Digital Detection of Group A Streptococcus using Colorex Strep A CHROMagar and WASPLab Chromogenic Detection Module


  • Despite the availability of several diagnostics tools for the diagnosis of Group A Streptococcus (GAS) pharyngitis, culture remains one of the primary methods in use today and is still considered the gold standard for the detection of GAS from pharyngeal samples.
  • However, in larger volume laboratories, screening for GAS by culture can be cumbersome and streamlined approaches using automated plating instrumentation, smart incubation and image analysis could be helpful.
    • This study evaluates the capability of the WASPLab® Total Laboratory Automation System (TLA) (Figure1) (Copan  Diagnostics, Murrieta, CA) PhenoMATRIX™ Chromogenic Detection Module (CDM) to automatically detect and interpret orange GAS colonies on a novel chromogenic agar called Colorex Strep A Agar (CHROMagar, Paris, France) To date, the Colorex Strep A Agar is not Food and Drug Administration (FDA) cleared.

There have been 2 excellent published studies demonstrating the ability of the WASPLab® CDM software in detecting and sorting positive and negative cultures based on pigmentation production on chromogenic  media


  • 250 cultures were manually examined by medical technology staff after 24 hours of incubation.
  • Plates examined by the CDM software after secondary  manual review had a sensitivity of 100% and a specificity of 96.4% (Table 1).
  • Plates examined manually by technologist (not assisted by CDM software) after secondary manual review had a sensitivity of 96.5% with a specificity of 100% (Table 2).
  • Of the 57 cultures that grew orange colonies on the Colorex Strep A Agar, 51 were confirmed as GAS by MALDI-TOF MS.
  • 6 cultures grew orange colonies; but 4 cultures were not identified as GAS and unfortunately, 2 cultures were not tested by MALDI-TOF MS.
  • 5 specimens that were initially PCR positive did not grow in culture on the Colorex A Strep Agar.
  • Isolation of GAS in blood agar plates had a sensitivity of 78.9% and specificity of 73.6%