Preincubation of Cervical Swabs in Lim Broth Improves Performance of ICON Rapid Test for Detection of Group B Streptococci

Abstract

Objective: The purpose of this study was to determine whether an enrichment method would improve the performance of an enzyme imunoassay test, the ICON Strep B, for detection of group B streptococci (GBS) in vaginoperineal swabs. Methods: The study was done in 3 phases. First, in 250 maternity patients, 2 swabs per patient were tested simultaneously by an overnight selective broth culture method (Lim broth) and the ICON assay. Forty-five (18%) specimens were positive for GBS by culture. The ICON assay detected only 2 (4%) of the positives. Second, in 391 maternity patients, a single swab was cultured as above. However, during the overnight incubation of the Lim broth, 0.5 ml aliquots were removed and tested by ICON assay at 4, 6, 8, 10, and 12 h post-incubation. Seventy-two specimens (18%) were positive by culture. The ICON assay detected 20% of the positives at 4 h, 46% at 6 h, 70% at 8 h, 94% at 10 h, and 100% at 12 h post-incubation. Third, 97 high-risk patients with the diagnosis of preterm labor (PTL)/or preterm premature rupture of the membranes (PPROM) were sampled. Three specimens per patient were obtained: a single swab that was cultured as before and 2 double swabs, of which 1 was tested directly using the ICON test and the other was placed directly in Lim broth and incubated overnight. The aliquots of broth were tested by the ICON assay at 2, 4, 6, and 8 h post-incubation. Twenty-four specimens were positive by culture. Results: The direct ICON test detected only 4 (17%) of the positives. The ICON assay performed on the enriched samples detected 4% of the positives at 2 h, 21% at 4 h, 58% at 6 h, and 100% at 8 h post-incubation. Conclusions: These data indicate that the ICON assay may be used with 100% sensitivity and specificity to detect GBS-colonized high-risk mothers within 8 h if the initial sample size is doubled and the enrichment broth is used in the performance of the ICON assay.