Septic Arthritis in an Afebrile Young Infant With a Urinary Tract Infection

Although guidelines exist for the evaluation of serious bacterial infections in febrile young infants, these infections are less likely to be considered in those who are afebrile. We discuss the case of an afebrile, well-appearing, previously healthy 5-week-old girl who presented with left knee swelling and was found to have an Escherichia coli urinary tract infection complicated by E coli septic arthritis. This case underscores the importance of considering serious bacterial infections in afebrile, well-appearing infants less than 2 months old and maintaining a low threshold for diagnostic testing. Furthermore, this case highlights a unique pathophysiology of septic arthritis from a urinary source, emphasizing the need for providers to consider urine testing in cases of young infants with septic arthritis.