Which scenario favors broth microdilution over disk diffusion in antimicrobial susceptibility testing?

• A. When results are needed quickly.
• B. When precise MIC values are needed for dosing decisions or for organisms/drugs with unclear or absent disk diffusion breakpoints.
• C. There are many isolates to test.
• D. Testing yeasts.

Answer: (B)

When you need a precise measure of how much drug will inhibit the organism, broth microdilution is the method of choice. It tests a range of antibiotic concentrations in broth and gives you an exact minimum inhibitory concentration (MIC). That exact MIC is crucial for dosing decisions, especially for drugs with narrow therapeutic windows or when the achievable drug exposure must be matched to the organism’s susceptibility.

Disk diffusion, by contrast, yields only categorical results (susceptible, intermediate, resistant) based on zone sizes around a disk. Those interpretations rely on established breakpoints. If those breakpoints don’t exist or are unclear for a particular organism–drug combination, or if the diffusion characteristics are unreliable, you can’t depend on disk diffusion for accurate guidance. In such cases, broth microdilution provides actionable numeric data and aligns with standardized guidelines, making it the preferred choice.