Emergence of Fluoroquinolone Resistance in Outpatient Urinary Escherichia coli Isolates

Background Because of high rates of trimethoprim-sulfamethoxazole resistance in Escherichia coli, Denver Health switched to levofloxacin as the initial therapy for urinary tract infections (UTIs) in 1999. We evaluated the effects of that switch 6 years later. Methods Levofloxacin prescriptions per 1000 outpatient visits and levofloxacin resistance in outpatient E. coli were evaluated over time. E. coli isolated in 2005 were further characterized by specimen source and antimicrobial susceptibilities. Risk factors for levofloxacin-resistant E. coli UTI among nonpregnant adult outpatients were evaluated in a case-control study. Results Between 1998 and 2005, levofloxacin use increased from 3.1 to 12.7 prescriptions per 1000 visits (P < .01) and resistance in outpatients increased from 1% to 9% (P < .01). Although prescriptions for sulfonamide antibiotics decreased by half during the same period, E. coli resistance to trimethoprim-sulfamethoxazole increased from 26.1% to 29.6%. Levofloxacin-resistant E. coli were more likely resistant to other antibiotics than levofloxacin-susceptible isolates (90% vs 43%, P < .0001). Risk factors for levofloxacin-resistant E. coli UTI were hospitalization (odds ratio for each week of hospitalization, 2.0; 95% confidence interval, 1.0-3.9) and use of levofloxacin (odds ratio, 5.6; 95% confidence interval, 2.1-27.5) within the previous year. Conclusion Fluoroquinolone prescriptions increased markedly after an institutional policy change for empiric treatment of UTI, and a rapid increase in fluoroquinolone resistance among outpatient E. coli followed. Risk factors for infection with resistant E. coli were recent hospitalization and levofloxacin use. Risk factors should be considered before initiating empiric treatment with a fluoroquinolone.