Sensitivity of the BD MAX group B Streptococcus assay increases with direct testing of specimens collected in Copan ESwab collection devices vs. enriched culture specimens
Publication Date: 06/17/2015
Conference or Journal: Presented at 2015 ECCMID, Copenhagen
Author(s): Eric T. Beck, Garrett C. Reymann, Blake W. Buchan, and Nathan A. Ledeboer
US Centers for Disease Control guidelines indicate that molecular assays for group B streptococci (GBS) be performed on enriched culture specimens, which negates the rapid turnaround time of these assays. The purpose of this study was to evaluate whether specimens collected with ESwab collection devices ([ESwab] regular flocked swab with 1 mL liquid Amies transport medium; Copan Diagnostics Inc, Murrieta, CA) can be used for directl testing in the BD MAX™ GBS Assay ([MAX] BD Diagnostics, Quebec, Canada). In this study, mock specimens were prepared to determine the optimal input volume of liquid Amies transport medium into the MAX™ GBS Assay. In addition, studies were performed to determine whether Eswabs yield equivalent sensitivity to enriched culture specimens in the MAX™ GBS Assay.
- Pool LIM broth cultures inoculated with vaginal-rectal swabs that showed growth, but were negative for the presence of GBS..
- Prepare 0.5 McFarland preparation of GBS.
- Dilute GBS to 1.5 x 106 – 1.5 x 101 CFU/mL in the pooled GBS negative cultures.
- Prepare mock specimens by dipping Eswabs or standard collection swabs (BBL™ CultureSwab™ Dual Swab with Liquid Stuart Medium; BD Diagnostics, Sparks, MD) into dilutions.
- Test different volumes and concentrations of Eswab specimen in the MAX™ GBS Assay to determine optimal input volume.
- Prepare mock specimens with four standard collection devices and four ESwabs for each dilution.
- Inoculate standard swabs into sterile Lim broth, incubate overnight at 35°C , and test in the MAX assay according to the package insert.
- Vortex ESwabs and add 500 µL of transport medium to the sample buffer tube and test immediately in the MAX assay.
500 µL of Eswab transport medium appears to be the optimal input volume for the MAX™ GBS Assay. The limit of detection (the least concentrated dilution in which at least 3 of 4 replicates were positive) of the MAX™ GBS with ESwabs and Lim Broth cultures was 1.5 x 102 and 1.5 x 104 CFU/mL, respectively.
- Routine GBS screening is performed at 35-37 weeks gestational age, but testing closer to time of delivery would likely increase the negative predictive value of the result.
- In order to perform GBS testing at time of admission must have access to a faster, more sensitive GBS testing methodology.
- The current study shows that direct testing of Eswabs (without broth enrichment) would decrease the turnaround time of the MAX™ GBS Assay by approximately 90% (24 to 2 hours).
- In addition to decreasing TAT, Eswabs increased the sensitivity over 100 fold compared to the current methodology.
- This study was performed with mock specimens, but the results merit further clinical investigation.
*Materials and Support for this study provided by Becton Dickinson and Company