Association of Serum Eosinophilic Cationic Protein With Disease Severity in Infant Bronchiolitis

OBJECTIVES: This study aimed to evaluate serum eosinophilic cationic protein (ECP) levels in young children with acute bronchiolitis compared with healthy controls and to investigate their association with clinical severity and tobacco smoke exposure. We hypothesized that ECP would not function as a diagnostic biomarker but might reflect inflammatory phenotypes associated with severe disease. METHODS: In this prospective cross-sectional study, 96 children aged 1 to 24 months with acute bronchiolitis and 96 age-matched and sex-matched healthy controls were enrolled. Serum ECP levels and routine laboratory parameters were measured at presentation. Within the bronchiolitis cohort, associations between serum ECP levels and clinical severity indicators-including oxygen requirement, ICU admission, mechanical ventilation, and prenatal or passive tobacco smoke exposure-were evaluated. Correlations between ECP and systemic inflammatory markers were analyzed using Spearman correlation analysis. Multivariable binary logistic regression was performed to identify independent predictors of severe bronchiolitis. RESULTS: Serum ECP levels did not differ significantly between young children with bronchiolitis and healthy controls, indicating limited diagnostic utility. However, within the bronchiolitis group, significantly higher ECP levels were observed in young children requiring ICU admission or mechanical ventilation and in those with prenatal or passive tobacco smoke exposure. Serum ECP levels demonstrated significant positive correlations with total leukocyte count and leukocyte subpopulations, including neutrophils, monocytes, and eosinophils, as well as with serum glucose levels. Age-stratified analyses revealed distinct leukocyte profiles across infancy, whereas serum ECP concentrations remained comparable between age groups. In multivariable analysis adjusting for age and tobacco smoke exposure, serum ECP was not independently associated with severe disease, whereas tobacco smoke exposure emerged as a strong independent predictor of severe bronchiolitis. CONCLUSIONS: Although serum ECP does not seem to have independent prognostic utility, its elevation in severe and smoke-exposed cases suggests that it may reflect distinct inflammatory phenotypes in bronchiolitis rather than function as a direct clinical risk stratification tool.