Evaluation of two rapid tests for detection of maternal endocervical group B streptococcus: enzyme-linked immunosorbent assay and gram stain.

OBJECTIVE

To establish the reliability of two rapid tests for detection of group B streptococcus, the new enzyme-linked immunosorbent assay (ELISA) and the Gram stain. In addition, we wished to determine whether the combination of these tests would increase test sensitivity.

METHODS

During a 2-year period, cervical secretions from 660 patients who presented with either premature rupture of membranes or preterm labor at a gestational age of 37 weeks or less were tested by three methods: the Equate ELISA test, a Gram stain smear, and a culture for group B streptococcus. Results of the rapid tests were compared with the corresponding culture results.

RESULTS

Cultures were positive for group B streptococcus in 50 patients, a prevalence of 7.5%. The sensitivity for the Equate ELISA test was 24% and for the Gram stain, 20%. Specificity was 95 and 89%, respectively. When results of these tests were combined, the sensitivity increased to 26%. Both rapid tests yielded a high rate of false positives. In more than 50% of false-positive cases, bacteria other than group B streptococcus were isolated.

CONCLUSION

Neither the Gram stain nor the Equate ELISA test is sensitive enough to be used as a screening test for the identification of group B streptococcus. Furthermore, the combination of both rapid tests yielded only a slight increase in sensitivity. The high false-positive rate of the ELISA test seems to be related to a cross-reactivity of the anti-group B streptococcal antibody with other cervical bacteria.