Quantitative Cultures for Diagnosing Ventilator-Associated Pneumonia: A Critique

Abstract

The diagnosis of ventilator-associated pneumonia has been clouded by uncertainty, because a reference standard has never been established. The use of invasive procedures to obtain respiratory tract samples for culture, with quantitation of the bacteria isolated, has been the approach most commonly advocated. Quantitation of bacteria from lower respiratory tract specimens can be used to distinguish colonization from infection. We review the invasive procedures (bronchoalveolar lavage, protected specimen brushing, nonbronchoscopic bronchoalveolar lavage, and blinded bronchial sampling), the methods of quantitation used, the types of catheters used, the sample collection methods, and the criteria used as cutoffs for the quantitative cultures. Quantitation of lower respiratory tract samples is inherently unstable from a mathematical perspective, given the variability in the volume of fluid instilled and reaspirated and the magnitude and complexity of the area being sampled. We also briefly review the use of quantitation for bacterial infections other than pneumonia, including urinary tract infection and catheter-related bacteremia. The variability in both the methods and reference criteria in the studies reviewed show that the quantitation approach is neither standardized nor evidence based.

Bronchoscopic and Nonbronchoscopic Methods of Airway Culturing in Tracheostomized Children

“…For example, cystic fibrosis practitioners tend to use higher thresholds for BAL quantitative cultures (usually 10 5 CFU/ mL), 4,18 whereas the threshold of 10 4 CFU/mL is considered to be acceptable for the diagnosis of lower respiratory tract infections in non-cystic fibrosis patients. 5 Different diagnostic thresholds for positive quantitative cultures have been established for BP-BAL, B-BAL, and TA by previous investigators 7,16,19 and were used in our study. The lowest threshold of 10 2 for BP-BAL as applied in our study was also used by Timsit et al, 20 and it provided a higher sensitivity in determining significant positivity of the cultures obtained, although specificity was somewhat decreased.…”

Use of Quantitative Cultures and Reduced Duration of Antibiotic Regimens for Patients with Ventilator-Associated Pneumonia to Decrease Resistance in the Intensive Care Unit

“…Ventilator-associated pneumonia (VAP) is responsible for approximately half of the infections acquired in the ICU and represents one of the principal reasons for the prescription of antibiotics in this setting [10]. Invasive diagnostic methods, including bronchoalveolar lavage (BAL) and/or protected specimen bronchial brushing (PSB) with quantitative cultures, could improve the identification of patients with true bacterial pneumonia and facilitate decisions of whether to treat [11,12]. These techniques also permit a streamlined choice of antibiotics for patients with proven bacterial infection, once the results of respiratory tract cultures become available, based on the identity of the specific pathogens and their susceptibility to specific antibiotics, to avoid prolonged use of a broader spectrum of antibiotic therapy than is justified by the available information.…”

Collection of tracheal aspirate: safety and microbiological concordance between two techniques

“…For example, if only a small volume of liquid is recovered by aspiration, then, the bacteria count may be mistakenly larger due to the smaller volume of diluent. On the other hand, if a larger volume of liquid is recovered, the concentration will be lower at the expense of increased dilution (2) .…”

“…It is easily performed at the bedside, relatively simple, minimally invasive and inexpensive, has a proven acceptable accuracy and requires minimal investment for the training of health professionals (1) . Some authors (2)(3) point out, however, that further evidence is required for the management of TA, especially with regard to the techniques of microbiological collection and processing of the samples, because there are different protocols and some unresolved issues. Bacterial and fungal colonization of the internal surface of the endotracheal tube (ETT) in the form of biofilm is one of the factors which contributes most to harvesting of non-representative samples, the occurrence of false results or questionable cultures with no diagnostic value, given the possibility of contamination of the sample during the collection phase.…”