Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1419 page)

   Gram stain examination of sputum should be performed to ensure that poor-quality specimens are not processed for routine sputum culture. A number of screening strategies have been proposed. Sputum specimens are scored on the basis of the presence and quantity of PMNs and SECs.

   Acceptable specimens are usually inoculated onto SBA, chocolate and MacConkey agar.

   If anaerobic infection is suspected, specialized techniques, like needle aspiration, are required to exclude contamination by the patient’s endogenous flora.

   
Turnaround time

   Cultures are incubated for 48–72 hours.

   Additional time is required for isolation, identification, susceptibility, and other testing, as required.

   Interpretation

   
Expected results:
Light or rare growth of normal endogenous respiratory tract flora (or no growth).

   
Positive results:
Positive cultures must be interpreted carefully in the context of Gram stain results and other laboratory findings, imaging studies, and clinical presentations.

   
Negative results:
A negative culture does not exclude LRT infection. Poor specimen quality and transport conditions, or heavy contamination, may prevent the isolation of fastidious pathogens. Uncommon fastidious LRT pathogens, such as
Bordetella pertussis
, are not detected reliably by routine sputum culture.

   Limitations

   Noninvasive and minimally invasive techniques for specimen collection may result in contamination of the specimen with the patient’s endogenous upper respiratory flora. Because LRT infections are commonly caused by a patient’s flora, such contamination may compromise the interpretation of sputum cultures.

   The sensitivity and specificity of routine sputum cultures are relatively low for diagnosis of LRT infections. Diagnoses may be improved by submission of blood cultures, urinary antigen tests (e.g.,
S
.
pneumoniae
), serology, and molecular diagnostic techniques and tests for other types of LRT pathogens, such as
Mycoplasma
species.