Automated Detection of Streptococcus pyogenes Pharyngitis by Use of Colorex Strep A CHROMagar and WASPLab Artificial Intelligence Chromogenic Detection Module Software

Abstract

Colorex Strep A agar (CHROMagar, Paris, France) was evaluated with PhenoMATRIX chromogenic detection module (CDM) software (Copan Diagnostics Inc., Murrieta, CA) to detect group A Streptococcus (GAS) from throat specimens. The software results were compared to those of manual plate image reading. In addition, GAS PCR testing was performed on all specimens. True-positive specimens were defined as culture-positive (by either PhenoMATRIX CDM or manual reading) specimens confirmed as GAS by matrix-assisted laser desorption ionization-time of flight mass spectrometry plus any culture-negative specimens that were positive by both initial and repeat PCR testing. Of 480 specimens, 96 were considered true-positive specimens. Software reading of the chromogenic agar for suspected colonies detected 110 orange colonies, whereas technologist reading interpreted only 93/110 specimens (84.5%) as positive. None of the 361 cultures interpreted as negative by the PhenoMATRIX CDM software was positive by manual reading. In comparison with true-positive results, the sensitivity and specificity were 96.9% and 100% for PCR testing, 87.5% and 97.7% for technologist reading of chromogenic agar, 90.6% and 94.0% for software reading of chromogenic agar, 83.3% and 97.7% for technologist reading for β-hemolysis on blood agar, and 39.5% and 83.1% for technologist reading for β-hemolysis on blood agar accompanied by any zone of inhibition around a bacitracin-impregnated disk, respectively. The software had the most accurate results of the non-molecular testing methods, detecting all suspected colonies on the chromogenic agar and identifying 3 additional true-positive specimens that were missed by manual reading. The PhenoMATRIX CDM software and the Colorex Strep A agar can improve detection of GAS from throat specimens, and they compared favorably to molecular testing.

Keywords: WASPLab; chromogenic detection module; group A Streptococcus.

FIG 1

Example of Colorex Strep A agar growing GAS.

FIG 2

Example of orange colonies growing on Colorex Strep A agar that were not confirmed as GAS.

FIG 3

Example of very small, buried colonies in the first quadrant of group A streptococci that were detected only by the software. (a) Whole plate, with arrows pointing to two small orange colonies. (b) Magnified area (×4 magnification), with arrows showing the two orange colonies.