Associations Between Serum 25(OH)D Concentrations and Clinical Characteristics in Pediatric Patients

<b>Background/Objectives:</b> Vitamin D has an essential role in immune modulation and inflammatory control, particularly in respiratory infections. Despite widespread supplementation policies, hypovitaminosis D remains common in children and data linking vitamin D status to hospitalization outcomes in pediatric upper respiratory tract infections are limited, especially in Eastern Europe. <b>Methods:</b> We included 400 pediatric patients hospitalized between October 2020 and December 2024 for acute respiratory tract infections (ARTI), and we stratified them into a Normal Vitamin D group (NVD) with sufficient serum 25(OH)D concentrations and a Low Vitamin D group (LVD) with insufficient or deficient levels. Between-group comparisons for continuous variables were performed using non-parametric methods. <b>Results:</b> Children with insufficient or deficient 25(OH)D had a significantly longer duration of hospitalization compared with those with sufficient levels (mean 4.68 ± 2.59 days vs. 2.89 ± 1.81 days). The LVD group showed markedly lower serum vitamin D concentrations (mean 21.63 ± 5.56 ng/mL; median 22.29 ng/mL) compared with the NVD group (mean 47.60 ± 19.59 ng/mL; median 43.70 ng/mL). Markers of disease severity were consistently higher in vitamin D-deficient patients, including higher clinical scores (mean 3.77 ± 2.29 vs. 1.62 ± 1.89), elevated CRP levels (mean 3.50 ± 3.02 mg/L vs. 1.64 ± 1.59 mg/L), and increased O₂ therapy requirement (69.5% vs. 21.0%). Fever was more frequent in the LVD group (61.0% vs. 32.0%). An inverse correlation was observed between serum 25(OH)D concentrations and hospitalization duration, clinical score, and disease severity, with deficiency present across all age strata in the LVD group, while no cases of deficiency were observed in the NVD group. <b>Conclusions:</b> Low serum 25(OH)D concentrations are associated with increased disease severity and prolonged hospitalization.