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There are no specific diagnostic tests for aspiration pneumonia. The diagnosis is usually based on new findings of hypoxemia, pulmonary infiltrates in gravity-dependent lung regions, fever, and leukocytosis after an observed or suspected episode of vomiting or regurgitation in a patient at risk for aspiration. Most affected patients are febrile and tachypneic. Rales are present in about two thirds of patients, and cough, wheezing, or cyanosis is seen in one third.
Most patients with aspiration pneumonia are treated in the absence of a specific microbiologic diagnosis. The main barriers to establishing such a diagnosis are the difficulty in obtaining specimens of deep respiratory tract secretions without contamination by oral flora and the limited laboratory capacity for isolation of anaerobic organisms. Expectorated sputum is not a valid specimen for anaerobic culture because it is invariably contaminated with oral flora. However, sputum should be examined by Gram stain and culture for aerobic pathogens.
A sputum Gram stain showing numerous neutrophils and mixed flora (gram-positive bacilli and cocci and gram-negative rods) (figure 3) is suggestive of polymicrobial infection typically seen in aspiration pneumonia and is an important diagnostic clue. The main value of sputum culture in nonintubated patients with aspiration pneumonia is to identify serious pathogens (eg, gram-negative enteric bacilli) and provide information on their antimicrobial susceptibility, which can be used to adjust treatment. Blood, pleural fluid, transthoracic needle aspirates, and bronchoscopic-protected specimen brush samples, when available, are suitable specimens for anaerobic as well as aerobic culture.
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