What is the purpose of quality control in AST, and which organisms are standard QC strains?

• A. To ensure accuracy and reproducibility; standard QC strains include E. coli ATCC 25922 and S. aureus ATCC 25923
• B. To determine patient-specific susceptibility
• C. To measure antibiotic manufacturing quality
• D. To select which antibiotics to test on a given day

Answer: (A)

Quality control in antimicrobial susceptibility testing ensures that the results you report are accurate and reproducible across runs and laboratories. By running well-characterized reference strains with known susceptibility patterns, a lab can verify that all parts of the testing system—media, reagents, instruments, inoculum density, and interpretation criteria—are working correctly. If the control strains produce results within the predefined acceptable ranges, you can trust the patient results; if they don’t, the run is invalid and must be repeated with appropriate fixes.

Two standard QC strains widely used are Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923. These organisms have established MIC distributions and disk diffusion zone diameters for many antibiotics, serving as reliable benchmarks for methods like disk diffusion and broth microdilution. Their consistent, well-documented responses help detect trends or anomalies in testing conditions, ensuring the method remains accurate.

Other options describe patient-specific decisions, manufacturing quality, or choosing which antibiotics to test on a given day, which are not the function of quality control in AST.