Endotracheal Aspiration in the Diagnosis of Ventilator-Associated Pneumonia

Abstract

T he time-honored method of identifying bacterial pathogens that are potentially responsible for nosocomial pneumonia in patients receiving mechanical ventilation is the microscopic examination of specimens obtained by endotracheal aspiration (EA). This technique is the simplest noninvasive means of obtaining respiratory secretions from patients receiving mechanical ventilation; it is readily performed at the bedside and requires minimal training by health-care providers. This section focuses on clinical studies evaluating diagnostic procedures using endotracheal specimens (ie, cytologic examination, antibody coating, elastin fibers, Gram’s stain, and culture) in immunocompetent adults with suspected VAP. The cytologic examination of specimens containing a large number of leukocytes and a paucity of epithelial cells is likely to produce the most valid representation of infectious organisms. A test for the detection of the presence of antibody coating on bacteria has been developed in an attempt to distinguish organisms that are colonizing the lower respiratory tract from those that actually are infecting it. The test is based on the premise that an infection will elicit an antibody response in the host and that this response will be detectable on the microorganism. In addition, using 40% potassium hydroxide to detect elastin fibers has been promoted as a rapid and inexpensive way to demonstrate the destruction of lung parenchymal tissue that is caused by pneumonia. However, an analysis of endotracheal specimens obtained by aspiration has been diagnostically inadequate. Several qualitative articles have reviewed the use of endotracheal specimens to diagnose VAP. Among the challenges for investigators and clinicians are the following: distinguishing upper from lower respiratory tract infection; distinguishing infection from colonization and contamination; standardizing aspiration collection methods and microbiological techniques; and interpreting test properties in light of the host’s immune status, the pathogenic load, and the effect of prior antimicrobial therapy. Newer bronchoscopic methods for diagnosing VAP have become the focus of recent investigations, conferences, and professional documents. Invasive approaches have not necessarily been adopted by clinicians,2 at least in part because of procedural access, cost, and the absence of compelling evidence that treatment based on the derived data changes clinical or economic outcomes. Thus, many physicians continue to use endotracheal specimens and other clinical features in diagnosing VAP.