: apixaban and rivaroxaban are affected by CYP3A4 and P-glycoprotein induction, while dabigatran and edoxaban remain vulnerable to P-glycoprotein induction. SPS advises prescribing an anticoagulant for which monitoring is available, such as warfarin, or changing the interacting medicine. NICE recommends a vitamin K antagonist when a DOAC is unsuitable. Warfarin is therefore the best option because its effect can be assessed and titrated using INR monitoring. His carbamazepine must remain unchanged, so anticoagulation-service monitoring is essential during initiation and throughout treatment. The apparent convenience of a DOAC does not outweigh the clinically important risk of inadequate stroke prevention from this interaction.

Reference: NICE NG196: Atrial fibrillation: diagnosis and management — recommendations 1.6.3 to 1.6.5 (2021; updated March 2025) — https://www.nice.org.uk/guidance/NG196/chapter/recommendations NHS Specialist Pharmacy Service: Managing interactions with direct oral anticoagulants (DOACs) — Carbamazepine, phenytoin and phenobarbital (Published 5 January 2024; updated 20 June 2025) — https://sps.nhs.uk/articles/managing-interactions-with-direct-oral-anticoagulants-doacs/ Apixaban 5 mg film-coated tablets — Summary of Product Characteristics, section 4.5 (2026) — https://www.medicines.org.uk/emc/product/100160/smpc