Rapid diagnosis by antigen detection for groupB Streptococcus carriage in pregnant women

Abstract number: P1153

Raglio A., Arosio M., Grigis A., Peretto M., Fanti D., Latino M.A., Goglio A.

Objectives: In 2002, the Centers for Disease Control and Prevention (CDC) recommended that all pregnant women be screened for carriage of group B Streptococcus (GBS) at between 35 and 37 weeks of gestation by pre-enrichment overnight in Todd-Hewitt broth followed by subculture on a blood agar plate. Not all women, however, perform the test during pregnancy. Rapid methods are available for screening at the time of birth. The purpose of our study was to evaluate the performance of an immunochromatographic test to detect GBS antigen in pregnant women 24 hours before standard culture method.

Methods: Between 1 May and 30 September 2008, 176 pregnant women were monitored, antenatally or at delivery, by placing 142 vaginal and 153 rectal swab specimens into Todd-Hewitt broth with nalidixic acid and colistin (Biomerieux). After overnight incubation, the broth was subcultured onto ChromId Agar strepto B (Biomerieux) and the detection of GBS antigen was performed directly from the enrichment broth, according to the kit procedure (Bionexia GBS). The discordant results were analyzed by a real-time PCR, GeneXpert GBS assay (Cepheid).

Results: We examined 176 pregnant women, 63 positive and 113 negative for GBS culture. Antigen confirmed all the culture positive results. Among 113 negative cultures, 9 cases were positive by antigen. Taking as reference the culture, antigen showed 100% sensitivity, 92.1% specificity, NPV 100% and PPN 87.5%.

We tested by PCR 7 samples with discordant results: 4 resulted positive and 3 negative.

Conclusions: The detection of GBS antigen by immunochromatographic test (Bionexia GBS) allowed to identify all the positive cases and showed a NPV of 100%. PCR results suggested that the immunochromatographic test may be more sensitive than culture and allowed to a specificity increase. The reading of antigen was always easy, only in 6 cases with the growth of 3–4 GBS colonies onto Agar ChromId strepto B, the colorimetric reaction of sample-line resulted weak and this could lead to equivocal interpretation. GBS antigen detection was rapid, reliable, easy-to-perform and able to identify all the colonised pregnant women already within 24 hours after sample collection.