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High-risk antimicrobial prescriptions among ambulatory patients on warfarin
M A Lane1, S T Devine, J R McDonald
1Infectious Diseases Division, Washington University School of Medicine, St Louis, MO 63110, USA. mlane@dom.-wustl.edu
What Is Known And Objective:
Warfarin is a potent anticoagulant with many drug-drug interactions, including antimicrobials. There is limited data on the frequency of prescription of high-risk antimicrobials to patients on warfarin. To examine the frequency of prescriptions for potentially interacting antimicrobials in ambulatory patients on warfarin and the impact of warfarin on the prescription of high-risk antimicrobials.
Methods:
A retrospective cohort study of patients with pharmacy benefits who had ≥1 claim for an oral antimicrobial between 1 January 2008 and 31 December 2008 was conducted, utilizing a pharmacy benefits database. Demographic data including age, gender, chronic disease score (CDS) and geographic location were determined. Warfarin users were defined as any patient with ≥1 claim for warfarin during the follow-up period. Antimicrobials considered high risk for potential interaction with warfarin based on existing literature included trimethoprim/sulfamethoxazole, levofloxacin, ciprofloxacin, metronidazole and fluconazole. Multivariate analysis was used to determine the impact of warfarin use and other factors on high-risk antimicrobial prescription.
Results And Discussion:
A total of 4,568,150 patients with ≥1 claim for antimicrobials during 2008 were analysed. Of them, 110,192 (2·4%) also had one or more claims for warfarin. Among all antimicrobial prescriptions in warfarin users, 42·6% were for high-risk antimicrobials. The mean number of antimicrobial prescriptions was 3·0 in warfarin users versus 2·4 in warfarin non-users (P-value <0·001). After adjusting for age, gender, CDS and geography, the odds of exposure to high-risk antimicrobials was 42% lower (OR 0·58; P-value <0·001) in warfarin users compared with warfarin non-users.
What Is New And Conclusions:
A high percentage (42·6%) of antimicrobial prescriptions among warfarin users were for high-risk antimicrobials that carry excess bleeding risk. Although clinicians were somewhat less likely to prescribe high-risk antimicrobials to warfarin users compared with non-users, the incidence of co-prescription remains high.
Insights
High-risk antimicrobial prescriptions are common in warfarin patients, despite a slightly lower likelihood of co-prescription. This study highlights the need for careful management of drug interactions in patients using warfarin.
Area of Science:
- Pharmacology
- Drug Interactions
- Antimicrobial Therapy
Background:
- Warfarin is an anticoagulant with significant drug-drug interaction potential, particularly with antimicrobials.
- Limited data exists on the frequency of high-risk antimicrobial prescriptions in patients concurrently using warfarin.
Purpose of the Study:
- To determine the frequency of potentially interacting antimicrobial prescriptions in ambulatory patients on warfarin.
- To assess the impact of warfarin use on the prescription patterns of high-risk antimicrobials.
Main Methods:
- Retrospective cohort study using a pharmacy benefits database (2008).
- Included patients with oral antimicrobial claims; warfarin users defined by ≥1 warfarin claim.
- Identified high-risk antimicrobials based on literature (e.g., trimethoprim/sulfamethoxazole, fluoroquinolones, metronidazole, fluconazole).
- Multivariate analysis adjusted for demographics and chronic disease score.
Main Results:
- Analyzed 4,568,150 patients; 110,192 (2.4%) were warfarin users.
- 42.6% of antimicrobial prescriptions in warfarin users were high-risk.
- Warfarin users had slightly more antimicrobial prescriptions (3.0 vs. 2.4).
- Adjusted analysis showed a 42% lower odds of high-risk antimicrobial exposure in warfarin users (OR 0.58).
Conclusions:
- A substantial proportion (42.6%) of antimicrobial prescriptions in warfarin users involved high-risk agents associated with increased bleeding risk.
- While clinicians were less likely to prescribe high-risk antimicrobials to warfarin users, co-prescription rates remain high.
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