Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1432 page)

   MacConkey agar for nonfastidious gram-negative bacilli, including
P
.
aeruginosa
and
S
.
maltophilia
.

   
B
.
cepacia
–selective agar.

   Chocolate agar for isolation of
H
.
influenzae.

   Cultures for mycobacterial, fungal, viral, or other respiratory pathogens are also recommended in addition to bacterial cultures.

   
Turnaround time:
Cultures are examined daily for 96 hours. Several days are required for isolation, susceptibility testing, and identification of suspected isolates.

   Interpretation

   Patients with CF often show respiratory tract colonization that changes little over time, even in response to antimicrobial therapy. The interpretation of cultures demonstrating such “abnormal flora” may be challenging; clinical and therapeutic decisions must be based on a variety of clinical and other factors, in addition to culture results.

   The workup and interpretation of CF respiratory cultures are typically based on several factors, including type of specimen submitted, organism(s) isolated, and the predominance of a specific pathogen compared to other flora.

   Limitations

   Although rapidly growing mycobacteria and mold may be isolated with CF respiratory cultures, special cultures are needed for sensitive detection of nontuberculous mycobacteria,
Aspergillus
species and other molds, and viruses that may cause acute respiratory infections in these patients. It is difficult to differentiate isolates that represent chronic colonization versus acute exacerbation on the basis of laboratory criteria.

   
Common pitfalls:

   Clinicians must order special throat or lower respiratory cultures specifically designed for evaluation of patients with CF; routine cultures are not optimized for evaluation of the flora typically isolated from such specimens. Laboratories should not apply sputum rejection criteria based on Gram stain screening recommended for routine sputum cultures submitted from other patients.