Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1363 page)

   Pathogens present in the specimen in concentrations greater than approximately 10
³
–10
⁴
organisms per milliliter should be detected by Gram stain and will typically yield moderate or heavy growth in culture. Concentration of CSF and sterile fluid specimens, using techniques such as centrifugation, improves detection of microorganisms by Gram stain.

   Any type of organisms seen by Gram stain should be isolated by culture after appropriate processing. Monitoring the correlation of Gram stain and bacterial culture results, therefore, may be used as an important quality assurance (QA) tool. Detection of an organism by Gram stain for which culture yields no comparable isolate suggests that additional cultures, like anaerobic or mycobacterial, may be required.

   
Negative results:

   Infections may be associated with low concentrations of pathogens (<10
³
organisms/mL). For example, in adults with overwhelming bacteremia and sepsis, the concentration of organisms in the bloodstream is typically approximately 1–10 organism/mL, well below the detection level by Gram stain microscopy.

   PMNs and other signs of inflammation may increase suspicion of infection in smears negative for microorganisms.

   Limitations

   Some pathogenic microorganisms fail to stain avidly by the Gram stain technique. Special modifications or stains may improve detection, like the use of fuchsin as a counterstain for the Gram stain, or acridine orange as a fluorogenic alternative to the Gram stain. Poor specimen collection, such as not sampling the site of infection, may give false-negative or misleading results.

Suggested Reading

Winn WC Jr, Allen SD, Janda WM, et al.
Koneman’s Color Atlas and Textbook of Diagnostic Microbiology
, 6th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006.

GROUP B STREPTOCOCCUS VAGINAL–RECTAL CULTURE SCREEN

   Definition

   Group B
Streptococcus
(GBS) infection is the leading cause of early-onset neonatal sepsis. Bacteremia, multiorgan disease, and meningitis are all possible manifestations of neonatal GBS infection. Maternal GBS carriage in the GU or GI tract is the primary risk factor for neonatal infection. The CDC and relevant professional organizations have recommended screening pregnant women for GU and GI carriage of GBS and using culture results as primary guidance for the use of intrapartum antimicrobial prophylaxis for prevention of neonatal infection.

   Special Collection and Transport Instructions

   Swab specimens should be collected at 35–37 weeks’ gestation.

   Swab the lower vagina/vaginal introitus, followed by the rectum (i.e., through the anal sphincter). Two swabs or a single swab may be used. If two swabs are used, they should be submitted to the laboratory together as a single specimen.