A comparison of the yield of positive antenatal group B Streptococcus cultures with direct inoculation in selective growth medium versus primary inoculation in transport medium followed by delayed inoculation in selective growth medium

Abstract

OBJECTIVE: Our purpose was to compare the yield of positive group B Streptococcus cultures with standard medium for transport of culture swabs compared with use of selective medium during transport. STUDY DESIGN: Cultures of introitus, perineum, and rectum were obtained on prenatal patients; one was placed in standard transport medium, and the other directly in selective growth medium. Swabs in standard transport medium were plated for routine culture and then transferred to selective growth medium, Todd-Hewitt broth, in the laboratory. RESULTS: A total of 307 of 1222 (25.1%) patients had a positive result by any method. With direct inoculation into selective growth medium at the time of sampling, 4.6% of positive cultures were missed. With delayed inoculation into selective growth medium, 16.3% were missed ( p < 0.001). Without use of selective media (routine culture), 31.9% were missed ( p < 0.001). CONCLUSIONS: Use of standard transport medium with subsequent transfer into selective growth medium results in a significantly decreased yield of positive group B Streptococcus cultures. (Am J Obstet Gynecol 1996;175:155-7.)