Diagnostic multiparametric models and antibiotic practices in febrile infants younger than 90 days

Most studies in febrile infants evaluated individual laboratory data to predict diagnoses of febrile infants, not taking into consideration the combination of data. In this study we aimed to analyze the diagnostic accuracy - both individually and in combination - of C-reactive protein (CRP), procalcitonin (PCT), white blood cell count (WBC), and neutrophil count (ANC), along with clinical findings, in predicting whether a young febrile infant has a viral infection, urinary tract infection (UTI) or serious bacterial infection (SBI). This is a retrospective, observational, single center, cohort study during a period of 7 years (2018-2024). A total of 615 patients were included in the study, of which 352 male (57.3%) with a median age of 51 days (IQR). For the detection of SBI, AUC values were greater for PCT (OR 1.130; 95% CI: 1.062-1.203, p < 0.001; AUC 0.75) than for CRP (OR 1.017; 95% CI: 1.008-1.026, p < 0.001; AUC 0.64). The comparison of ROC curves for CRP and PCT suggested no statistically significant difference between the two markers in detecting viral infections, UTI without bacteremia, UTI with bacteremia and SBI. The AUC values of the markers were always higher if used in combination with urinalysis and clinical findings. In addition, antibiotic sparing procedures were implemented in a third of patients with viral infections and oral shift frequently implemented, without negative outcome.

Our findings suggest that single biomarkers are not useful in discriminating viral and bacterial infections, but a combination of CRP (or PCT), urinalysis, and clinical findings, may be more accurate.

• Febrile infants under 90 days of age are at high risk for serious bacterial infections, often leading to invasive testing and antibiotic treatment. However, single biomarkers like CRP or PCT have limited diagnostic accuracy.

• Combining biomarkers with clinical findings and urinalysis improves diagnostic precision, potentially reducing unnecessary treatments and supporting safer, individualized management strategies in this population.