Could Polymerase Chain Reaction–Based Methods Differentiate Pneumonitis From Bacterial Aspiration Pneumonia?

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We read with interest the article published in a recent issue of Critical Care Medicine by Lascarrou et al (1) about antibiotic therapy in comatose mechanically ventilated patients following aspiration. The authors concluded that only half of patients with suspected bacterial aspiration pneumonia (BAP) had the diagnosis confirmed by telescopic plugged catheter sampling. In addition, stopping empirical antibiotic therapy when sampling recovered no microorganism was safe.
The authors should be congratulated for having provided these interesting data on this important clinical problem. However, some clarifications are needed to better understand the study and improve the design of future trials. The frequency of BAP was probably underestimated as patients at higher risk for aspiration, such as those with chronic neurologic disease impairing laryngeal mobility, were excluded. Could the authors explain why these patients were excluded? In addition, stroke is a major cause for coma and aspiration (2). Could the authors provide the number of patients with stroke included in this study (1) and the frequency of BAP in this specific subgroup?
The authors chose to cover anaerobes, which may play a role in BAP. However, no anaerobic microorganisms were identified by microbiological cultures despite rapid transfer of samples under optimal conditions to the microbiology laboratory. Could the authors provide the microbiological technique used to identify these microorganisms? Previous studies showed that specific microbiological techniques might be required to identify these bacteria.
Microaspiration of contaminated oropharyngeal and gastric contents is the main mechanism in the occurrence of ventilator-associated pneumonia (VAP) (3). Based on the definition of BAP used by the authors, it is very difficult to differentiate it from early onset VAP. However, this is probably not important, as these two infections are treated similarly. The measurement of microaspiration markers, such as alpha amylase and pepsin, is accurate in diagnosing microaspiration in mechanically ventilated patients (4). However, these markers could not differentiate pneumonitis from BAP, as these conditions frequently coexist. One elegant method to rapidly differentiate pneumonitis from BAP is to use polymerase chain reaction (PCR)–based techniques. During the last decade, several studies investigated the accuracy of these methods in diagnosing lower respiratory infections, such as community-acquired pneumonia or VAP. The results of these studies are encouraging, suggesting that these methods could now be used at the bed of the patients, and the results could be obtained in few hours (5). In fact, the excellent negative predictive value of PCR-based methods would be very interesting to differentiate pneumonitis from BAP, to avoid or early withdraw antibiotic treatment in patients with negative results. Further randomized controlled trials are needed to investigate the impact of a PCR-based strategy on the use of antimicrobials in patients with suspected BAP. Experts all agree that every effort should be made to reduce antimicrobial consumption in the ICU and to break the vicious circle of multidrug resistant bacteria emergence and reduce its negative effects on mortality and morbidity in critically ill patients.
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