Group B Streptococcus Colonization Detection, NAA With Reflex to Susceptibilities*

Antepartum detection of group B β-hemolytic streptococci in patients who are at high risk for anaphylactic shock due to penicillin allergy.

A positive result does not necessarily indicate the presence of viable organisms. Patients who have used systemic or topical (vaginal) antibiotic treatment in the week prior as well as patients diagnosed with placenta previa should not be tested using this assay.

In the US, group B Streptococcus (GBS) remains a leading cause of early-onset neonatal sepsis. The most common mode of acquisition by the neonate is exposure to the maternal genital flora in utero through ruptured membranes or by contamination during passage through the birth canal.

Infection is manifested in two major forms, early-onset septicemic infection manifest in the first few days of life and late-onset meningitis that occurs during the first few months of life. The Centers for Disease Control and Prevention (CDC) and the American Congress of Obstetricians and Gynecologists (ACOG) practice guidelines recommend universal antepartum screening at 35 to 37 weeks of gestation. These guidelines specify co-collection of a vaginal and rectal swab specimen to maximize sensitivity of GBS detection. Per the CDC and ACOG, swabbing both the lower vagina and rectum substantially increases the yield of detection compared with sampling the vagina alone. Rates of maternal colonization have not changed, but universal antepartum screening along with the use of intrapartum antibiotic prophylaxis has resulted in a decrease of early-onset GBS disease. In November 2010, the CDC published revised guidelines recommending that, in addition to culture, antepartum vaginal/rectal specimens could be tested using a nucleic acid amplification (NAA) test following incubation in a selective enrichment broth medium to enhance the detection of GBS.

Susceptibility testing is not routinely performed, as this organism is universally susceptible to penicillin, and surveillance for resistance is not recommended. The CDC and the ACOG recommend reflex susceptibility testing to clindamycin and erythromycin (D-zone test to detect inducible clindamycin resistance) for group B Streptococcus only for those patients at high risk for anaphylactic shock due to penicillin allergy.

For those patients without likely evidence of severe anaphylaxis, the CDC and ACOG recommend the use of cefazolin as the agent of choice for intrapartum chemoprophylaxis without the need for susceptibility testing. For those patients with GBS that is resistant to clindamycin, treatment with vancomycin is recommended without the need for additional susceptibility testing. Even though it is tested together with clindamycin in the D-zone test, erythromycin is no longer an acceptable alternative for intrapartum GBS prophylaxis for penicillin-allergic women at high risk for anaphylaxis, and susceptibility results for this agent will not be reported.

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