Evaluation of the relationship between acute bronchiolitis severity score and the neutrophil-to-lymphocyte ratio

Objective: Acute bronchiolitis is one of the most common lower respiratory tract infections in children under 2 years and a leading cause of hospitalization. Although clinical scores (e.g., the Modified Tal Score) are widely used to grade severity, they are time-consuming and may vary across the disease course. The aim of this study was to evaluate the association between bronchiolitis severity and complete blood count (CBC) parameters obtained at admission, including the neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), mean platelet volume (MPV), and platelet count (PLT). Furthermore, the diagnostic and prognostic value of these parameters was analyzed in comparison with clinical severity scores. In this way, it was intended to investigate whether easily accessible and inexpensive hematological markers could contribute to clinical decision-making in the management of bronchiolitis. Materials and Methods: This single-center retrospective study reviewed records from Kırşehir Training and Research Hospital. Children aged 1–30 months who presented between 10/01/2021 and 06/30/2024 and were diagnosed with acute bronchiolitis were categorized as mild, moderate, or severe by the Modified Tal Score (mild n=68, moderate n=103, severe n=41). Mild cases (SpO₂ ≥94%, adequate feeding, no marked retractions) were generally discharged after outpatient follow-up; moderate cases (SpO₂ 90–94% with tachypnea/retractions and reduced oral intake) required 1–12 days of ward admission; severe cases (SpO₂ <90% with marked dyspnea, use of accessory muscles, agitation/lethargy) required intensive care, and one case was fatal. Healthy controls (n=49) were age-matched children without chronic disease or recent infection who presented for routine visits. Clinical/laboratory data were retrieved from hospital systems and patient files; missing items were verified with families when necessary. Results: A total of 261 children were included (212 bronchiolitis, 49 controls; 59.8% male). Severe cases were younger on average (p=0.001). With increasing severity, neutrophil count/percentage, WBC, CRP, NLR and PLR increased, while lymphocyte count/percentage decreased (all p<0.001). PLT (p=0.055) and MPV (p=0.842) did not differ between severity groups. On ROC analysis, NLR >0.615 discriminated mild bronchiolitis from controls (AUC 0.839; sensitivity 70.6%; specificity 81.6%), and NLR >0.765 discriminated severe cases from controls (AUC 0.923; sensitivity 80.5%; specificity 95.9%). In a high-sensitivity strategy (targets ≥90% and ≥80%), thresholds achieving these sensitivities were selected to maximize specificity; in comparisons involving severe cases, the Youden-J–optimized threshold increased specificity, whereas maintaining ≥90% sensitivity required lower NLR cut-offs at the expense of specificity. In this dataset, NLR outperformed PLR for distinguishing bronchiolitis from controls; however, for severity stratification among bronchiolitis groups. However, in distinguishing the mild–severe strata of bronchiolitis severity, PLR outperformed NLR. For hospital length of stay (LOS), thresholds of >1.5 days for the mild→moderate transition and >5.5 days for the moderate→severe transition were identified (AUC 0.992 and 0.859, respectively). In ordinal logistic regression, longer LOS (OR 39.4; p<0.001) and higher CRP (OR 1.06; p=0.005) independently predicted greater severity, while age was protective (OR 0.92; p=0.031). Although NLR and PLR showed strong univariable discrimination, both lost independent significance in the multivariable model (p>0.05). Conclusion: NLR and PLR are significantly higher in infants with bronchiolitis than in healthy controls and correlate positively with severity. However, NLR alone is limited for differentiating severity strata (mild–moderate–severe). From a clinical perspective, when CRP and clinical severity assessment are considered together, NLR appears more appropriate as an adjunct diagnostic marker, and PLR as an adjunct for severity stratification. In multivariable analysis, younger age and higher CRP independently predicted worsening clinical course, NLR and PLR lost their independent significance. Keywords: Bronchiolitis; MPV; NLR; PLR; PLT.