Evaluation of a chromogenic medium, CHROMagar StrepA, for the detection of Streptococcus pyogenes in throat swab...

Streptococcus pyogenes is a common bacterium that causes a wide spectrum of diseases, ranging from mild pharyngitis to life-threatening invasive infections. Rapid and accurate detection is essential for optimal patient management. In this study, the performance of a chromogenic medium, CHROMagar StrepA, was evaluated for the detection of S. pyogenes in throat swab samples. The evaluation was conducted in three successive phases. Phase 1 involved testing using pure bacterial isolates. Phase 2 consisted of a semi-quantitative comparison with conventional culture on COL-S medium (Columbia agar supplemented with sheep blood). Phase 3 assessed diagnostic performance in routine clinical practice, in parallel with a rapid antigen detection test (OSOM Strep A Test). Diagnostic performance metrics, including sensitivity, specificity, positive predictive value, and negative predictive value, were calculated. CHROMagar StrepA demonstrated 100% specificity and sensitivity for the detection of S. pyogenes when compared with conventional culture. It enables reliable, visually distinctive identification of S. pyogenes in culture, although certain non-Group A Streptococcus (GAS) streptococcal strains remain challenging to detect. These results support the use of CHROMagar StrepA as a culture-based approach for detecting S. pyogenes, while identifying limitations in the detection of non-GAS streptococci and selectivity that may require further optimization.

Accurate identification of Streptococcus pyogenes is essential for the appropriate management of patients with sore throat and for preventing complications. While rapid tests provide quick results, they may miss infections when bacterial levels are low, making culture methods still necessary in many cases. This study evaluates a chromogenic culture medium designed to simplify the detection of S. pyogenes by producing easily recognizable colonies. By improving visual interpretation and reducing reliance on technical expertise, this approach may help clinical laboratories work more efficiently and confidently. The findings provide practical insight into the strengths and limitations of a chromogenic medium compared with conventional culture and rapid testing, supporting informed decisions about its use in routine diagnostic practice.