Questions & Answers About Implementing the 2010 Guidelines for Laboratorians
Prenatal GBS Specimen Processing and Testing Procedures
Q: Are transport media containing charcoal acceptable?
A: Yes, swabs should be placed into nonnutritive transport media, which can include commercial systems with or without charcoal (e.g. Stuart’s and Amies with or without charcoal).
Q: Can our lab use the direct plating method?
A: Yes, direct plating of a vaginal/rectal swab is acceptable as long as the lab ALSO inoculates the swab in the selective broth medium. Direct plating can be helpful for speeding up reporting of results, but by itself has a high false-negative rate, so inoculation in selective enrichment broth is absolutely required. Remember: accurate results are far more important than rapid results for prenatal specimens.
Q: What broths are recommended for enrichment for GBS?
A: The new guidelines recommend selective enrichment broths such as Lim or TransVag, as well as pigmented selective enrichment broth as an alternative.
Q: Can I send out a negative GBS report if my chromogenic broth has no pigment?
A: No. GBS detection by most chromogenic broths is only possible for beta-hemolytic strains (labs should consult the package insert for the chromogenic broth they are using to determine the capabilities of their broth formulation). In recent studies, approximately 4% of invasive early-onset GBS isolates are non-beta hemolytic and would not produce a color change with chromogenic broth. All negative chromogenic broths should be subcultured to an appropriate agar plate and further tested, or the broth can be tested directly to determine if GBS is present.
Q: Do I need to look for non-hemolytic GBS?
A: Yes, in recent studies, approximately 4% of invasive early-onset GBS isolates are non-beta hemolytic. Pigmented broths and some chromogenic agars may only detect hemolytic GBS and should be further tested if no color change occurs. On blood agar, beta hemolysis may not always be apparent, so if whitish-gray colonies are identified and suspected of being GBS, then these should be further tested.
Q: Can our lab use a nucleic acid amplification test (NAAT) like PCR for prenatal (35-37 weeks) GBS screening?
A: Yes, as long as the test is done from an enrichment broth. Laboratories can use FDA-approved/cleared or other validated tests.
Q: Can NAAT testing be done directly from prenatal swabs?
A: No. All vaginal-rectal swabs must be inoculated in selective enrichment broth (e.g., TransVag broth, LIM broth) first and incubated for 18-24 hours prior to performing NAAT.
Q: When can a lab use NAAT for rapid testing directly from the swab?
A: A lab can use NAAT (e.g. PCR) for rapid intrapartum testing for women with unknown GBS status who present in active labor and have no risk factors that would otherwise be an indication for intrapartum GBS prophylaxis (i.e. <37 weeks gestation, temperature ≥100.4°F, or rupture of membranes ≥18 hours).Top of Page
GBS Susceptibility Testing
Q: Does susceptibility testing need to be done for all prenatal GBS specimens?
A: No. Susceptibility testing only needs to be done if the ordering physician has indicated that the patient has an allergy to penicillin and is at high risk for anaphylaxis.
Q: What methods are approved for susceptibility testing for GBS?
A: CLSI recommends disk diffusion or broth microdilution testing for susceptibility testing. Alternatively, commercial systems that are FDA-approved or cleared for testing of streptococci other than S. pneumoniae are also acceptable.
Q: Does susceptibility testing for GBS need to include a test for inducible clindamycin resistance?
A: Yes, susceptibility testing for GBS cultures must include a D-zone or other validated test for inducible clindamycin resistance.
Q: Can I use Etests for determining antibiotic susceptibility for GBS?
A: Yes. Etests are an FDA-approved alternative for determining antibiotic susceptibility for GBS. Labs using Etests should consult the package insert and interpret the results according to the established CLSI breakpoints for Streptococcus spp. beta-hemolytic group. An approved or cleared test for detection of inducible clindamycin resistance would still be required for testing erythromycin-resistant and clindamycin susceptible GBS determined by Etest from penicillin-allergic women at high risk for anaphylaxis.
Q: Are there any differences in D-zone testing for GBS compared to D-zone testing for other species, e.g., staphylococci?
A: Yes. Clindamycin and erythromycin disks must be placed 12 mm apart from each other when plating, which is different from the recommended 15-26 mm distance used in staphylococci resistance testing. Additionally, for streptococci testing, a disk dispenser CANNOT be used to place disks on the plate.
Q: Can I use the MicroScan panels I use for detecting inducible clindamycin for S. aureus?
A: No, the MicroScan ICd test should not be used for testing group B streptococci. It has been formulated for staphylococci and will not work for group B strep. It contains 4ug/mL of erythromycin and will inhibit the growth of most group B streps. The new CLSI broth method for beta-hemolytic streps contains only 1 ug/mL erythromycin. It is analogous to the different distances used for testing staph and strep in the D-Zone test - 15-26 mm for staph and 12 mm for streps.Top of Page
Q: Why did the guidelines change the reporting threshold for GBS from urine to ≥104 cfu/mL?
A: Studies of GBS bacteriuria have shown that it is a marker for heavy colonization and a risk factor for having an infant with early-onset GBS disease. However, most of these studies used a colony count cutoff of 104 cfu/mL or higher. It is difficult to determine what the significance of lower colony counts are, and reporting all colony counts can also be a significant burden on a lab. In light of this, the new guidelines require that labs report all GBS cfu/mL of 104 or higher.
Q: Should our lab work to identify even a very small amount of GBS that may be present in a urine culture?
A: It is no longer recommended to report low colony counts. If a lab has identified the presence of GBS in the urine that is less than 104 cfu/mL, and would like to report it, that is appropriate if it is their protocol.Top of Page
- Page last reviewed: May 23, 2016
- Page last updated: May 23, 2016
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