Nosocomial bacterial pneumonia has been difficult to diagnose . Frequently, the criteria for diagnosis are fever, cough, and development of purulent sputum, in conjunction with radiologic evidence of a new or progressive pulmonary infiltrate, a suggestive Gram stain, and positive cultures of sputum, tracheal aspirate, pleural fluid, or blood. Although clinical findings in conjunction with cultures of sputum or tracheal specimens may be sensitive for bacterial pathogens, they are highly nonspecific, especially in patients receiving mechanically assisted ventilation ; conversely, cultures of blood or pleural fluid have very low sensitivity .