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

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.

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 1.9 to 3.0 at a weekly warfarin dosage of 52.5-54.5 mg) was 10.74. One scheduled warfarin dose was withheld and 5 mg of phytonadione administered; one day later, the INR was 3.4 (goal, 2.5-3.5). INR values remained stable for several weeks until the patient again received a 1-g ceftriaxone injection for an infection (she was also prescribed oral cefuroxime and phenazopyridine); four days later, the INR was 16.99. Again, the scheduled warfarin dose was withheld and 5 mg of phytonadione administered. One day later, the INR had declined to 4.6 but remained above the target range; therefore, warfarin was withheld for a second day, after which the patient received 7.5 mg of warfarin sodium daily for two days, resulting in an INR decrease to 2.1. The patient continued to receive 7.5 mg of warfarin sodium daily, and at one-week follow-up the INR value (2.5) was within the therapeutic range.

A 67-year-old American Indian woman with previously stable INR values during long-term warfarin therapy after mitral valve replacement surgery had INR elevations on two occasions after receiving ceftriaxone for urinary tract infections.