Drug Susceptibility Testing: methods and meaning

Drug susceptibility testing (DST) determines the effectiveness of specific antimicrobial agents against pathogens, particularly bacteria and fungi. It guides treatment, tracks the emergence and spread of antimicrobial resistance, and underpins laboratory quality control.

Methods - **Broth dilution** — the pathogen is exposed to varying antimicrobial concentrations in liquid medium to determine the minimum inhibitory concentration (MIC). - **Agar dilution** — the same principle on solid media. - **Disk diffusion (Kirby–Bauer)** — antibiotic disks are placed on inoculated agar; the zone of inhibition is measured and interpreted. - **E-test** — a gradient strip; the point where growth intersects the strip indicates the MIC. - **Molecular methods** — PCR-based detection of genetic markers associated with resistance.

Interpretation Results are reported against breakpoints defined by international standards organisations (CLSI/EUCAST), which are periodically revised; laboratories must always apply the current standard. Molecular detection of a resistance marker and phenotypic resistance are related but not always identical — phenotypic confirmation remains important where therapy depends on it.