Background: Biomarkers could potentially help in early diagnosis of various infections, including VAP. SPD is primarily expressed and secreted by type II alveolar cells (Clara cells) but is also detected in the tracheal and bronchial glands of the lower airways. Firstly SPD was identified in respiratory tract, but current studies demonstrate that the expression of SPD is in almost all mucosal surfaces, including epithelial cells in exocrine ducts, the mucosa of gastrointestinal and genitourinary tract and in tear fluid. SPD has a well demonstrated and important role in pulmonary innate immunity, especially in protection against Gram-negative bacteria.