With pressure mounting from legislators, insurers and consumer advocates to drive ventilator-associated pneumonia (VAP) rates to zero, the article by Conway Morris and colleagues (see page 516) in this issue of Thorax is a timely reminder that VAP rates are uncertain estimates rather than concrete measures of patient morbidity.1 Conway Morris and colleagues show that the reported VAP rate of an Intensive Care Unit (ICU) is heavily dependent upon its favoured diagnostic technique. ICUs that exclusively use bronchoalveolar lavage (BAL) to diagnose VAP are liable to report VAP rates that are 76% lower than those that exclusively use endotracheal aspirates. This laxity in the VAP definition confers a risk that some well-intended initiatives may decrease VAP rates yet provide little benefit to patients and perhaps even put some at risk.

Conway Morris and colleagues base their estimate of varying VAP rates upon the comparative yield of simultaneous BAL and endotracheal aspirate cultures taken from 53 critically ill patients with clinical syndromes suggestive of pneumonia. All patients had radiographic infiltrates, pyrexia or an abnormal leucocyte count, and clinical signs of chest infection such as increased volume or purulence of sputum, crepitations and deterioration in oxygenation. In these patients, there was a clear step function in culture results: 89% of endotracheal aspirate qualitative cultures were positive (defined as any growth), 51% of endotracheal quantitative cultures were positive (defined as ⩾10⁶ colony-forming units (CFU)/ml), and 21% of BAL cultures were positive (defined as ⩾10⁴ CFU/ml).

Each of these culture thresholds qualifies as VAP using the definitions of the Hospitals in Europe Link for Infection Control through Surveillance (HELICS) system. Consequently, depending upon one’s choice of diagnostic technique, measured VAP rates can vary fourfold. HELICS does assign different VAP codes to patients depending upon which culture technique and threshold was used …