How will you support more accurate GBS screening?

The adoption of new clinical practice relies on clinical evidence and knowledgeable champions. Despite documented increases in test sensitivity, the potential to reduce adverse clinical outcomes and greater laboratory efficiency, approximately 81% of labs do not use a nucleic acid amplified test (NAAT) for GBS screening.^1,2 Fortunately, lab leaders are emerging as champions of this screening tool for mothers and infants.

Group B Streptococcus (GBS) is a Gram-positive bacterium that colonizes the lower genital tract of approximately 25% of women in the US.³ If left untreated, GBS can cause maternal and fetal infection, and early-onset neonatal disease (EOD), which can endanger the lives of newborns.⁴ Mortality from EOD has fallen steeply in the past 50 years, thanks to US screening protocols.⁵ Screening tests have evolved beyond culture testing to incorporate molecular NAATs for even greater clinical and operational benefits.

The limitations of traditional culture-based testing

There is considerable consensus among US professional societies including the American Society for Microbiology (ASM), ACOG (American College of Obstetricians and Gynecologists), and the Center for Disease Control (CDC) that GBS screening is recommended between 36 0/7 and 37 6/7 weeks of gestation.^5,6,7 While traditional testing by culture alone is frequently used to meet these screening recommendations, data support the use of highly sensitive NAATs for GBS screening.

When GBS is detected, intravenous antibiotics administered to the mother during labor can prevent most cases of GBS EOD in newborns.⁷ However, ASM guidelines suggest that the sensitivity of culture tests may be as low as 53-70%.⁶ Labs can look towards a more sensitive screening protocol by implementing NAAT methods.

Using NAAT to increase test accuracy

NAAT technology is a powerful ally in reducing false negative results to ensure that GBS-positive mothers receive critical prophylaxis, while GBS-negative mothers avoid unnecessary antibiotics. In a 2019 study of PCR NAATs for GBS detection, authors from The University of California San Diego concluded that the method is highly sensitive and should be considered the preferred method for prenatal GBS screening.⁸

Intravenous penicillin is the current antibiotic of choice for intrapartum prophylaxis in GBS-positive women.⁶ Fewer than 1% of the population is estimated to be truly allergic to penicillin, but for this small cohort of pregnant women, it is necessary to perform GBS antimicrobial susceptibility testing (AST) to guide appropriate alternate antibiotic therapy.^6,9  Since AST requires isolation of viable organisms and growth in culture, laboratories could consider implementing a modified algorithm to include parallel NAAT and culture with AST in these rare cases.