Effect of delayed inoculation of selective media in antenatal detection of group B streptococci

Abstract

Objective: To determine accuracy of group B streptococcal culture swabs immediately inoculated after sampling compared with swabs transported to the laboratory and inoculated subsequently. Methods: Lower genital tract specimens were collected by sweeping two rayon-tipped swabs across the lower vagina and rectum of gravidas at 35–37 weeks’ gestation. One swab was placed immediately in selective growth medium (immediate inoculation group). The other was placed in standard transport media, sent routinely to the clinical laboratory, and transferred subsequently to selective growth medium within 2 hours (delayed inoculation group). Results: Matched specimens were collected from 374 women during the study period. Ninety-three women had positive cultures, a colonization rate of 24.9%. Concordant results were found in 364 of 374 (97.3%). Six women had negative results by the immediate method but positive by the delayed method, and four women had positive immediate cultures but negative by the delayed method. There was no statistically significant difference between the two methods in the detection of positive cultures. Conclusion: Use of immediate inoculation instead of delayed inoculation in culturing group B streptococcus specimens does not result in an improved detection rate.