Comparison of two culture media and three sampling techniques for sensitive and rapid screening of vaginal colonization by group B streptococcus in pregnant women.

The Centers for Disease Control and Prevention (CDC) recommend universal screening of all pregnant women between 35 and 37 weeks of gestation for group B streptococci (GBS) by use of a selective broth medium. Recent reports suggest that Granada medium can be used for rapid and direct visual identification of GBS colonies. However, studies comparing the Granada medium method to the selective broth method are few, and while some report comparable sensitivities, others have found significant differences in detection rates between the two methods. This prospective study compared a method using Granada agar to a Todd-Hewitt broth method with subculture to blood agar in order to determine which GBS detection method is more sensitive and less labor-intensive and has a more rapid turnaround time. Detection rates for three sampling techniques (rectovaginal, vaginal only, and cervical only) were also compared. Consecutive specimens for GBS screening received over a 6-month period from 1,635 pregnant women were included. Overall, GBS was detected in 390 (23.8%) women. The Granada medium gave positive results for 348 of these women, and the selective broth gave positive results for 385, indicating sensitivities of 89.2% for the Granada medium and 98.7% for the selective broth. These findings show that the Granada medium method is less sensitive than the selective broth method and should not replace it as the only method for screening pregnant women for GBS. However, the Granada medium method reduced detection time to 1 day and also reduced the use of ancillary tests in approximately 90% of positive cases. Additionally, no significant differences were noted in the detection rates with rectovaginal, vaginal, and cervical specimens.