Because clinical suspicion of HAP/VAP is overly sensitive, further diagnostic strategies are required for optimal management. The presence of a new or progressive radiographic infiltrate plus at least two of three clinical features (fever > 38℃, leukocytosis or leukopenia, and purulent secretions) are the most accurate criteria for starting empiric antibiotic therapy (41). The invasive diagnostic approach allows the etiologic cause of pneumonia to be defined by semi-quantitative cultures of lower respiratory tract specimens (PSB or BAL) with initial microscopic examination. Careful examination of a Gram-stain for polymorphonuclear leukocytes, macrophages, squamous epithelial cells, and the morphology and staining of the bacteria may improve the diagnostic accuracy when correlating with culture results (42,43). Conversely, a negative tracheal aspirate in a patient without a recent (within 72 hours) change in antibiotics has a strong negative predictive value (94%) for VAP (44). This algorithm provides an approach to the evaluation of VAP using either the clinical or invasive approach(45). The decision to discontinue antibiotics, using this algorithm, may differ depending on the type of respiratory tract sample that is collected and whether the culture results are reported in quantitative or semi-quantitative terms.