Persistent Anticoagulation and Severe Upper Gastrointestinal Bleeding in a Patient With Factor V Leiden: A Case of the Warfarin Reservoir Phenomenon.

We report a case of a 58-year-old man with atrial fibrillation who presented with hematemesis and melena despite reporting that he had discontinued warfarin therapy three months earlier. On admission, he was hypotensive and tachycardic, with laboratory tests showing hemoglobin of 6.8 g/dL and INR of 4.2. A plasma assay confirmed detectable warfarin levels, suggesting a persistent anticoagulant effect despite discontinuation. Endoscopy revealed a bleeding gastric ulcer, which was treated successfully with endoscopic hemostasis. The patient required a blood transfusion, vitamin K, and fresh frozen plasma for INR reversal. Further work-up revealed a heterozygous factor V Leiden mutation, creating a paradoxical clinical situation of both thrombotic predisposition and major hemorrhage. This case highlights the importance of recognizing unusual pharmacokinetic persistence of warfarin, the so-called "warfarin reservoir" phenomenon, and the clinical challenge posed by coexisting inherited thrombophilia. Awareness of this scenario can aid in timely diagnosis, appropriate reversal strategies, and multidisciplinary decision-making for long-term anticoagulation management.