Diagnosis of nosocomial pneumonia.

Nosocomial pneumonia occurs in 0.6% of hospitalized patients. The usual causative agents are gram-negative bacilli, Staphylococcus aureus, Streptococcus pneumoniae, and anaerobic bacteria. In immunocompromised hosts, the differential diagnosis also includes fungi, mycobacteria, viruses, Nocardia, and Pneumocystis carinii. Important risk factors for the development of nosocomial pneumonia include prolonged mechanical ventilation, thoracic or upper abdominal surgery, altered mental status, underlying immunosuppression, chronic obstructive pulmonary disease, and the use of antacids or histamine type 2 blockers. Colonization of the oropharynx and tracheal secretions with gram-negative aerobic bacteria is common in hospitalized patients with or without pneumonia. The diagnosis of nosocomial pneumonia is usually based on the clinical features of dyspnea, cough, fever, purulent sputum production, new pulmonary infiltrates, hypoxemia, and leukocytosis. However, the clinician must recognize that the presence of these features is neither sensitive nor specific in the diagnosis of nosocomial pneumonia. Microbiologic diagnosis is also difficult because blood cultures are usually negative, and cultures of tracheal secretions, although usually sensitive, are not specific. Invasive procedures may prove useful, but most have yet to be studied in large groups of patients with nosocomial pneumonia.