GBS screening is strongly recommended for pregnant women at 34-36 weeks or at the time of delivery. Although PCR based rapid tests may become the standard of care test because of their high sensitivity, specificity and rapid turnaround time (TAT), faster turnaround time is not required for ante partum screening of GBS and cost becomes an important consideration.
We verified the Illumigene GBS assay (Meridian) and compared it with the use of Carrot Broth (enrichment broth) and the Brilliance GBS agar (Thermo Fisher) for the detection of GBS from pregnant women.
The Illumigene GBS assay is based on the loop-mediated amplification technology. This assay targets a highly conserved 213 base pair sequence of the Streptococcus agalactiae genome. 239 samples of LVS/vaginal swabs were inoculated in the Carrot Broth (enriched broth culture). After 18-24 hours incubation the broths were tested with Illumigene assay and subculture onto the Brilliance GBS agar incubated at 37C overnight. Carrot broth will produce orange or brick red colour if haemolytic strain of GBS is present.
Illumigene assay has 100% sensitivity and 93.4% specificity. Brilliance GBS agar has 100% sensitivity and 93.43% specificity. Observable colour change in Carrot broth has 70.4% sensitivity and 94.9% specificity.
Although Illumigene assay has very good sensitivity, specificity and better TAT, the extra cost is not justified to our laboratory to perform this assay. The use of enriched broth and chromogenic agar is sufficient to detect GBS from ante partum women screening. Conventional culture by chromogenic agar is still cost and labour saving.