Key result
Warfarin dose adjustment based on VKORC1 genotype and plasma concentration monitoring successfully increased PT-INR from 1.40 to 1.99 in a patient taking carbamazepine.
Why the study?
Does genetic testing and plasma concentration monitoring help adjust warfarin dosing in a patient with carbamazepine interaction?
Case Report (n=1)
No
Does genetic testing and plasma concentration monitoring help adjust warfarin dosing in a patient with carbamazepine interaction?
Genetic testing for VKORC1 and CYP2C9, along with plasma concentration monitoring, can guide warfarin dose adjustments in patients with complex drug interactions like carbamazepine.
Answers are grounded in this paper and cite their sources.
May inform individualized warfarin dosing in carbamazepine users; hypothesis-generating pending prospective validation.
WHAT IS KNOWN AND OBJECTIVE: Carbamazepine is known to interact with warfarin. We report on a case of this interaction and on its management using the patient's genetic information. CASE SUMMARY: The case concerns a 74-year-old Japanese woman with a mood disorder and a central retinal vein occlusion. She was on therapy that included carbamazepine and had started to take warfarin. However, the patient's prothrombin time expressed as the international normalized ratio (PT-INR) was 1·40 despite taking a dose three times higher than the average. The patient's S-warfarin concentration was 0·15 μg/mL and R-warfarin was 0·52 μg/mL. Her cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex, subunit 1 (VKORC1), genotypes were *1/*1 and -1639GA, respectively. The VKORC1 genotype indicated that she would require an even higher dose. We proposed a further increase in dose and the patient's PT-INR rose to 1·99. WHAT IS NEW AND CONCLUSION: The patient required a high warfarin dose because of the VKORC1 genotype, and induction of CYP2C9 by carbamazepine. We improved the patient's pharmacotherapy based on her genetic information.
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Aomori et al. (2014) conducted a case report in Central retinal vein occlusion and mood disorder requiring warfarin and carbamazepine (n=1). Warfarin dose adjustment using genetic information and plasma concentration monitoring was evaluated on PT-INR. Warfarin dose adjustment based on VKORC1 genotype and plasma concentration monitoring successfully increased PT-INR from 1.40 to 1.99 in a patient taking carbamazepine.
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