Sputum and tracheal aspirates are of only limited diagnostic value because of the frequency of bacterial colonization. Patients with suspected ventilator-associated pneumonia commonly have Gram-negative organisms in their sputum, but not in samples from which pneumonia can be reliably diagnosed (postmortem or pleural aspirates). However, the isolation of Staphylococcus aureus or anaerobic Gram-negative organisms in sputum is mirrored in 80 per cent of cases by similar isolation in diagnostic cultures. The absence of Gram-negative organisms from purulent sputum excludes them as a cause of pneumonia. Similarly, the culture of tracheal aspirates has an almost 100 per cent sensitivity but only a 29 per cent specificity for the diagnosis of pneumonia. When pneumonia is confirmed, the same organism has usually been cultured in tracheal aspirates but, despite the polymicrobial nature of nosocomial pneumonia, there is a higher incidence of polymicrobial cultures from tracheal aspirates (ChastreandFagon 1994).
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