BACKGROUND: Lower respiratory tract infection (LRTI) is a leading cause of morbidity and mortality among children globally. Copeptin, released by the pituitary gland, serves as a biomarker for various conditions and, as a neuroendocrine stress hormone, is useful in acute conditions. This study aimed to determine the role of copeptin levels in LRTI in children and whether it can reliably predict pneumonia severity. MATERIALS AND METHODS: This prospective case-control study was performed between April and October 2023. The study included four groups: (i) patients diagnosed with bronchiolitis (Group 1, n=25), (ii) patients diagnosed with mild to moderate pneumonia (Group 2, n=25), (iii) patients diagnosed with severe pneumonia (Group 3, n=25), and (iv) a control group (Group 4, n=26). RESULTS: Copeptin values differed significantly between the groups (p<
0.001 for all comparisons). Copeptin demonstrated a sensitivity of 87.4 % and specificity of 82.2 % for distinguishing between patients with bronchiolitis and pneumonia, using a cut-off value of >
0.586 ng/ml. For the identification of patients with severe pneumonia versus those with mild to moderate pneumonia, copeptin exhibited a sensitivity of 97.9 % and specificity of 94.7 % with a cut-off value of >
1.215 ng/ml. The copeptin level exhibited a positive correlation with fibrinogen and FAR levels while demonstrating a negative correlation with albumin levels (r=0.354, CONCLUSIONS: Copeptin demonstrates potential as a predictor of disease severity in children with pneumonia. It can also serve as a valuable tool to guide physicians in differentiating between bronchiolitis and pneumonia, as well as in diagnosing severe pneumonia.