By Kathryn Radigan, MD

Attending Physician, Division of Pulmonary and Critical Care, Stroger Hospital of Cook County, Chicago

Dr. Radigan reports no financial relationships relevant to this field of study.

SYNOPSIS: Tracheal aspirate cultures identified plausible pneumonia pathogens in more than half of newly intubated mechanically ventilated patients suffering from severe community-acquired pneumonia.

SOURCE: McCauley LM, Webb BJ, Sorensen J, Dean NC. Use of tracheal aspirate culture in newly intubated patients with community-onset pneumonia. Ann Am Thorac Soc 2016;13:376-381.

Since community-acquired pneumonia (CAP) is common and often life-threatening, early and appropriate antibiotic treatment is essential. Although experts recommend tracheal aspirate to optimize the treatment of patients with CAP, there is no literature to support its diagnostic usefulness. To evaluate the use of tracheal aspirate culture in identifying pneumonia pathogens, McCauley et al identified all patients with suspected community-onset pneumonia with International Classification of Disease, Ninth Revision (ICD-9) codes and radiographic evidence of pneumonia within the EDs of two university-affiliated Utah hospitals. Between December 2009 and November 2010 and between December 2011 and November 2012, patients who were diagnosed with pneumonia and intubated within 24 hours of ED arrival were identified electronically. As a part of immediate post-intubation care, standing orders instructed the respiratory therapists to obtain a tracheal aspirate for culture. Semiquantitative cultures had to meet acceptability standards (< 10 epithelial cells per low-powered field) and be quantified as rare, 1+, 2+, 3+, or 4+, based on the number of quadrants on the agar plate that demonstrate growth. All organisms present in a semi-quantitative amount > 1+ and any organism identified as predominant or only growth (regardless of the total amount of growth) were identified and included in the study. Investigators excluded organisms if identified as normal oral flora. The authors reviewed the electronic medical record to collect the results of other microbiological studies.

Of the 2,011 patients presenting with pneumonia, 94 were intubated and 84 received a tracheal aspirate. Of the 84 patients who underwent a tracheal aspirate, 47 featured a pulmonary pathogen identified by tracheal aspirate culture. Eighty patients also submitted blood cultures and 71 underwent pneumococcal and Legionella urinary antigen testing. Fifty-five patients underwent viral polymerase chain reaction (PCR) testing. Out of all the specimens evaluated by the variety of diagnostic mechanisms, researchers confirmed a microbiological diagnosis in 55 patients. The tracheal aspirate culture was the only positive test in 32 out of 82 patients. These patients otherwise would have been classified as culture-negative. Overall, 40 patients underwent de-escalation of antibiotic therapy, and 16 patients received targeted therapy.

Tracheal aspirate cultures added significant diagnostic value to other routine microbiological tests and identified many patients who otherwise would be considered culture negative.

COMMENTARY

Pneumonia is one of the leading causes of hospitalization and death in the United States and often associated with considerable morbidity and mortality, especially in older adult patients and those presenting with significant comorbidities.1 Early and appropriate treatment has been shown to improve outcomes substantially.2 This is especially important as the rates of drug-resistant pathogens and broad-spectrum antibiotic usage increase. Since it is difficult to achieve a microbiological diagnosis, McCauley et al examined the diagnostic usefulness of tracheal aspirate at the time of intubation in patients suffering from severe CAP.

Endotracheal aspirate, using a clean suction catheter, is a noninvasive method to obtain a deep respiratory sample in recently intubated patients. Bronchoalveolar lavage (BAL) or a protected specimen brush sample are alternative options to confirm the microbiological diagnosis in this setting. Interestingly, there has been extensive research into the use of tracheal aspirate cultures as a noninvasive method to diagnose ventilator-associated pneumonia (VAP). In this particular setting, it often is difficult to discriminate between organisms responsible for infection vs. colonization. Since the endotracheal aspirate cultures within the current study were performed immediately after intubation for severe CAP, one presumes there was not sufficient time for colonization. There also is concern that the results within this study were reported semi-quantitatively vs. quantitatively. At least in the diagnosis of VAP, one might believe that semi-quantitative cultures of endotracheal aspirate may be poorly concordant with quantitative cultures obtained via non-bronchoscopic BAL. Regardless, failure to identify potential multiple drug-resistant pathogens with semiquantitative cultures would be less concerning in a CAP population. In addition, even though there is concern that semi-quantitative cultures may promote excessive antibiotic usage, there was substantial de-escalation of antibiotics in this study.3

Regardless of whether one favors semiquantitative cultures, researchers confirmed a microbiological diagnosis in almost two-thirds of patients with the tracheal aspirate culture. It was the only positive test in almost 40% of patients. Since evolving literature supports the notion that viruses cause CAP more often than bacteria,4 results may have been even more striking if viral PCR had been completed in all patients as opposed to two-thirds. Nonetheless, one of the major benefits to this study is that endotracheal culture was a part of standing orders and initiated as a part of the immediate post-intubation care. As there is often a struggle to order, collect, and process culture data in a timely manner, standing post-intubation orders may improve yield and confirm a microbiological diagnosis. As such, it is clear that tracheal aspirate cultures offer important additive diagnostic value to other routine tests performed in the setting of severe CAP while facilitating appropriate antibiotic therapy. Further studies may determine whether tracheal aspirate culture in patients suffering from severe CAP makes a significant difference in hospital stay, morbidity, or mortality.

REFERENCES

  1. Prina E, Ranzani OT, Torres A. Community-acquired pneumonia. Lancet 2015;386:1097-1108.
  2. File TM Jr. New diagnostic tests for pneumonia: What is their role in clinical practice? Clin Chest Med 2011;32:417-430.
  3. Fujitani S, Cohen-Melamed MH, Tuttle RP, et al. Comparison of semi-quantitative endotracheal aspirates to quantitative non-bronchoscopic bronchoalveolar lavage in diagnosing ventilator-associated pneumonia. Respir Care 2009;54:1453-1461.
  4. Jain S, Self WH, Wunderink RG, et al. Community-acquired pneumonia requiring hospitalization among U.S. adults. N Engl J Med 2015;373:415-427.