What is the role of quality control (QC) in disk diffusion testing?

• A. To determine the identity of the organism.
• B. To verify test performance; zone diameters must fall within established QC ranges for the control strains.
• C. To measure the color of colonies.
• D. To shorten incubation.

Answer: (B)

Quality control in disk diffusion testing is about confirming that the test is performing correctly. In this method, antibiotic discs are placed on a standard agar plate inoculated with a standardized bacterial suspension, and after incubation you measure the zones of inhibition. QC uses control strains with known, established zone-diameter ranges. When the observed zones for these controls fall within those ranges, you can trust that the test conditions—media quality, disc potency, inoculum density, and incubation conditions—are appropriate, so the results for patient isolates are reliable. If QC results fall outside the accepted ranges, it signals a problem and you should troubleshoot or repeat the test to prevent incorrect susceptibility interpretations. This role is not about identifying the organism, assessing colony color, or shortening incubation; it’s about validating the test system’s performance.