Evaluation of A New Chromogenic Group B Streptococcus Agar and Use of Digital Analysis for Detection of Group B Streptococcus in Vaginal/Rectal Swabs


Detection of Group B Streptococcus (GBS) colonization during pregnancy aids in the prevention of early onset GBS disease in newborns. A new Group B chromogenic culture medium combined with segregation software (SSW) developed for the WASPLabTM system (Copan Diagnostics) may aid in the accurate detection of GBS colonization as well as enhanced laboratory workflow management. We evaluated the LIM broth/ChromID® Strepto B agar (STRB, bioMerieux) for detection of GBS along with the WASPLab SSW for automated digital analysis for separation of negative and positive culture results. The comparative method was Carrot broth/GBS Detect medium (Hardy Diagnostics). Vaginal/rectal swabs were processed by the WASPLab system using both methods with initial inoculation into the enhancement broths which were incubated off-line at 35-37oC overnight, followed by WASP inoculation of the plates and incubation at 35-37oC for 20 h (Detect) and 24 h (STRB). WASPLab digital images of GBS Detect (beta hemolysis) and STRB (pink to red colonies considered positive) were visually examined. STRB also had digital images analyzed by the SSW to automatically segregate negatives from positives. All positives were confirmed as GBS or not GBS by phenotypic methods. There were 245 samples acceptable for comparison with 152 negative by Detect and STRB and 87 positive by both methods. There were 3 cultures positive by Detect that were negative by STRB and 2 cultures that were negative by Detect and positive by STRB. Sensitivity was 96% and specificity 99%, however the adjusted specificity was 100% with the 2 STRB positive/ Detect negative cultures considered to be true positives. The 245 STRB plate images were analyzed by SSW after initial visual examination. There were 89 STRB visually examined and determined to be positive with 100% detected as positive by SSW with no false negatives. Of the remaining 156 STRB negative plates, 124 (79%) were negative by SSW. There were 32 positives indicated by SSW that were determined to be negative for GBS by additional phenotypic testing. These were determined to be SSW false positive most likely due to blue/purple colonies with a pink hue in the medium. The STRB is equivalent to the Detect for detection of GBS. The SSW is a powerful tool for automatic separation of positive and negative cultures to help laboratory workflow.