Performance of a new DNA probe for the detection of group B streptococcal colonization of the genital tract

Abstract

To test the performance characteristics of a new DNA probe designed for the rapid identification of heavy colonization of the genital tract with group B streptococci. Vaginal and combined vaginal-perianal samples were collected from 193 pregnant women and cultured on colistin-nalidixic acid agar plates. Bacterial growth was classified semiquantitatively. Specimens were also tested by a new DNA probe in two formats: a direct assay performed on the swabs soon after collection and an assay performed after the swabs were incubated for 24 hours in an enriched culture medium. The agar cultures were positive in 36 of 193 patients (18.6%, 95% confidence interval 13.2–24). Nineteen women were lightly colonized, and 17 were heavily colonized (at least 10 4 colonies/mL). The combined vaginal-perianal swabs yielded positive results more often than the vaginal swabs alone (26 versus 16, χ 2 = 24, P < .01). In its direct form, the assay had only 8.3% sensitivity in identifying colonized women. In heavily colonized women, the sensitivity of the assay increased slightly to 12%. After a 16–24-hour incubation, the sensitivity of the assay was 81%. The direct assay is insufficiently sensitive for clinical use. The delayed assay offers no advantage over standard cultures.