Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1444 page)

   Collection of infected tissue or aspirate, at least 1 g, is recommended. Collection using swabs is not recommended.

   Collection and transport under anaerobic conditions are recommended, especially for closed wounds. Specimens should be transported to the laboratory within 2 hours. Specimens may be held at 4°C for a short time if transport is delayed.

   Use

   Most specimens for bacterial wound cultures should be examined by Gram staining. Specimens from superficial wounds showing a significant numbers of epithelial cells are likely to be contaminated by endogenous flora unrelated to the infection.

   Specimens are inoculated onto supportive, enriched, and selective/differential media.

   Specimens are inoculated onto supportive and enriched nonselective media, like SBA and chocolate agar media, and selective media, like MacConkey, PEA, and CNA agar. Some laboratories include a broth medium, like thioglycollate broth, to routine cultures. Anaerobic medium is inoculated for appropriate specimens collected and submitted under anaerobic conditions.

   
Turnaround time:
Cultures are incubated for 48–72 hours. Additional time is required for isolation, identification, susceptibility testing, and further characterization, as needed.

   Interpretation

   
Expected results:
No growth.

   
Positive results:
Cultures of infected wounds often show growth of several types of organisms. Cultures must be interpreted carefully: Mixed cultures may be caused by colonization by endogenous flora or contamination due to poor specimen collection technique. However, mixed cultures may represent synergistic polymicrobial infections, especially when anaerobes are isolated.

   
Negative results:
Negative culture decreases the likelihood of active bacterial infection.

   Limitations

   Negative cultures may be caused by prior antimicrobial treatment, infection caused by a fastidious pathogen, or collection of the sample from a site other than that of active infection.