What is the purpose of disk diffusion quality-control strains in testing?

• A. To ensure the test system yields expected zone diameters for reference strains.
• B. To measure exact MIC values in patient samples.
• C. To determine the pharmacokinetics of the antibiotic.
• D. To adjust the antibiotic concentration.

Answer: (A)

Disk diffusion quality-control strains verify that the testing system is performing correctly by producing expected zone diameters for reference strains. These reference strains have well-established zone-size ranges for each antibiotic, so running them under the same conditions as patient isolates checks every part of the process—media quality, antibiotic potency, disc storage, the inoculum density, and the incubation environment. When the observed zones fall within the approved ranges, the run is considered valid and patient results can be trusted. If they don’t, it signals a problem—such as degraded discs, incorrect inoculum, or incubation issues—and the test should be investigated or repeated. The purpose is to ensure reliability and accuracy of the assay, not to measure exact MIC values or adjust antibiotic concentrations.