Which statement best differentiates disk diffusion from MIC testing?

Which statement best differentiates disk diffusion from MIC testing?

• A. Disk diffusion yields MIC values; MIC testing yields qualitative S/I/R.
• B. Disk diffusion yields qualitative S/I/R by zone diameters; MIC testing yields numeric MIC values.
• C. Breakpoints are not used with either method.
• D. E-test does not provide MIC values.

Answer: (B)

The key idea is how the two methods report results and how those results are interpreted. In disk diffusion, you place antibiotic disks on a bacterial lawn and measure the zone of no growth around each disk. The size of that zone is then compared to established breakpoints to categorize the isolate as susceptible, intermediate, or resistant. This makes the result qualitative because you’re reporting a category based on zone diameter, not a specific concentration. In contrast, MIC testing determines the lowest concentration of antibiotic that prevents visible growth, giving a numeric MIC value (for example, 0.25 μg/mL). That numeric value is then interpreted against breakpoints to decide S/I/R, but the essential output of the test is the actual MIC number, not a zone. Breakpoints are used with both methods to interpret results, and E-tests do provide MIC values by showing where growth is inhibited along a strip.

The key idea is how the two methods report results and how those results are interpreted. In disk diffusion, you place antibiotic disks on a bacterial lawn and measure the zone of no growth around each disk. The size of that zone is then compared to established breakpoints to categorize the isolate as susceptible, intermediate, or resistant. This makes the result qualitative because you’re reporting a category based on zone diameter, not a specific concentration.

In contrast, MIC testing determines the lowest concentration of antibiotic that prevents visible growth, giving a numeric MIC value (for example, 0.25 μg/mL). That numeric value is then interpreted against breakpoints to decide S/I/R, but the essential output of the test is the actual MIC number, not a zone.

Breakpoints are used with both methods to interpret results, and E-tests do provide MIC values by showing where growth is inhibited along a strip.