465. Antimicrobial Use in Ambulatory Patients on Warfarin
Session: Poster Session: Hospital-acquired and Transplant Infections
Friday, October 30, 2009: 12:00 AM
Room: Poster Hall A
Background: Warfarin interacts with many antimicrobials, creating the potential for serious bleeding events. The extent of this risk is unknown.
Methods: The study period was January 1, 2008 to December 31, 2008. The study population included all subjects who received pharmacy benefits management from Express Scripts and received at least one prescription for > 30 days during the study period. Age, gender, chronic disease score, geographic location and prescriber specialty were obtained from Express Scripts databases. Antimicrobials of interest with known warfarin interactions included trimethoprim/sulfamethoxazole (TMP/SMX), levofloxacin, ciprofloxacin, metronidazole and fluconazole. Their use was compared in warfarin-exposed and -unexposed groups. Multivariate analysis was used to identify risk factors for receipt of an antimicrobial of interest.
Results: A total of 174,938 warfarin-exposed and 4,459,341 -unexposed patients met inclusion criteria. The average number of antimicrobial prescriptions was similar in warfarin-exposed and -unexposed groups (3.02 ± 3.00 prescriptions vs. 2.37 ± 2.41 prescriptions). Warfarin-exposed patients received a total of 332,978 antimicrobial prescriptions during the study period, of which 93,509 (28.1%) were for antimicrobials of interest. Warfarin-unexposed patients received a total of 10,562,704 prescriptions during the study period, of which 2,593,491 (24.6%) were for the antimicrobials of interest. In a multivariate model, age and higher chronic disease score were associated with receiving an antimicrobial of interest. Warfarin-exposure was associated with lower risk of receiving an antimicrobial of interest(OR 0.56, 95% CI 0.56-0.57).
Conclusion: Although warfarin-exposed patients were less likely than -unexposed patients to receive an antimicrobial of interest, 28.1% of antimicrobial prescriptions to patients on warfarin were for antimicrobials known to interact with warfarin, putting them at risk for bleeding events.
Scott Devine, PhD1, Michael Lane, MD, MSc2, Jay McDonald, MD2 and  M. A. Lane, None. 
S. T. Devine,
Express Scripts, Inc Role(s): Employee, Received: Salary.
J. R. McDonald, None., (1)Express Scripts, Inc., St. Louis, MO, (2)Washington University School of Medicine, St. Louis, MO