How Accurate Are Tracheal Aspirates at Diagnosing Nosocomial Pneumonia?

Pneumonia is either the first or second most common ventilator-associated event (VAE) seen in the pediatric intensive care unit. The reported incidence varies widely among studies (from 0.3 to 45.1 per 1,000 ventilator days). The lack of a consistent, gold-standard definition for what had been traditionally referred to as ventilator-associated pneumonia (VAP) plays a large part in this variance.

The U.S. Centers for Disease Control and Prevention (CDC) defines VAP as a positive bacterial culture from a tracheal aspirate (more than 104 colony-forming units [cfu] from a sample obtained via bronchoalveolar lavage) and/or purulent secretions (more than 25 neutrophils and fewer than 10 epithelial cells per low-power field) in a patient who requires mechanical ventilation and has acutely worsening oxygenation. However, other investigators have defined VAP by the presence of pathogenic bacteria in tracheal aspirate in conjunction with new chest radiographic findings and fever or leukocytosis. Studies by Srinivasan and colleagues and Carcillo et al have found that the presence of pneumonia is associated with high antimicrobial use and increased morbidity, mortality, hospital length of stay, and costs. The Society of Critical Care Medicine, in partnership with the Critical Care Societies Collaborative, also has been working closely with the CDC to develop the VAE surveillance definition algorithm, representing a purposeful departure from VAP toward more purposeful, objective measures of conditions and complications occurring in adult patients on mechanical ventilation. The findings of the VAP Surveillance Definition Working Group, which has only focused on adult patients, were published in the November 2013 issue of Critical Care Medicine. It is clear, the development and use of a standardized definition is essential.

In a study by Willson et al, the authors begin examining current definitions by seeking the specificity of tracheal secretion cultures and Gram stains in the diagnosis of VAP and correlating them with prospectively collected clinical signs and symptoms. Tracheal secretions were collected daily from all intubated pediatric patients who were expected to remain on a ventilator for more than 48 hours. Samples were collected for up to 14 days after intubation. Initially, during phase one of the project, the samples were collected using new, sterile suction catheters (n=213 samples). Later, during phase two, in-line suction catheters were used (n=122 samples), and if these samples had more than 104 cfu/mL bacteria, they were taken to the microbiology lab for identification. Finally, clinical data such as age, gender, white blood cell count, reports of increasing tracheal secretions, and chest radiograph reports (when available) were recorded.

Sixty-one patients were recruited for this study, providing a total of 335 samples. In phase one, the rate of positive cultures (i.e., more than 104 cfu/mL) rose from 11% during the first three days of intubation to 37% to 53% during days 4 to 10 or more. In phase two, 47% of patients had positive cultures on days 1 to 3. This rose to between 84% and 93% on days 4 to 10 or beyond. In addition, the number of phase-two subjects who met CDC criteria for purulence ranged from 44% to 63% over the entire course of time spent on the ventilator. The main pathogens identified in phase two were Staphylococcus aureus and Gram-negative rods.

Perhaps more important than the ubiquitous presence of bacteria and leukocytes in tracheal aspirate samples was the lack of correlation between clinical criteria (e.g., worse oxygenation, infiltrates on chest radiograph, fever) and positive bacterial cultures using logistic regression. Combinations of two or more clinical criteria were also not associated with positive bacterial cultures.

The diagnosis of VAP in intubated pediatric and adult patients carries serious repercussions, both individually and institutionally. However, defining VAP has proven difficult, and various criteria have been used by investigators. The results of this study by Willson et al demonstrate serious problems with the current definitions of VAP and with delineating infection in mechanically ventilated pediatric patients — even as early as one to three days after intubation — problems which may not be reconciled by simply including clinical criteria.

This Concise Critical Appraisal is authored by SCCM member Daniel E. Sloniewsky, MD. Each installment highlights journal articles most relevant to the critical care practitioner. Dr. Sloniewsky is an associate professor in the Department of Pediatrics at the Stony Brook Long Island Children’s Hospital in Stony Brook, NY, where he is board certified in pediatrics and pediatric critical care. He completed his fellowship training at Children’s Memorial Hospital and Northwestern University in Chicago. His major interests are acute pediatric pulmonary disease, transfusion medicine and ethics. He is also actively involved in resident education, Pediatric Advanced Life Support and Pediatric Fundamental Critical Care Support instruction.

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