Warfarin poisoning with delayed rebound toxicity

Background: Intentional poisoning with warfarin is not the same as over-anticoagulation, for which guidelines exist. The coagulopathy resulting from a warfarin overdose is reversed with vitamin K₁, the dose and timing of which is often guided by experience with the management of over-anticoagulation with warfarin therapy, rather than acute overdose. Case Report: We report a case of a 50-year-old man who ingested an unknown amount of his warfarin, venlafaxine, and paracetamol. He presented with an international normalized ratio (INR) of 2.5, which steadily increased over 24 h to 7, despite receiving an initial 1 mg of vitamin K₁. He was then treated with 5 mg vitamin K₁, and once the INR returned to 4.5, 40 h post ingestion, he was discharged home. He was also treated with a full course of acetylcysteine for the paracetamol overdose. The following day his INR rebounded to 8.5 and he suffered a spontaneous epistaxis requiring readmission; he was treated with low titrated doses of vitamin K₁. The warfarin concentration was 74.6 μg/mL 26 h post ingestion and decreased to 3.7 μg/mL over 72 h. Why Should an Emergency Physician Be Aware of This? Our case highlights the risk of a rebound elevated INR even 3 days after acute warfarin overdose despite treatment with vitamin K₁. Understanding the pharmacokinetics of vitamin K₁ in comparison with warfarin, repeat INR testing, and continued treatment with oral vitamin K₁ may help avoid complications of rebound coagulopathy in warfarin overdose.