Direct detection of group B streptococci from vaginal specimens compared with quantitative culture

Abstract

Determination of prenatal vaginal carriage of group B streptococci (GBS) is important in the management of newborns. A pronase extraction-latex particle agglutination method (Streptex; Wellcome Diagnostics, Dartford, England) was used to rapidly detect GBS species-specific antigen directly from vaginal specimens. It was compared with quantitative and broth enrichment cultures. A total of 434 vaginal swab specimens were obtained before delivery. GBS cultures were positive for 14.7% of the specimens (64 of 434). Colony counts ranged from 2 to >106 CFU per swab. The sensitivities of the direct antigen analysis were 19% (12 of 64) for all cultures and 63% (12 of 19) for specimens heavily colonized with GBS (>104 CFU per swab). The specificity of the antigen test was 99.7%, with only one false-positive. There were three false-negative tests with colony counts of >106 CFU per swab. The predictive values were 92% for a positive antigen test and 88% for a negative antigen test. The direct immunochemical detection of GBS antigen can be useful in a population of heavily colonized women. Direct latex particle agglutination does not appear to be salutary for a lightly colonized population and does not appear to be able to replace either culture or antigen detection after growth amplification at this time.