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.