Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1331 page)

   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.

   Interpretation

   
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 >10
³
CFU/mL for bronchial brush. Growth of a respiratory pathogen is expected at a concentration of >10
⁴
CFU/mL for visually guided BAL or >10
⁵
–10
⁶
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.

   Limitations

   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.

   
Common pitfalls:

   The predictive value of cultures is markedly decreased for patients with any antibiotic therapy prior to the procedure.