Waiting Until Fever Clears Before UTI Ultrasound Cuts False Positives in Children

Febrile urinary tract infections are among the most common serious bacterial infections of early childhood. When an infant or toddler is hospitalized with one, standard practice in many institutions is to perform a renal and bladder ultrasound during the admission - the American Academy of Pediatrics recommends the scan for children ages 2 months to 2 years to check for anatomic abnormalities in the kidneys, bladder, or urinary tract. What has not been clearly established is when during the hospitalization that ultrasound should happen.

A study published February 17 in Hospital Pediatrics examined that timing question in a multi-site retrospective dataset of approximately 300 children. The finding was consistent: ultrasounds performed within the first 24 hours of a child's last fever were significantly more likely to return abnormal results - and many of those abnormalities resolved on their own, suggesting they were caused by the fever rather than a genuine structural problem.

The cascade that false positives trigger

The clinical consequence of a false-positive renal ultrasound is not merely another note in the chart. When an ultrasound shows abnormal findings, the standard next step is a voiding cystourethrogram (VCUG) - a procedure that requires catheterization, uses radiation, and is distressing for young children and their families. A VCUG is entirely appropriate when a child has a true anatomic abnormality. When the abnormality seen on ultrasound was temporary, caused by fever-related inflammation or vascular changes, the VCUG adds burden without benefit.

"Families are already under tremendous stress when their child is hospitalized," said lead researcher Melanie Marsh, MD, assistant professor at Wake Forest University School of Medicine and clinician at Advocate Children's Hospital in Illinois. "Our findings suggest that a short delay in imaging may help avoid unnecessary testing while still ensuring children receive safe, appropriate care."

Study design and findings

The retrospective study drew on medical records from approximately 300 children hospitalized for febrile UTIs between 2018 and 2022 across five sites: Advocate Children's Hospital in Illinois, Atrium Health's Levine Children's and Wake Forest Brenner Children's hospitals in North Carolina, Aurora Children's Health in Wisconsin, and Nationwide Children's Hospital in Ohio. The multi-site design strengthens the generalizability of the findings compared to single-institution studies.

The critical comparison was between ultrasounds performed during active fever or within 24 hours of fever resolution versus those done later in the hospitalization, closer to discharge. Early ultrasounds produced significantly more abnormal results. Crucially, the study also found that waiting to perform the ultrasound until later in the stay did not prolong hospitalization. The delay carried no apparent clinical cost.

"Ultrasounds performed later in the hospital stay had less risk of false positives," Marsh said. "And that means less chance of additional and invasive tests."

Implications for current guidelines

The AAP's current guidance recommends the ultrasound in this age group but does not specify optimal timing within a hospital stay. Marsh noted that updated guidance from the AAP on post-UTI imaging is anticipated. In the interim, the study offers practical, quantitative support for a specific practice change: performing the ultrasound after fever has resolved and closer to discharge rather than on admission day.

"We're due for updated guidance for physicians surrounding imaging following a UTI diagnosis," Marsh said. "In the meantime, we wanted to figure out the best time for a renal and bladder ultrasound."

Limitations to keep in mind

The study is retrospective - meaning imaging timing was determined by clinical practice at five institutions rather than by random assignment, which introduces the possibility of selection bias. Children who received early imaging may have differed systematically from those imaged later in ways that the analysis could not fully account for. The roughly 300-patient sample, while multi-site, is modest in size for drawing definitive conclusions about specific timing cutoffs. Prospective studies with more controlled timing comparisons would provide stronger evidence before this finding is formally incorporated into national guidelines.

The authors acknowledge that their findings point toward a practical change but that additional studies are needed to confirm them in broader populations.