CD64 and CD169 as Translational Biomarkers for Discriminating Bacterial from Viral Infections in Febrile Infants ≤90 Days of Age: From Immune Pathophysiology to Bedside Application

Management of febrile infants ≤90 days remains one of the most critical scenarios in pediatric emergency medicine. Current clinical algorithms prioritize bacterial risk stratification (serious/invasive bacterial infections, SBI/IBI), while viral epidemiology is inconsistently integrated, despite viral pathogens accounting for the majority of febrile episodes. This doctoral research is grounded on the hypothesis that integrating three dimensions — host immune response, timing from fever onset, and viral epidemiology — can improve diagnostic accuracy over the still-dominant static, bacterium-centered paradigm. The research program was structured into four components: evaluation of the timing-dependent performance of conventional inflammatory biomarkers; retrospective epidemiological and virological analyses; prospective validation of an interferon signature as a host-response marker; and development/validation of CD64 and CD169 as translational biomarkers. The core study — a single-center, prospective investigation conducted at the Pediatric Emergency Department of IRCCS Istituto Giannina Gaslini — enrolled 61 infants (29 non-BI, 12 BI, 20 healthy controls), analyzing neutrophil CD64 and monocyte CD169 expression by flow cytometry. The CD64/CD169 ratio showed the best diagnostic performance in discriminating bacterial from non-bacterial infections, with an AUC of 0.989 (sensitivity 100%, specificity 96.6%, cut-off ≥1.27), outperforming individual biomarkers and conventional markers (PCT AUC ~0.94, CRP AUC ~0.90). The non-BI group displayed a CD169-predominant profile and the BI group a CD64-predominant profile, consistent with the opposite biological direction of host response to viral versus bacterial stimuli. Overall, findings indicate that the current bacteria-centered approach is safe but lacks specificity; viral infections should occupy a central role in the diagnostic paradigm, being etiologically predominant and capable of modulating — without abolishing — bacterial risk; and host-response biomarkers, particularly the CD64/CD169 combination, represent a promising, biologically grounded path toward a dynamic diagnostic framework, potentially reducing invasive procedures, inappropriate antibiotic use, and unnecessary hospitalizations, pending confirmation in larger cohorts.