MONTEREY, CALIF. — A newly approved rapid test for group B streptococcus colonization in pregnancy can be performed by labor and delivery nurses and generates results in about an hour and a half, Dr. Rodney K. Edwards reported.
The Xpert GBS assay is the first rapid test approved for group B streptococcal (GBS) screening at the point of care and may improve GBS detection and prophylactic treatment at the time of labor, potentially reducing the incidence of early-onset neonatal GBS infection, he said at the annual meeting of the Infectious Diseases Society for Obstetrics and Gynecology.
The test was approved by the Food and Drug Administration on July 25 and is commercially available now.
Conventional GBS screening by culture at 35–37 weeks' gestation misses subsequent colonization. Results aren't available for 14–48 hours, so culture isn't helpful in assessing GBS colonization in previously unscreened women in labor. Because GBS colonization can be intermittent, culture results from 35–37 weeks' gestation have a positive predictive value of 67%–85% for GBS colonization at the time of labor.
A previously approved rapid GBS test, the IDI-Strep B test, must be analyzed in laboratories, which may not be available around the clock, added Dr. Edwards of the University of Florida, Gainesville. He has been a speaker for Cepheid, the company that makes both rapid GBS assays and that funded the study.
The Xpert GBS assay compared favorably with culture and had greater sensitivity and a better negative predictive value than the IDI-Strep B test in a prospective study of 784 pregnant women seen at six medical centers. Vaginal/rectal swabs from each patient were analyzed by Xpert GBS assay, culture, and IDI-Strep B test. Labor and delivery nurses performed the Xpert GBS assay on 548 women in labor. Laboratory workers analyzed swabs from these patients by culture and IDI-Strep B test, and used all three screening tests on samples from the 418 intrapartum patients.
The prevalence of GBS colonization was 24%. Compared with culture, the Xpert GBS assay was 91% sensitive, which is above the Centers for Disease Control (CDC) and Prevention's recommendation that a rapid intrapartum screening test for GBS be at least 85% sensitive, he noted. The 95% confidence interval for the Xpert GBS assay's sensitivity did not cross 85%.
The assay had a specificity of 96%, a positive predictive value of 88%, a negative predictive value of 97%, and an accuracy rate of 95% compared with culture.
The IDI-Strep B test's 79% sensitivity and 94% negative predictive value compared with culture were significantly less accurate than the results obtained by the Xpert GBS assay. The IDI-Strep B test's 95% specificity, 84% positive predictive value, and 92% accuracy rate were comparable to results in those categories from the Xpert GBS assay.
The Xpert GBS assay will cost $45 per test. “Whether or not that is something worth doing at that price, that's up to interpretation,” Dr. Edwards said. Although the cost is higher than for culture, “I think it compares favorably to other rapid tests that we perform on labor and delivery units such as fetal fibronectin.”
The test is made to be processed using a GeneXpert Dx system, which costs about $20,000.
One physician in the audience suggested that replacing culture screening with Xpert GBS screening would require doing an intrapartum assay on every woman. “It's a paradigm shift on labor and delivery” units, he said.
Dr. Edwards said that initially the assay would be used for women in labor without a prior screening culture—“people who come in for premature rupture of membranes or preterm labor, or unregistered patients,” he suggested. An eventual replacement of the assay for the current screening strategies could significantly increase the work of labor and delivery nurses. The nurses at his institutions liked doing the assay in the study, however, because they felt that it improved clinical care. “Our nurses now miss it and continue to ask me, 'When is that machine coming back?'”
The assay is a qualitative, automated real-time polymerase chain reaction (PCR) test with fluorogenic detection of the amplified DNA. Unlike other PCR tests, it doesn't require that the sample be separately prepared and is designed to purify, concentrate, detect, and identify targeted nucleic acid sequences from unprocessed samples.
The study's analysis excluded results from another 244 swabs—12 from patients who were enrolled more than once, 10 from patients with “unresolved” results after two attempts at Xpert GBS assay, and 222 that were vaginal/perianal swabs instead of vaginal/rectal swabs recommended by the CDC.
The investigators did analyze results from the excluded swabs, however, and found that the Xpert GBS assay was significantly less sensitive using vaginal/perianal swabs, compared with vaginal/rectal swabs. “I have no idea why this is the case. It doesn't make sense to me,” and an additional study is planned comparing screening of vaginal/rectal and vaginal/perianal samples, he said. The sensitivity of culture did not differ significantly between types of swabs.