Rapid GBS Assay Is Equal to Standard Cultures : The polymerase chain reaction-based diagnostic costs more, but cuts turnaround time from days to hours.


NEW ORLEANS — A rapid polymerase chain reaction-based diagnostic to detect group B streptococcus infection during pregnancy is as specific and sensitive as are standard cultures and delivers results in a matter of a few hours, according to a small comparative study.

Dr. Lori Goranson, a resident at Dartmouth-Hitchcock Medical Center in Lebanon, N.H., presented data from 99 women who came to the center's outpatient clinic.

The Centers for Disease Control and Prevention has issued a call for a rapid assay for group B strep that is available 24 hours a day, 7 days a week, and that can be easily performed without a sophisticated lab or highly trained personnel, Dr. Goranson said at the annual meeting of the American College of Obstetricians and Gynecologists.

The agency recommends that all women be tested for GBS by 35–37 weeks' gestational age, she said. Laboring women who present with unknown GBS status, however, are empirically given antibiotics as a precaution. As a result, thousands of women are likely being treated unnecessarily, Dr. Goranson said.

There are a little more than 4 million live births a year in the United States, she said.

Given that 10%–30% of women are colonized with GBS, that 13% of women present preterm, and that 8% are without prenatal care, as many as 600,000 may inappropriately receive antibiotics, according to Dr. Goranson.

She and her colleagues at Dartmouth aimed to determine whether the Xpert GBS test could fulfill the CDC's parameters. The Xpert was approved by the Food and Drug Administration in 2006. Two vaginal/rectal swabs were taken from 99 women; one swab was tested using standard culture and the other with the Xpert device.

The swab is placed into a cartridge with reagents. It takes 2 minutes from sampling to the start of analysis, said Dr. Goranson. Results generally are available within 75 minutes, although the average in the Dartmouth study was 83 minutes. That compares with an average of 2.3 days to receive a culture result, she said.

The average maternal age was 29.7 years and the average gestational age at collection was 36 weeks.

Twenty-five of 99 specimens cultured positive, for a prevalence rate of 25%, which was consistent with the literature, noted Dr. Goranson.

Seventy-four of 99 (75%) cultured negative. With the PCR-based Xpert system, 27 of 99 specimens were positive, and 72 of 99 were negative. The overall agreement rate between the two tests was 96%. There were discordant results. Three of the four PCR-positive results cultured negative. One PCR-negative swab cultured positive.

The Xpert test had 96% sensitivity and 96% specificity, with a negative predictive value of 99%, and a positive predictive value of 88%.

The test is highly sensitive and specific, easy to use, and produces rapid results, concluded Dr. Goranson. Though they are not yet using the Xpert system clinically at Dartmouth, it could eventually be used to support standard culture or as a substitute for women presenting preterm with an unknown GBS status, she said.

It is more expensive than standard culture—probably about 1.5 times as much, said Dr. Goranson. The device costs about $65,000 and cartridges cost about $45 each, she said. But the Xpert system can also be used to conduct rapid enterovirus, methicillin-resistant Streptococcus aureus, and other diagnostics, she said.

Dr. Goranson stated that she had no financial conflicts of interest to report.

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