According to research published early online in Pediatrics, children exposed to amoxicillin within 60 days of a urinary tract infection (UTI) are at increased risk for ampicillin-resistant microbes.

Researchers evaluated the impact of previous antimicrobial exposure on the development of antimicrobial resistance in 533 children aged 6 months to 6 years with their first UTI in a retrospective cohort study. The researchers examined the relationship between antimicrobial resistance in UTI isolates and exposure to specific antimicrobial agents (amoxicillin, amoxicillin-clavulanate, cefdinir, trimethoprim-sulfamethoxazole, and azithromycin) in the previous 120 days. The following results were reported:

• 8%, 14%, and 21% were exposed to antimicrobial agents within 30, 60, and 120 days before the UTI, respectively.
• Amoxicillin exposure within 30 days (odds ratio [OR]: 3.6 [95% confidence interval (CI): 1.6–8.2]) and 31 to 60 days (OR: 2.8 [95% CI: 1.0–7.5]) before UTI both were associated with ampicillin resistance.
• Exposure to amoxicillin >60 days before the UTI was not associated with ampicillin resistance. Amoxicillin exposure within 30 days of UTI was also associated with amoxicillin-clavulanate resistance (OR: 3.9 [95% CI: 1.8–8.7]).

The researchers conclude that antimicrobial prescribers should consider this association when selecting empiric antimicrobial agents for a new UTI and should use strategies to reduce unnecessary antimicrobial use to avoid development of resistant bacteria.