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Subject: Bronchial Culture (Bal or Brush), Quantitative
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Quantitative bacterial cultures of specimens collected bronchoscopically (BAL or protected brush) are usually submitted for the evaluation for ventilator-associated pneumonia (VAP). The diagnosis of VAP is challenging, requiring a combination of clinical, imaging, and laboratory studies. Cultures are assessed in comparison to thresholds established by the laboratory in collaboration with clinicians.
Protected brush and BAL specimens are collected by a trained physician using standard procedures.
The brush is inserted through a plugged catheter via the biopsy channel of the bronchoscope. After expulsion of the plug, the brush is used to collect cells and secretions from the distal airways.
The brush end should be removed, using sterile technique, and placed in a small volume (1 mL) of nonbacteriostatic saline for transport.
BAL specimens are collected by a trained physician using standard procedures. The procedure and placement of the tip may be done under direct visualization or “blindly” through an endotracheal tube (mini-BAL).
The bronchoscope should be wedged in the terminal airways to ensure sampling of alveolar contents; return from the procedure should be 10–100 mL, sampling approximately 1 mL of alveolar secretions.
Samples should be transported to the laboratory as quickly as possible, using standard protocols for bacterial cultures.
Known volumes of the specimen (or specimen dilutions) are inoculated onto solid agar media, including SBA, chocolate, and MacConkey agar (and other media as required for uncommon pathogens, such as Legionella); quantitative results are reported on the basis of the number of colonies isolated.
Protected brush: The brush is vigorously agitated in the saline transport fluid to release trapped microorganisms. The saline is then used to prepare dilutions for media inoculation.
BAL: A measured aliquot of BAL fluid is used to prepare dilutions for media inoculation.
After incubation, the concentration of each type of organism is calculated using the colony count on the solid media, volume inoculated onto the solid media, and the dilution of the original specimen. Cultures are interpreted on the basis of the isolate identification, quantity of isolate in culture, and the presence of other flora, especially endogenous flora of low pathogenicity.
Turnaround time: Incubation for 48 hours. Additional time is required for pathogen isolation, identification, susceptibility testing, and further characterization, if needed.
Expected results: A low quantity of endogenous upper respiratory flora is often seen.
Positive results: In patients with pneumonia, growth of a respiratory pathogen is expected at a concentration of >103 CFU/mL for bronchial brush. Growth of a respiratory pathogen is expected at a concentration of >104 CFU/mL for visually guided BAL or >105–106 for blind mini-BAL.
Negative results: False-negative cultures may be caused by prior antimicrobial therapy. Detection of pneumonia caused by certain fastidious pathogens may require inoculation of special media. Heavy contamination of the specimen with endogenous flora may mask the growth of the causative pathogen.
Quantitative culture of protected brush specimens has only moderate to good performance, with the PPV and NPV of 74% and 85%, respectively. Quantitative culture of BAL has only moderate to good performance, with a PPV of 83–91% and NPV of 87–89%. The presence of intracellular organisms in >5% of cells is associated with higher specificities. Tissue histopathology and quantitative culture of biopsy are considered the “gold standard” for diagnosis.
The predictive value of cultures is markedly decreased for patients with any antibiotic therapy prior to the procedure.
Candida species are common contaminants and should not routinely be identified to species level.