Blood Test Ratio Accurately Distinguishes Bacterial From Viral Infections in Febrile Newborns

A fever in a baby under three months old is a medical emergency. The immune system is still forming, and what starts as a simple infection can turn critical within hours. But here is the dilemma that has haunted pediatric emergency rooms for decades: when a tiny infant arrives burning with fever, how do you know if bacteria or a virus is responsible? The answer matters enormously. Bacterial infections need antibiotics immediately. Viral infections typically resolve on their own with time and care. Guess wrong, and you either miss a life-threatening infection or you treat a harmless virus with powerful drugs and invasive procedures the child did not need.

Researchers in Genoa, Italy, presented a potential solution this week at the European Emergency Medicine Congress. Dr. Tommaso Bellini and his team at IRCCS Istituto Giannina Gaslini have developed a blood test that measures two molecules—CD64 and CD169—and combines them into a single ratio. The logic is elegant: CD64, found on neutrophils, rises in response to bacterial infection. CD169, found on monocytes, rises in response to viral infection. When you measure both and compare them, the pattern tells you which type of infection is present.

The study involved 60 babies brought to the pediatric emergency department. Some had fever; others had non-infectious conditions and served as controls. Blood samples were analyzed using flow cytometry, a specialized technique that measures individual cell characteristics. The results were striking. Babies with no infection had an average ratio score of 1.10. Those confirmed to have viral infections averaged 0.33. Those with bacterial infections averaged 16.22. The separation was clean enough that the test achieved 100 percent sensitivity—it caught every single case of bacterial infection—and 96.6 percent specificity, meaning false alarms were rare.

Dr. Bellini explained the mechanism plainly: during a viral infection, one marker is high while the other is low. During a bacterial infection, the pattern reverses. By combining them into a ratio, the test achieved what current standard biomarkers cannot—a way to distinguish serious bacterial infection from self-limiting viral illness without generating the false positives that drive unnecessary treatment. Those false positives carry real consequences. They lead to lumbar punctures, prolonged antibiotic courses, and hospital admissions for infants who have only a virus and need nothing more than monitoring and supportive care.

But the test has a significant limitation. Flow cytometry requires specialized equipment and trained personnel. It is not available at the bedside or in most emergency departments. It lives in specialist laboratories. The next phase of research aims to translate this into something faster and cheaper—a point-of-care test that could be run in the emergency department itself, giving doctors an answer in minutes rather than hours.

The research is continuing. The initial cohort of 60 babies will expand to 75. More importantly, Bellini's team has launched a larger trial called DISCERN—Discriminating Infection via Surface CD-marker Expression Ratios iN infancy—that will assess the test's accuracy in 300 to 500 babies across multiple centers. Dr. Maša Sorić, head of emergency medicine at University Hospital Merkur in Zagreb and not involved in the research, noted the stakes. A fever in a newborn sends parents into the emergency department terrified. Current practice treats every case as potentially serious until proven otherwise, which means invasive tests, antibiotics, and long hospital stays. If these results hold up in a larger trial, the implications extend beyond individual infants. Antibiotic resistance is a growing threat, driven partly by overuse of these drugs. A test that allows doctors to prescribe antibiotics only when truly necessary could help preserve their effectiveness for the infections where they are genuinely lifesaving.