The effect of screening-to-labor interval on the sensitivity of late pregnancy culture in the prediction of group B streptococcus colonization at labor: a prospective multicenter cohort study.

INTRODUCTION: To evaluate the effect of increasing screening-to-labor interval on the performance of group B streptococcus (GBS) screening by late pregnancy enriched culture compared to intralabor real time polymerase chain reaction (RT-PCR).

MATERIAL AND METHODS: GBS colonization was determined from 2,624 women with singleton pregnancies by culture at 35 to 37 weeks of gestation and at the beginning of labor by culture and RT-PCR from recto-vaginal swab samples.

RESULTS: GBS colonization rates were 29.0% in late pregnancy culture, 29.7% in intralabor culture, and 28.2% in intralabor PCR. Intrapartum culture used as a reference, the late pregnancy culture had an overall sensitivity of 89.2% (95% CI 88.0-90.4%) and specificity of 96.5% (95% CI 95.8-97.2%), and intrapartum PCR of 91.5% (95% CI 90.4-92.6%) and 98.5% (95% CI 98.0-99.0%), respectively. However, up to four weeks after screening the sensitivity of late pregnancy culture was equivalent or higher than that of RT-PCR. RT-PCR was invalid in 0.9% of women. Between late pregnancy screening and labor, GBS colonization changed from negative to positive in 3.2% and from positive to negative in 2.5% of women.

CONCLUSIONS: The late pregnancy enriched culture and intrapartum RT-PCR have comparable sensitivity in the detection of GBS when culture screening is conducted no more than four weeks before labor. Late pregnancy culture sampling should be postponed to at least 37 pregnancy weeks. This article is protected by copyright. All rights reserved.