Complete Blood Count-Derived Inflammatory Markers in Pediatric Lower Respiratory Tract Infection: A Narrative Review

Abstract

Lower respiratory tract infections (LRTIs) are a leading infectious cause of death in children worldwide. Complete blood count (CBC)-derived inflammatory ratios are low-cost and widely available, yet their reported performance varies. We reviewed 46 PubMed-indexed studies (2016–2026) in children aged 0–18 years with viral, bacterial, or mycobacterial LRTI. The findings were mapped across diagnosis, etiological differentiation, severity, and outcome prediction. The neutrophil-to-lymphocyte ratio (NLR) was the most investigated marker, with the broadest evidence base for severity assessment and for separating bacterial from viral disease, although its performance remained pathogen-, population-, and endpoint-dependent; it was elevated in severe pertussis, tuberculosis, Mycoplasma pneumoniae, Streptococcus pneumoniae, and SARS-CoV-2 infection, whereas findings in respiratory syncytial virus infection were conflicting. The platelet-to-lymphocyte ratio had the next broadest evidence base for severity, and the systemic immune-inflammation index was consistently elevated in Mycoplasma pneumoniae and SARS-CoV-2 infection. No single ratio performed consistently across all etiologies. Ratios governed by one dominant immunopathogenic mechanism behaved predictably; those whose numerator and denominator respond to the same process did not. Cut-offs were neither standardized, age-specific, nor externally validated, and most estimates derived from retrospective cohorts in few countries; reported values therefore represent association and discrimination rather than validated prediction. Standardized, age-specific thresholds are needed before these markers can support routine risk stratification.