Blood Test Differentiates Bacterial and Viral Infections in Febrile Infants
Posted on 29 Sep 2026
Fever in infants younger than 3 months is treated as a medical emergency because infections can become life-threatening while the immune system is still developing. Emergency department teams must quickly distinguish serious bacterial infections that require antibiotics from viral illnesses that usually need monitoring and supportive care. To support this early differentiation, a new blood test uses paired immune-cell markers to help distinguish bacterial from viral infection in febrile newborns.
The investigational test compares the blood levels of CD64 and CD169, two molecules that rise in response to different infection types. CD64 is found on neutrophils, a type of white blood cell, and is thought to increase in bacterial infection. CD169 is found on monocytes, another type of white blood cell, and is thought to increase in viral infection.
Researchers tested blood samples from 60 babies brought to the pediatric emergency department at IRCCS Istituto Giannina Gaslini in Genoa, Italy. Some infants were being assessed and treated for fever, while others were being evaluated for noninfectious conditions. The samples were analyzed using flow cytometry, which enables assessment of individual cell characteristics.
The researchers combined CD64 and CD169 measurements into a ratio for each baby. The average ratio was 1.10 in babies with no infection, 0.33 in babies ultimately confirmed to have a viral infection, and 16.22 in babies confirmed to have a bacterial infection. The test showed 100% sensitivity for bacterial infection and 96.6% specificity, indicating that false positives were very low in this initial cohort.
The research is continuing and is expected to include data on 75 babies. A larger multicenter prospective study, called DISCERN, or Discriminating Infection via Surface CD-marker Expression Ratios iN infancy, is also beginning. That study will assess test accuracy in 300 to 500 babies.
“In very young babies with fever, distinguishing serious bacterial infection from self-limiting viral illness is a major diagnostic dilemma. Our standard biomarkers can help show when there might be a bacterial infection, but they also create 'false positives,' which drives overtreatment. This can mean unnecessary lumbar punctures, antibiotics and hospital admissions for infants who turn out to have only a virus,” said Dr. Tommaso Bellini, pediatric emergency physician at the IRCCS Istituto Giannina Gaslini.
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IRCCS Istituto Giannina Gaslini
