Research Article: Intrapartum PCR assay versus antepartum culture for assessment of vaginal carriage of group B streptococci in a Danish cohort at birth

Date Published: July 5, 2017

Publisher: Public Library of Science

Author(s): Mohammed Rohi Khalil, Niels Uldbjerg, Poul Bak Thorsen, Jens Kjølseth Møller, Jose Melo-Cristino.


The aim of this study was to compare the performances of two strategies for predicting intrapartum vaginal carriage of group B streptococci (GBS). One strategy was based on an antepartum culture and the other on an intrapartum polymerase chain reaction (PCR). We conducted a prospective observational study enrolling 902 pregnant women offered GBS screening before delivery by two strategies. The Culture-strategy was based on vaginal and rectal cultures at 35–37 weeks’ gestation, whereas the PCR-strategy was based on PCR assay on intrapartum vaginal swab samples. An intrapartum vaginal culture for GBS was used as the reference standard from which the performances of the 2 strategies were evaluated. The reference standard showed a GBS-prevalence of 12%. The culture-strategy performed with a sensitivity of 82%, specificity of 91%, positive predictive value (PPV) of 55%, negative predictive value (NPV) of 98%, and Likelihood ratio (LH+) of 9.2. The PCR-strategy showed corresponding values as sensitivity of 83%, specificity of 97%, PPV of 78%, NPV of 98%, and LH+ of 27.5. We conclude that in a Danish population with a low rate of early-onset neonatal infection with GBS, the intrapartum PCR assay performs better than the antepartum culture for identification of GBS vaginal carriers during labor.

Partial Text

Even though early-onset neonatal infection with Group B streptococci (EOGBS) is rare, it still constitutes a health problem in countries where the prevalence of EOGBS disease is 2 in 1,000 live births, and the mortality rate is 50% [1]. As EOGBS [2] occurs only among the group of neonates who are born by the 10–35% of women colonized vaginally with GBS [3–8], the Centers for Disease Control and Prevention (CDC), USA, in 2002 recommended universal culture screening of all pregnant women between 35 and 37 weeks’ gestation in order to give intrapartum antibiotics to the screen positives [9, 10]. The implementation of this strategy was followed by a decrease in the EOGBS rate from 1.5 to 0.4/1,000 live births [9]. This decrease must be categorized as a success, however, one might wonder why the EOGBS rate in some other countries including Denmark is only 0.1–0.4/1,000 live births [11] even though they have not implemented this antepartum culture-based screening program.

A total of 2,343 pregnant women attending the prenatal clinic at Lillebaelt Hospital, Kolding, Denmark (with an average of 3,200 deliveries per year) over a 15 months’ period between April 2013 and June 2014 were invited to participate in this prospective observational study. One thousand three hundred sixty-four (n = 1,364) declined to participate, leaving 979 participants in the final cohort (Fig 1). Detailed information on oral antibiotic use during pregnancy was obtained from registered data in both patient hospital records and the Danish Medical Agency’s Register of non-hospitalized patient use, which included records on all drug prescriptions filed at any Danish pharmacy. Five patients received antibiotics after week 35 of gestation and were therefore excluded (Fig 1). Further, sixty women withdrew from the study at the time of birth for various reasons. Twelve were lost for follow-up. Thus, 902 sets of patient samples were available for comparisons between antepartum culture (culture-based screening) and PCR analysis (PCR-based screening) with intrapartum culture.

All 979 enrolled women had an antepartum swab obtained as part of the culture-strategy and 902 (92%) had an intrapartum swab as part of the culture-based reference standard and the PCR-strategy. The intrapartum vaginal GBS colonization rate detected by culture was 11.5% (reference standard). By comparison, the culture-based strategy found 9.4% (85/902) GBS-positive women by combining results from antepartum vaginal and rectal swab cultures (7.4% by vaginal swab samples and 8.9% by rectal samples) (Table 1), and the PCR-strategy (intrapartum vaginal swab sample) found 12.2% GBS-positive women (Table 2).

We evaluated two screening strategies for identification of vaginal GBS colonization in a Danish cohort of laboring women, using an intrapartum culture as the reference standard. The antepartum culture-strategy achieved a LR+ of 9.2, whereas the intrapartum PCR-strategy achieved a LR+ of 27.5.