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BEST PRACTICES :: GROUP B STREP


Consider NAATs for detection of GBS in pregnant women


By David T. Pride, MD, PhD G


roup B streptococcus (GBS), a bacterium passed from mother to child during labor and child- birth, remains a considerable public health concern both in the United States and abroad.1,2


In


the U.S., the Centers for Disease Control and Pre- vention (CDC) has called for improvements of up to 90 percent in screening sensitivity; but culture- based methods have not yet approached this goal.2-4 Our laboratory at The University of California, San Diego (UCSD) compared three polymerase chain reaction (PCR)-based nucleic acid amplification tests (NAATs) in their abilities to detect GBS samples from pregnant women, and found that all three have significantly greater sensitivities and reduced turn- around time (TAAT), relative to our standard of care culture-based testing. Therefore, considering NAATs as a new gold standard in the detection of GBS may be warranted.


GBS remains a global public health concern Recent global estimates suggest GBS is responsible for 409,000 maternal/fetal/infant cases annually, including 205,000 cases of infants with early-onset


disease (EOD), leading to septicemia, meningitis, or pneumonia within the first week after delivery.5 GBS is also estimated to be responsible for 147,000 stillbirths and infant deaths, and neurodevelopmen- tal impairment in another 10,000 annually.5,6 Vaginal-rectal GBS colonization has been reported to occur in about 18 percent of pregnant women globally and about 25 percent in the U.S., according to the CDC.7,8


In the U.S., GBS infection is the


leading cause of infant morbidity and mortality, and of bacterial meningitis and septicemia in a newborn’s first week of life.12


Long-term disabilities may include


retardation, hearing or vision loss, and potentially death.10,11


Culture-based screening presents pros and cons Since 2002, the CDC has recommended universal GBS screening at 35-37 weeks of pregnancy. The preferred choice since then has been culture-based methods using vaginal-rectal specimens, which has resulted in a dramatic decrease in the incidence of EOD, despite the carrier rate remaining steady at ~20 percent.2 The primary and highly effective strategy for pre- venting EOD is intravenous antibiotic administration during labor in women who test positive for GBS.2 Still, EOD due to GBS remains a problem,13,14


given


a large percentage (81 percent) of neonates who develop EOD are born to GBS-negative mothers, which suggests an inadequate sensitivity of culture- based screening in the form of false-negatives.3,14 A promising alternative to culture-based screening is PCR-based NAATs, several of which have been approved by the Food and Drug Administration (FDA). However, adoption of NAATs has not been widespread, and culture-based tests remain the gold standard.


Comparing three commercially available NAATs We sought to determine the performance of three FDA-approved GBS NAATs: (1) the Hologic Pan- ther Fusion GBS assay; (2) the Luminex Aries GBS assay; and (3) the Cepheid Xpert GBS LB assay; the goal to compare their sensitivity and specificity against one another and to culture. We collected 500 vaginal-rectal samples from women at 35-37 weeks of pregnancy and enriched them in Lim broth (Todd Hewitt broth, Copan) for 16-24 hours. After enrichment, an aliquot of the Lim broth was used for culture per UCSD’s standard of care test- ing. Residual enriched Lim broth specimens were aliquoted into multiple tubes for testing with each NAAT method.


24 JULY 2019 MLO-ONLINE.COM


Transmission from mother to newborn


occurs at an estimated rate of 40-73 percent with about one to two percent of colonized newborns developing EOD.9-11


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