Evaluation of the GeneXpert GBS assay in determining groupB streptococcus colonisation

Abstract number: R2165

Park J.S., Cho D.H., Yeo I.K., Yang J.H., Kim M.Y., Shin S.M., Joo S.I., Ra E.K., Park S.S., Kim E.C.

Objectives: It is recommended that all pregnant women be screened for group B streptococcus carriage at between 35 and 37 weeks of gestation. However intrapartum screening test for GBS could reveal more exact status of GBS colonisation at the time of delivery and would obviate the need for prenatal screening. One of the latest development in the field of the molecular diagnosis of GBS is a fully automated real-time PCR assay, the GeneXpert GBS (GXGBS; Cepheid, Sunnyvale, CA, USA), of which total analysis time is 75 min. In this study, we prospectively evaluated the performance characteristics of GXGBS and in comparison with selective culture in pregnant women. In addition, this study is designed to provide data on the rates of maternal carriage of GBS in Korean women and characteristics of isolated GBS.

Methods: We also performed analytical sensitivity, specificity and precision of GXGBS. We compared GXGBS and standard selective culture with Todd-Hewitt broth in 175 pregnant women at 35–37 week gestation. Specific identification of colonies was done by Vitek-2 and Lancefield grouping. Further serotyping was done using conventional and molecular methods.

Results: For GXGBS, the 95% detection limit was 1.41×10³ CFU/swab and no cross-reactivity was found. The CVs of within-run, between-run and total precision were 2.8%, 2.6%, and 3.3%, respectively. Detection rate of GBS carriers by GXGBS was 11.4% (20/175), compared with 8.6% (15/175) of selective culture method. The sensitivity, specificity, positive and negative predictive values of the GXGBS compared with selective culture were 86.7, 95.6%, 65.0%, and 98.7%, respectively, The most frequent serotypes were Ia, III, and V (72.2%). All strains were susceptible to penicillin, but a considerable proportion was resistant to erythromycin (44.4%) and clindamycin (44.4%).

Conclusion: GXGBS is a highly sensitive and specific tool for the detection of GBS. Simple preparation and rapid results are attractive points of this assay. GXGBS might be substitutive test for the detection of GBS in antenatal and intrapartum screening test.