The diagnosis of pneumonia in mechanically ventilated patients is difficult, and still there is no "gold-standard" diagnostic method. It is usually based on the combination of clinical, radiological, and microbiological criteria defined by Centers for Disease and Control (CDC) ( Table  3) . But these criteria have low sensitivity and specificity. The systemic signs (fever, leukocytosis, etc.) of infection can be seen by any condition in ICU (pulmonary edema, pulmonary infarction, after surgery, trauma, devascularized tissue, open wounds, etc.). Investigators reported that the clinical diagnosis of VAP is associated 30-35% falsenegative and 20-25% false-positive results [57] [58] [59] . And also, ICU patients do not always have systemic signs of infection due to their underlying disease (chronic renal failure, immunosuppresion, etc.). Radiological infiltration has limited value, mimicking by cardiogenic pulmonary edema, noncardiogenic pulmonary edema, adult respiratory distress syndrome (ARDS), atelectasis, pulmonary contusion, which are not uncommon in ICU [29] . In an autopsy-proven VAP study, Wunderink et al. [1] reported that no radiographic sign had a diagnostic accuracy greater than 68%. The presence of air bronchograms was the only sign that correlated well with pneumonia, correctly predicting 64% of pneumonias in the entire group. The upper respiratory tract of patients is colonized with potential pulmonary pathogens a few hours after intubation [48, 61] . Consequently, isolation of pathogens from tracheal secretions do not always indicate pulmonary infection. But a positive Gram's stain may guide the initial antibiotic therapy. However prior antibiotic and corticosteroid therapy can reduce the sensitivity of this technique [62, 63] .