• Assisted ventilation
• pneumonia

Ventilator-associated pneumonia (VAP) is reported to occur in up to 20–27% of mechanically ventilated patients, and impacts healthcare in terms of patient morbidity, mortality and expenditure.1–3 The concept of VAP seems straightforward—that is, alveolar inflammation due to an infectious agent that was not present at the time of initiation of mechanical ventilation. However, the diagnosis remains difficult. Importantly, this difficulty in diagnosis of VAP leads to potential over-/underprescription of antibiotics and misguided treatment.

The American Thoracic Society guidelines of 20053 suggest that the use of readily available clinical data is adequate to inform the diagnosis of VAP. However, while such an approach has the advantage of being straightforward to apply, when compared with postmortem histological specimens the resultant sensitivity and specificity were 69% and 75%, respectively,4 indicating the need for better diagnostic systems. More complex scoring systems combining clinical and microbiological data, for example the clinical pulmonary infection score5 and national nosocomial infection surveillance system,6 have been proposed for the diagnosis of VAP, but have also been criticised for non-specificity.7 The addition of quantitative culture results from endotracheal secretions, or more complex samples including protected specimens, brushings or bronchoalveolar lavage fluid (BAL),3 to the clinical scoring systems for diagnosing VAP goes some way to reducing the potential overdiagnosis of VAP.8 Quantitative microbiological cultures are limited by their reduced sensitivity in patients who have been treated with antimicrobials in the preceding 3 days.9 10 Finally, laboratories do not routinely test for atypical pathogens. Recent work has highlighted a significant prevalence of anaerobic bacteria,11 viruses12 and fungi13 14 in intubated patients, although their role in VAP remains unclear.

Ideally, therefore, in diagnosing VAP we could distinguish colonisation from infection causing pulmonary inflammation, and remain confident of a negative diagnosis in …