Acute respiratory infections (ARIs) are among the leading causes of hospitalization, morbidity, and mortality in children worldwide, particularly among critically ill patients admitted to pediatric intensive care units (PICUs). Vitamin D has an important role not only in bone metabolism but also in regulation of innate and adaptive immune responses, especially within the respiratory tract. Recent evidence suggests that vitamin D deficiency may be associated with increased susceptibility to respiratory infections, greater disease severity, prolonged hospitalization, and higher mortality rates in critically ill children.

This observational cross-sectional study aims to assess serum vitamin D status among critically ill children admitted to the PICU with acute respiratory infections and to evaluate the relationship between vitamin D levels, severity of respiratory illness, and risk of mortality. The study will include pediatric patients aged 1 month to 16 years admitted to the Pediatric Intensive Care Unit of Assiut University Children's Hospital.

Clinical evaluation and laboratory investigations, including serum 25-hydroxyvitamin D levels, serum calcium, and alkaline phosphatase, will be performed within the first 24 hours of admission. Severity of respiratory distress will be assessed using the Pediatric Respiratory Severity Score (PRESS), while mortality risk will be evaluated using the Pediatric Risk of Mortality Score (PRISM III).

The findings of this study may contribute to better understanding of the role of vitamin D in critically ill children with respiratory infections and may support future strategies for early risk assessment and improved clinical management in pediatric intensive care settings.

Acute respiratory infections remain a major global health problem and are a leading cause of morbidity and mortality among pediatric populations, particularly in low- and middle-income countries. Critically ill children admitted to pediatric intensive care units (PICUs) with severe respiratory infections are at increased risk of complications, prolonged hospitalization, and mortality. Identifying modifiable risk factors associated with disease severity and poor outcomes is therefore of significant clinical importance.

Vitamin D is a fat-soluble steroid hormone traditionally recognized for its role in calcium and phosphorus homeostasis and skeletal development. However, increasing evidence has demonstrated that vitamin D also has immunomodulatory functions through its effects on innate and adaptive immunity. Vitamin D receptors are expressed in immune cells, including macrophages and respiratory epithelial cells, supporting its role in host defense against respiratory pathogens.

Recent studies have suggested an association between vitamin D deficiency and increased susceptibility to acute respiratory infections, sepsis, and adverse outcomes in critically ill children. Nevertheless, available evidence remains inconsistent, and data from pediatric intensive care populations in Egypt and similar settings are limited.

This cross-sectional observational study will be conducted at the Pediatric Intensive Care Unit of Assiut University Children's Hospital. The study will include children aged 1 month to 16 years admitted with acute respiratory infections. Patients with chronic illnesses affecting vitamin D metabolism, severe immunodeficiency, congenital heart disease, malnutrition, rickets, or recent vitamin D supplementation will be excluded.

At PICU admission, all eligible participants will undergo detailed history taking, complete clinical examination, and laboratory investigations. Serum 25-hydroxyvitamin D [25(OH)D] levels will be measured within the first 24 hours of admission as the standard indicator of vitamin D status. Vitamin D levels will be categorized according to international clinical practice guidelines into deficiency, insufficiency, sufficiency, and toxicity.

Severity of respiratory illness will be evaluated using the Pediatric Respiratory Severity Score (PRESS), a validated clinical scoring system for respiratory distress in children. Mortality risk will be assessed using the Pediatric Risk of Mortality Score (PRISM III), which incorporates physiological and laboratory parameters obtained during the first 24 hours of PICU admission.

The primary outcome of the study is the prevalence of vitamin D deficiency among critically ill children with acute respiratory infections. Secondary outcomes include the association between serum vitamin D levels and both respiratory disease severity and mortality risk.

Data will be statistically analyzed using SPSS software. Appropriate statistical tests will be selected according to the type and distribution of data, and statistical significance will be considered at p < 0.05.

This study may provide important insights into the clinical significance of vitamin D deficiency in critically ill pediatric patients and may support future preventive and therapeutic strategies aimed at improving outcomes in children with severe respiratory infections.

Inclusion Criteria:

• Children aged between 1 month and 12 years.
• Admitted to the PICU with a primary diagnosis of acute respiratory infection.
• Diagnosis confirmed by clinical symptoms and radiological evidence.

Exclusion Criteria:

• Children with known chronic bone diseases or metabolic disorders.
• Patients receiving vitamin D supplementation prior to admission.
• Patients with chronic renal or hepatic failure.

The study involves pediatric patients admitted to the Pediatric Intensive Care Unit (PICU) at a tertiary care hospital who meet the diagnostic criteria for acute respiratory infections.