Evaluation of INR follow-up in warfarin-treated patients after antibiotic initiation in the emergency department

Purpose:   The purpose of this study was to determine if warfarin-treated patients prescribed interacting antibiotic therapy at the Durham Veteran Affairs Medical Center (VAMC) emergency department (ED) had INR follow-up within one week of antibiotic initiation.  

Methods:   A retrospective chart review of patients on warfarin therapy who were prescribed an antibiotic in the Durham VAMC ED and filled it at the Durham VAMC pharmacy from July 1, 2009 through June 30, 2011 was designed.  The study population included patients on warfarin monitored in a Durham VAMC anticoagulation clinic who were prescribed an antibiotic identified to interact with warfarin (amoxicillin, amoxicillin-clavulanate, azithromycin, ciprofloxacin, clarithromycin, doxycycline, fluconazole, metronidazole, moxifloxacin, and trimethoprim-sulfamethoxazole) in the Durham VAMC ED. 

Results: Of the 85 patients included in our study, 40% had INR follow-up within 7 days of antibiotic initiation.  Communication between the ED and anticoagulation providers regarding the drug interaction was documented in the medical record for 25.9% of evaluated patients.  Five patients had minor bleeding events and three experienced a critical INR.  No patients included in our study had a major bleeding event or required the administration of vitamin K. 

Conclusion: Fewer than half of warfarin-treated patients prescribed an interacting antibiotic in the ED had INR follow-up completed within 7 days of antibiotic initiation.   Communication alerting the anticoagulation providers occurred in only a quarter of evaluated patients.    These results suggest the need for improved communication between providers to ensure appropriate INR monitoring in patients receiving antibiotics that may interfere with warfarin.