Elevated international normalized ratio values associated with concomitant use of warfarin and ceftriaxone.

Purpose: The case of a patient receiving long-term warfarin therapy who experienced elevated International Normalized Ratio (INR) values on two occasions after injections of ceftriaxone is reported.

Summary: An elderly woman (age, 67 years) with multiple comorbidities who had been receiving warfarin therapy for about 8 years was given an intramuscular injection of ceftriaxone 1 g for the treatment of a urinary tract infection. Four days later, her INR (which had recently ranged from 1.