© American Society for Clinical Pathology, 2018. All rights reserved. Rat poisoning should be considered in the differential diagnosis of a patient presenting with signs and symptoms of vitamin K deficiency without a more likely explanation. However, confirming this diagnosis may be difficult due to the varying sensitivities of available assays. A 40-year-old Caucasian woman presented to our hospital with chronic abdominal pain, hematuria, and a history of diarrhea of unknown etiology, despite an extensive work-up. Her laboratory evaluation results were consistent with vitamin K deficiency. Because she reported that she had not ingested warfarin, rat poisoning was suspected; however, the results of the first assay were negative. A second specimen was sent to another reference laboratory with a more sensitive assay, and the diagnosis of brodifacoum poisoning was confirmed. The patient was treated with oral vitamin K. If a patient presents with unexplained signs and symptoms of vitamin K deficiency, toxicological evaluation should be performed and repeat testing may be warranted, depending on the sensitivity of the original testing method.