Clinical relevance of urine cultures in febrile infants under 3 months of age with negative urine dipsticks

Introduction The clinical significance of positive urine cultures in febrile infants with negative dipsticks remains debated. The aim of this study was to investigate the extent to which negative urine dipsticks in febrile infants <3 months of age correlate with clinically relevant urinary tract infections (UTI). Methods This prospective observational study included all febrile infants (<3 months of age, temperature ≥38.0 °C) visiting the emergency department between October 2022, and November 2023, who had a negative urine dipstick. Infants with a prior UTI or known susceptibility to UTI were excluded. Positive cultures were considered clinically irrelevant in cases of mixed growth, growth of 1000 cfu/ml, asymptomatic bacteriuria with the same bacteria and resistance pattern within 3 months, or negative inflammatory markers together with spontaneously resolved fever without antibiotic treatment. Results 111 infants were included. The median age was 1 month (IQR 1–2), and 42 (38%) were female. Positive urine cultures were found in 44 infants (40%), of which 24 (22%) showed mixed growth. Among the remaining 20 (18%) isolated positive cultures, 12 (11%) were of insignificant growth (1000 cfu/ml), 17 (15%) had fever that resolved spontaneously without antibiotic treatment, and 8 (7%) were asymptomatic bacteriuria. No infant had a clinically relevant UTI. Conclusion Clinically relevant UTIs are rare in febrile infants <3 months of age with negative urine dipsticks. Urine cultures should not be obtained in all febrile infants with fever, but be reserved for those with an indication of UTI or serious bacterial infection.