Modified use of real-time PCR detection of group B Streptococcus in pregnancy.

The CDC recommends antenatal screening of vaginal/rectal samples for Streptococcus agalactiae at 35-37 weeks' gestation, with intra-partum antibiotic prophylaxis for positive cases. We tested a modified use of the Cepheid Xpert GBS real-time PCR kit on enrichment cultures from 554 vaginal/rectal swabs compared to the current subculturing gold standard method. Swabs were inoculated on polymyxin nalidixic acid agar plates, and Todd-Hewitt enrichment broth cultures were examined daily for growth. Todd-Hewitt broth culture was also used for Xpert GBS. There was 92.06 % agreement between the subculture and PCR methods. Sensitivity of Xpert GBS was 100 %, specificity was 89.40 %, positive predictive value was 75.96 % and negative predictive value was 100 %. Colonization incidence was higher with younger (≤24 years) or older (≥35 years) maternal age. Modified use of the Cepheid Xpert GBS would assist rapid diagnosis of S. agalactiae colonization and facilitate timely and appropriate assignment to intra-partum antibiotic prophylaxis.