Assessment of Serum Vitamin D Status and Its Association With Disease Severity and Mortality Risk Among Critically Ill Children With Acute Respiratory Infections: A Cross-Sectional Study

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.