A 64-day-old male is brought to the ED for fever to 100.9°F at home. He’s well-appearing, feeding normally, and has mild nasal congestion and cough. No vomiting, rash, or decreased urine output. Vitals are stable. A respiratory viral panel returns positive for RSV. Your intern asks, “Since he’s over 60 days and RSV-positive, do we still need to check urine or labs?”
A positive viral test in a febrile infant feels reassuring—but should it change your workup?
As respiratory viral panels become more accessible and widely used in emergency departments, clinicians are increasingly asking whether a well-appearing infant with a documented viral infection still needs labs, cultures, or empiric treatment. If the child is less than 2 months, most clinicians are following the AAP febrile infant guidelines and doing blood and urine testing, maybe even thinking about doing an LP. How much does being a week older really change things?
In this post, we’ll dive into the evidence behind serious bacterial infection (SBI = meningitis, bacteremia, or UTI) risk in febrile infants who test positive for viruses—here’s a quick look at 2 relevant studies, and how this should (or shouldn’t) change your practice.
Spoiler: A virus may lower the risk, but it doesn’t rule it out.
The Evidence:
#1: The Risk of Serious Bacterial Infection in Febrile Infants 0-90 Days of Life With a Respiratory Viral Infection. Pediatr Infect Dis J. 2019 Apr;38(4):355-361. doi: 10.1097/INF.0000000000002165. PMID: 30882724.
Serious bacterial infection (SBI = meningitis, bacteremia, or UTI) occurred in 13% of viral-positive vs 24% of viral-negative infants ≤90 days. But…it was not statistically significant, so the authors asked whether certain viruses made this risk more or less.
They found that infants with mucosally restricted viruses (RSV, flu, rhinovirus, coronavirus) were at lower risk for SBI than those with systemic viruses (enterovirus, adenovirus) or no virus.
#2: Risk of Bacterial Coinfections in Febrile Infants 60 Days Old and Younger with Documented Viral Infections. J Pediatr. 2018 Dec;203:86-91.e2. doi: 10.1016/j.jpeds.2018.07.073. Epub 2018 Sep 6. PMID: 30195552; PMCID: PMC7094460.
In about 3000 febrile infants ≤60 days old, infants testing positive for a virus were significantly less likely to have an SBI as compared to those testing negative (4% for viral positive vs 13% for viral negative).
When stratified by age group, those ≤28d without a virus had an age-specific risk ratio of 4.0 – BUT, 4.2% of those infants with a virus still had an SBI.
Conclusions:
- For 2-3m olds with typical viral symptoms, especially for those who look well and test positive for RSV or flu, I typically do not do blood or urine testing as their risk of SBI is lower
- There will be some 2-3m olds with viruses that will also have UTI or SBI – so if the child looks sicker than your typical viral patient, consider doing urine or blood testing
- Study #2 reinforces why we do what we do for <2m old febrile infants – even with a virus, their overall risk of SBI is high enough that it warrants testing
- To consider: For those children less than but CLOSE to 2m old with clear viral symptoms, is there an opportunity for pediatricians to manage them on the outpatient side and avoid sending them to the ER?
Dr. Joshua Belfer, MD, is a Pediatric Emergency Medicine physician at the Children’s Hospital of Philadelphia, and is the Founder and Editor-in-Chief of HipPEMcrates. He can be reached at HipPEMcrates@gmail.com.
