The overall prevalence of GBS colonization varies depending on the studied population and the test used for screening. This variability may be related to various climatic, biological, sociocultural, geographic, and methodological determinants [12, 13]. In Brazil, prevalence has been reported to range from 9 to 36% [13–15]. In one Brazilian study  that compared culture and PCR, only 9.5% of samples were positive for GBS by culture, while 32.6% were positive when using PCR methods. Our study population was restricted to patients of a public hospital that serves as a referral center for high-risk pregnancies, which may explain our finding of a much higher prevalence (51.1%) than is usually reported in the literature, considering the qPCR method. In fact, this is one of the highest prevalence values ever reported among Brazilian women. In a 2011 study  conducted at the same hospital as the present investigation but using the conventional PCR agarose gel technique, the prevalence of GBS was 27.0%—much lower than that found in the present study. However, this difference can be justified by the higher sensitivity of qPCR.
In a study  carried out among women in the Southeast region of Brazil, the prevalence of GBS by the culture method was around 18%, although only vaginal samples were collected. In an Italian sample of pregnant women , about 20% were GBS-positive with the culture method. The prevalence of GBS by culture in the present study (14.3%) was similar to the results of previous studies [4, 14, 16]. The low prevalence of GBS positivity by culture methods as compared to other modalities may be justified by issues of technical execution, as the culture technique does not always follow CDC recommendations, or perhaps because this method has a much lower sensitivity than PCR-based methods.
According to the CDC , cultures are the gold-standard method for GBS screening in pregnant women at 35–37weeks of gestational age. The CDC guidelines also cite other laboratory tests for GBS detection, including PCR methods. PCR-based approaches are gaining a promising role in GBS detection, largely due to their higher sensitivity [6, 13, 18, 19]. A European consensus statement noted that failure to treat GBS-positive mothers could lead to serious adverse neonatal outcomes. Thus, it seems reasonable to consider methods with higher sensitivity even if they are associated with more false-positive results.
On comparison of the Xpert GBS to qPCR, we found a high degree of agreement on negative results (93% specificity), but only reasonable agreement on positive results (53% sensitivity). These discordant results can be justified by the higher sensitivity of the combined sample enrichment/qPCR method. As the maternal pathogen load that characterizes actual risk of neonatal infection is unknown, it is unclear whether a real need exists to increase the sensitivity of rapid tests or if their current parameters are sufficient to support clinical decision-making.
Considering culture as the gold standard, the Xpert GBS showed a sensitivity of 62% and a specificity of 76% in this study. According to Gavino , the sensitivity and specificity of the Xpert GBS were 95.8% and 64.5% respectively, while those of antenatal cultures were 83.3% and 80.6% respectively. Mueller  found a sensitivity of 85.7% and a specificity of 95.6% for the Xpert GBS compared to culture. These divergent results suggest that additional studies are needed to evaluate this method.
In a previous evaluation of the performance of the Xpert GBS rapid test when performed intrapartum, 13.4% of performed tests failed to yield a valid result on the first attempt (7.3% erroneous, 4.4% invalid, and 1.6% yielded no result). Another study  reported an invalid result rate of 13.6%, while Mueller  reported 13.4% after a 2-hour training session on thermocycler operation. In the present study, 90.4% of tests were valid; the remainder were 0.4% inconclusive, 7.8% erroneous, and 1.5% yielded no result. Although some of the errors found in this study may be justified by known problems with a batch of Xpert cartridges, the percentage of invalid tests is consistent with previous reports in the literature.
Positive GBS test results were not related to neonatal sepsis in this study. Considering that GBS infection can be very serious and affects approximately 2% of newborns whose mothers are colonized, a larger sample would almost certainly be needed to demonstrate this association; more probably, the absence of association found in this sample suggests that intrapartum antibiotic prophylaxis is effectively preventing neonatal infection by GBS.