Detection of group B Streptococcus by immunoassay following enrichment in LIM-selective broth medium

Rapid immunoassays have been developed to decrease the time to detection of Group B Streptococcus (GBS) carriage in pregnant women. In this study, a total of 162 pregnant women, considered to be high-risk obstetric patients, were seen in the Family Care Center at Memorial Hospital of Rhode Island, a 300-bed teaching hospital associated with Brown University Medical School. Vaginal and rectal specimens were taken and tested for GBS by using two rapid enzyme immunoassays (EIAs) that were compared with culture. Quidel and Hybritech ICON Group B strep tests were run following 4 h incubation in the selective enrichment LIM Group B strep broth; cultures were done both directly and after enrichment. Results with both EIAs were identical, with overall sensitivity, specificity, and positive and negative predictive values of 38%, 98%, 88%, and 84% respectively. However, when women having positive cultures were separated into moderately to heavily colonized (> or = 3 +) and lightly colonized (< or = 2 +) populations, the sensitivities were 82% and 19%, respectively. Although GBS assays are useful in the rapid diagnosis of heavily colonized women, culture following enrichment remains the most sensitive method for a lightly colonized population.