A major limitation of Group B Streptococcus (GBS) screening and prevention programs is a reliance on accurate laboratory detection of GBS carriage. However current laboratory methods lack sensitivity for GBS detection, resulting in a failure to target antibiotic prophylaxis to all infants at risk. Two thirds of neonatal GBS infections now occur in mothers who had tested negative for GBS carriage before delivery.
The aim of this study was to develop and evaluate in house PCRs targeting the sip and cfb genes of GBS for the detection of GBS carriage in late pregnancy and compare this to results from standard enrichment culture. A prospective trial of 1034 vaginal/rectal screening swabs was performed. Samples were incubated in enrichment broths for 24hrs prior to subculture on ChromID plates and PCR of the broth. The overall detection rates for culture, sip PCR and cfb PCR, were 174 (16.8%), 224 (21.6%) and 200 (19.3%) respectively, where a ct value of <36 was defined as PCR positive. All cfb PCR positives were also positive by sip PCR. Of the 24 positive only by sip, 10 were equivocal (ct 36-<40) by cfb PCR. An additional 25 were sip PCR equivocal, with 6 of these also being cfb PCR equivocal and 1 being cfb negative but culture positive. Of the 174 culture positives, 170 (98%) were positive by both PCRs. One of the 4 only positive by culture was sip PCR equivocal while 2 could not be confirmed on repeat culture. The final sample was not retested.
These results demonstrate that antenatal GBS detection by PCR is superior to enrichment culture with up to 22% of carriage only detected by PCR. Enhanced detection of GBS by PCR should further reduce the incidence of perinatal GBS infection.