Detection of group B streptococcus: comparison of an optical immunoassay with direct plating and broth-enhanced culture methods.

The aim of this study was to compare the diagnostic accuracy of an optical immunoassay (STREP B OIA, Biostar) to direct plating and broth-enhanced culture for the detection of group B streptococcus (GBS) colonization of the lower genital tract in pregnant women. GBS cultures from the lower genital tract were obtained in a prospective fashion using a dual swab transport system from patients with risk factors for perinatal GBS infection. One swab was used to inoculate a trypticase soy agar plate with 5% sheep blood (TSA) and then placed in Lim broth. The other swab was used to perform the Strep B OIA. Growth of GBS by either direct plating or broth-enhanced culture was used as the gold standard for determining GBS colonization. Of the 524 women in the study, 90 women had positive cultures (either TSA or Lim broth). The sensitivity, specificity, positive predictive value, and negative predictive value of the Strep B OIA were 47% (42/90), 96% (416/434), 70% (42/60), 90% (416/464). The sensitivity, specificity, positive predictive value, and negative predictive value of the TSA were 61% (55/90), 100% (434/434), 100% (55/55), 93% (434/469). The sensitivity, specificity, positive predictive value, and negative predictive value of Lim broth were 97% (87/90), 100% (434/434), 100% (87/87), and 97% (434/437). The sensitivity of the Strep B OIA to detect light GBS colonization and heavy GBS colonization, as determined by the TSA, was 53% (19/36) and 90% (17/19), respectively. The Strep B OIA and direct agar plate culture appear to be of limited clinical value due to their poor sensitivities. This study also demonstrates the need to use a selective medium such as Lim broth when assessing for GBS colonization of the lower genital tract.