There are many approaches to confirming ventilator-associated pneumonia (VAP). “Diagnosing VAP remains difficult and controversial,” writes one critical care provider. “The diagnosis can be made on the basis of radiographic findings, clinical findings, results of microbiological tests of sputum, or invasive testing such as bronchoscopy.” Providers should integrate any culture results with clinical evaluation before making a VAP diagnosis. The American Thoracic Society and the Infectious Diseases Society of America outline several (bacteriologic and clinical) diagnostic strategies for VAP. Hospitals should use diagnostic approaches appropriate for the individual patient—and that are feasible for their workflow.
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Quantitative cultures can help diagnose VAP. Bronchoscope-guided culture techniques, such as protected specimen brushings or BAL, can be used to collect specimens from the respiratory tract for antibiotic sensitivity testing. Bronchoscopes reach directly into the airway. This increases culture specificity, allowing providers to identify patients with a true lung infection.
While some consider BALs “essential” for VAP diagnoses, BALs are invasive, and as such carry risks including hypoxemia, bleeding, or arrhythmia. Endotracheal aspirate culture may offer a more practical approach to obtaining diagnostic cultures in critically ill patients. However, endotracheal aspirate cultures are not collected directly from the lung. Contamination from other flora is common in endotracheal cultures. Their low specificity makes it difficult to extrapolate results to VAP. Worse, VAP diagnoses based on endotracheal aspirate cultures alone could initiate inappropriate antibiotic treatment regimens that contribute to resistance.
One study compared BAL and endotracheal aspirate techniques for diagnosing VAP. Across 311 suspected VAP cases, Gram staining and semiquantitative culture results were comparable between diagnostic approaches. But, VAP incidences varied widely depending on which strategy was used for diagnosis. In the end, high BAL specificity and sensitivity led the authors to conclude BAL is preferred over aspirates “for the diagnosis, differential diagnosis, and treatment of VAP.”
Early recognition of causative pathogens is essential. Accurate microbial identification informs appropriate antibiotic therapy and thus may determine outcome. But providers should not rely on cultures alone for VAP diagnosis. Respiratory fluid sampling is necessary, but not always specific. Respiratory tract cultures for VAP diagnosis can be compromised by bronchiolitis, or oropharyngeal contamination. It’s important to pair any culture results with other clinical observations, as there is no “gold standard” VAP diagnosis technique. If a provider suspects VAP, they should move quickly to select an appropriate strategy, based on the most recent recommendations, to confirm diagnosis and initiate treatment. Current guidelines state “failure to initiate prompt appropriate and adequate therapy has been a consistent factor associated with increased mortality.”