Accuracy of rapid antigen detection test for group B streptococci in the indigenous vaginal bacterial flora

To test the sensitivity of the rapid group B streptococci (GBS) antigen test ICONR and compare its accuracy in women with vaginal enterococci or with non-specific disturbance of the lactobacillary flora.

The ICONR, aerobic culture and a microscopic wet mount evaluation were done on a vaginal sample in 254 unselected women presenting for routine gynecologic care in an academic hospital in Flanders, Belgium and tested by Chi2 [diagnostic odds ratio (DOR) and its 95 percent confidence limits].

Sensitivity of the test was 70%, specificity 99.5%. Prevalence of GBS was 10.6% overall, 23% in the group with abnormal vaginal flora and 7% in the normal group (p=0.002). Accuracy of the ICONR was not affected by abnormal vaginal flora, but was significantly lower in the presence of enterococci: the DOR decreased from 490 to 58, and the positive predictive value from 94 to 80%.

With a sensitivity of 70% the enzyme immunoassay ICONR does not appear to be suitable as a practical screening tool for detecting GBS carriage in normal or preterm laboring women. In the presence of enterococci the test performed less well, with a DOR falling by 8 to 9 fold. We presume this is due to lower specificity in vivo in the presence of enterococci, as non-specific disturbance of the lactobacillary flora did not interfere with test results.