Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1364 page)

   Transport to the laboratory within 24 hours in a nonnutritive transport medium; transport and store the specimen at 4°C prior to laboratory processing. Note if the patient is at risk for anaphylaxis to penicillin, an indication for susceptibility testing for GBS-positive specimens.

   Use

   Enrichment culture: Swabs are inoculated into selective broth medium, like Todd-Hewitt broth with gentamicin (8 μg/mL) and nalidixic acid (15 μg/mL) [TransVag broth], or colistin (10 μg/mL) and nalidixic acid (15 μg/mL) [Lim broth]. Commercially available enriched chromogenic broths may be used for the enrichment culture. Incubate broth cultures for 18–24 hours at 35–37°C in room air or 5% CO
₂
.

    For TransVag and Lim broth, subculture to an appropriate agar medium (e.g., SBA, CNA, GBS chromogenic agar). Examine subculture plates for colonies suggestive of GBS and perform confirmatory testing.

   Process chromogenic broth according to the manufacturer’s instructions.

   Broth enrichment is the most common method. Alternative testing of the broth enrichment culture includes specific latex agglutination or nucleic acid probe or PCR.

   Susceptibility testing should be performed on GBS isolates for patients with significant penicillin allergy; D-testing for inducible clindamycin resistance should be performed on isolates that are clindamycin sensitive but erythromycin resistant by routine testing.

   Direct plating of swabs may be performed in urgent circumstances, like when an unscreened woman presents in active labor, but enrichment culture should also be performed to ensure optimal sensitivity.

   History of early-onset neonatal GBS infection in a prior pregnancy and GBS bacteriuria or UTI at any time during a current pregnancy are indications for intrapartum prophylaxis, regardless of screening test results; screening is not recommended for these patients.

   Interpretation

   
Expected results:
10–30% of pregnant women are vaginal or rectal carriers of GBS. Carriage may be transient, intermittent, or persistant.

   
Positive results:
In the absence of intrapartum prophylaxis, 1–2% of infants born to GBS-colonized mothers develop early-onset neonatal infection.