Background: Acute bronchiolitis is one of the most common causes of hospitalization in young children. No data exist on monocyte-to-lymphocyte-ratio (MLR) and neutrophil-to-lymphocyte-ratio (NLR) as prognostic biomarkers for disease severity.

Aim: To characterize the NLR and MLR values in children hospitalized with RSV bronchiolitis and their prognostic value for severe clinical outcomes.

Methods: A retrospective cohort study that included children (age< 2 years) hospitalized due to RSV bronchiolitis, between 2018 and March 2022, with a complete blood count upon arrival. We divided the cohort into quartiles based on MLR and NLR. We examined associations between severity outcomes and quartiles.

Results: 2,038 children were included (median age of 4.4 months (IQR 1.9-9.8)). The median MLR was 0.14, 0.22, 0.30, and 0.47, and NLR was 0.37, 0.70, 1.16, and 2.29 in quartiles 1-4. Children with higher MLR  had a longer length of stay (LOS) in the hospital  (2.59, 2.62, 2.93, and 3.07 days, P < 0.001) in Q1-4, higher rate of hospitalization to the pediatric intensive care unit (PICU) (2.4%, 3.9%, 4.7%, 9.4%, P < 0.001), and lower minimal oxygen saturation (Q1 90%, Q4 87%, P < 0.001). Higher NLR was associated with lower minimal saturation and extended LOS. PICU admission was differentiated optimally by 0.34 for MLR and 0.67 for NLR. In a model accounting for age and sex, MLR quartiles predicted PICU admission (AUC=0.73; 95% CI, 0.681- 0.789)

Conclusion:  MLR and NLR are associated with severe clinical outcomes in hospitalized children with RSV bronchiolitis. MLR excelled in predicting PICU admission.