Background: Neonatal sepsis remains a major cause of morbidity and mortality, while its early diagnosis is challenging due to nonspecific clinical manifestations and the limited sensitivity of conventional diagnostic markers. Easily accessible biomarkers that can facilitate early diagnosis and predict disease severity and outcome are therefore urgently needed.

Aim: To evaluate the diagnostic and prognostic value of eosinopenia, neutrophil-to-lymphocyte ratio (NLR), serum lactate levels, and lactate clearance in neonatal sepsis and mortality.

Patients and Methods: This prospective cohort study included 200 neonates admitted to the Neonatal Intensive Care Unit of Minia University Children’s Hospital from April 2024 to December 2024. The study included 100 neonates diagnosed with early- or late-onset sepsis and 100 apparently healthy, age- and sex-matched controls. Complete blood count, eosinophil percentage, NLR, serum lactate, and arterial blood gas parameters were assessed. Serum lactate was measured initially and repeated after 6 hours, and lactate clearance was calculated. Receiver operating characteristic (ROC) curve analysis was performed to assess the diagnostic and prognostic performance of the studied biomarkers.

Results: Septic neonates demonstrated significantly lower eosinophil percentages and higher NLR and serum lactate levels compared with controls. Positive lactate clearance was observed in 90% of controls compared with only 57% of septic neonates. Eosinophil percentage showed the highest diagnostic performance, with an optimal cutoff of ≤0.09%, yielding an area under the curve (AUC) of 0.998, sensitivity of 100%, specificity of 99%, and overall accuracy of 99.5%. Initial and 6-hour lactate levels also demonstrated excellent diagnostic performance, with AUCs of 0.978 and 0.974, respectively. NLR showed moderate diagnostic performance, with an AUC of 0.642. Among septic neonates, 51% improved while 49% died. Both initial and 6-hour lactate levels were significantly higher among non-survivors, while positive lactate clearance was more frequent among survivors. The 6-hour lactate level demonstrated slightly superior prognostic discrimination for mortality compared with initial lactate, with an AUC of 0.806 versus 0.796, respectively. In contrast, neither eosinopenia nor NLR demonstrated significant prognostic value for mortality.

Conclusion: Eosinopenia is a highly accurate, readily available, and inexpensive diagnostic biomarker for neonatal sepsis. NLR provides moderate diagnostic utility but limited prognostic value. Elevated serum lactate and impaired lactate clearance reflect disease severity and are associated with increased mortality risk. Serial lactate measurements, particularly 6-hour lactate assessment, may provide valuable dynamic prognostic information and should be considered alongside conventional clinical and laboratory assessment in neonates with suspected sepsis.

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