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 The aim of this study was 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 with intrapartum real‐time polymerase chain reaction (RT‐PCR). Material and methods Group B streptococcus colonization was determined in 2624 women with singleton pregnancies by culture at 35‐37 weeks of gestation and at the beginning of labor by culture and RT‐PCR from recto‐vaginal swab samples. Results Group B streptococcus colonization rates were 29.0% in late‐pregnancy culture, 29.7% in intrapartum culture and 28.2% in intrapartum PCR. Intrapartum culture was 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 had sensitivity of 91.5% (95% CI 90.4%‐92.6%) and specificity of 98.5% (95% CI 98.0%‐99.0%). However, up to 4 weeks after screening, the sensitivity of late‐pregnancy culture was equivalent to or higher than that of RT‐PCR. The RT‐PCR was invalid in 0.9% of the 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 the women. Conclusions The late‐pregnancy enriched culture and intrapartum RT‐PCR have comparable sensitivities in the detection of GBS when culture screening is conducted no more than 4 weeks before labor. Late‐pregnancy culture sampling should be postponed to at least 37 weeks of gestation.