Antimicrobial-resistant Urinary Tract Infections in Febrile Infants: Prevalence, Associated Factors and Clinical Outcomes

Background: Antimicrobial resistance among pediatric urinary tract infections (UTIs) has become an increasing concern. However, data regarding resistant infections in febrile infants remain limited. Methods: We performed a retrospective single-center study including febrile infants younger than 24 months diagnosed with UTI in a pediatric emergency department in Spain between March 2022 and August 2025. Clinical, microbiologic and outcome-related variables were compared according to antimicrobial susceptibility. Bivariate and multivariate logistic regression analyses were performed to identify predictors of resistant infection. Results: Among 336 UTI episodes, resistant microorganisms were identified in 36 (10.7%). Escherichia coli was the predominant pathogen (303/336, 90.2%), accounting for 270/300 susceptible infections and 33/36 resistant infections. Multidrug-resistant and extended-spectrum β-lactamase-producing organisms accounted for most resistant isolates (28/36, 77.8%; and 14/36, 38.9%, respectively), and 8/36 (22.2%) fulfilled criteria for both phenotypes. No carbapenem-resistant organisms were identified. Baseline characteristics did not differ significantly between groups. No independent predictors of resistant infection were identified in the multivariate analysis. Resistant infections were associated with a higher proportion of adverse outcomes (33.3% vs. 15.3%; P = 0.01), mainly recurrences, although this finding was attenuated in the sensitivity analysis restricted to first episodes (27.3% vs. 14.9%; P = 0.08). Conclusions: Approximately 1 in 10 UTIs in febrile infants were caused by resistant microorganisms. No independent predictors were identified. Resistant infections were associated with a higher proportion of adverse outcomes, mainly recurrences, although this finding should be interpreted cautiously. Further prospective studies incorporating molecular typing techniques are needed to better characterize antimicrobial resistance dynamics in recurrent pediatric UTIs.