Rapid intrapartum detection of group B streptococcal colonization with an enzyme immunoassay.

OBJECTIVE

The purpose of this study was to determine the accuracy of a rapid immunoenzyme assay for detecting intrapartum colonization with group B streptococci.

STUDY

DESIGN

Three rayon-tipped swabs were used to collect specimens from the posterior vaginal wall of 424 preterm and term patients in labor. Three tests were performed on specimens obtained from the first 182 patients: semiquantitative culture on blood agar, culture in selective Todd-Hewitt broth, and ICON Strep B (Hybritech, San Diego) immunoconcentration assay. For the remaining 242 patients, the ICON test was performed only when the Todd-Hewitt broth culture was positive.

RESULTS

The prevalence of positive cultures was 23%. For the first 182 patients, the immunoassay had a sensitivity of 11%, a specificity of 100%, a positive predictive value of 100%, and a negative predictive value of 78%. The overall sensitivity for all 424 patients was 9%. In eight women with heavy colonization, defined as an inoculum of > 10(5) cfu/ml the sensitivity of the assay was 100%. In the study population there were three cases of group B streptococcal sepsis in infants whose mothers were only lightly colonized. None of these cases of colonization were detected by the assay.

CONCLUSION

The ICON immunoconcentration assay is very sensitive in identifying heavy group B streptococcal colonization of > 10(5) cfu/ml but quite insensitive in detecting lower levels of colonization. Thus it is not a suitable test for general screening.