nonsusceptibility

Nonsusceptibility is a term used in antimicrobial susceptibility testing to describe a microorganism that is not susceptible to a particular antimicrobial agent under standard testing conditions. It is commonly used as an umbrella term that includes both intermediate and resistant phenotypes, and it contrasts with susceptible, for which there is a high likelihood of therapeutic success with standard dosing. The exact interpretation depends on guidelines such as CLSI and EUCAST, which define breakpoints for each organism–drug combination.

In laboratory testing, isolates are evaluated by methods such as disk diffusion, broth microdilution, or gradient diffusion to determine a minimum inhibitory concentration (MIC). The reported category—susceptible, intermediate, or resistant—depends on interpretive criteria. When an isolate is nonsusceptible, its MIC is above the susceptible breakpoint or lies in the intermediate range. Nonsusceptibility also reflects intrinsic resistance inherent to certain species, in addition to acquired resistance mechanisms.

Clinically, nonsusceptibility informs therapy decisions. However, in vivo effectiveness depends on pharmacokinetics, pharmacodynamics, infection site, and host factors. Some intermediate phenotypes may respond to higher doses, alternative administration routes, or combination therapy, whereas resistant phenotypes often require a different agent.

Limitations include differences in breakpoints among guideline bodies, laboratory variability, and the imperfect correlation between in vitro results and clinical outcomes. Nonsusceptibility monitoring supports antibiotic stewardship and infection control by tracking emerging resistance patterns.