A super‐geriatric patient with gastrostomy underwent life‐threatening prothrombin time‐international normalized ratio prolongation by warfarin following fasting and antibiotic therapy

KEY CLINICAL MESSAGE: A 97‐year‐old woman with gastrostomy had a drastic enhancement for PT‐INR after starting antibiotic therapy. Possible causes include (1) vitamin K deficiency due to fasting and (2) a combination of warfarin and antibiotics. ABSTRACT: Geriatric and Asian‐descent patients are more sensitive to the effects of warfarin, a key anticoagulant drug. In this report, we present a 97‐year‐old bedridden woman being treated with warfarin for cardiogenic cerebral infarction and femoral neck replacement as part of in‐home medical care with a gastrostomy and was admitted to our hospital after developing pneumonia. We discontinued warfarin and started antibiotics, and her pneumonia‐related symptoms improved. Eleven days after restarting warfarin, the patient's PT‐INR surpassed the upper limit for measurement (over 10). We considered the mechanism might be triggered by (1) fasting, low nutrition status; and (2) antibiotics secondary to risk factors such as gastrostomy and being a super‐geriatric woman. We recommend careful monitoring of PT‐INR in patients treated with warfarin and antibiotics, especially in the setting of gastrostomy or older persons.