Susceptibility describes the likelihood that a bacterial pathogen will grow or be killed by a given antimicrobial agent.
Susceptibility is determined through in vitro testing methods, such as minimum inhibitory concentration (MIC) measurements. MICs are interpreted using standardised clinical breakpoints established by expert bodies such as the European Committee on Antimicrobial Susceptibility Testing (EUCAST) or the Clinical & Laboratory Standards Institute (CLSI). Breakpoints classify isolates into categories, including:
- susceptible (S),
- susceptible at increased exposure (I) or
- resistant (R)
This categorization reflects the probability of therapeutic success when appropriate dosing regimens are used. Susceptibility, therefore, provides a crucial link between in vitro laboratory measurements, pharmacokinetics/pharmacodynamics (PK/PD), and clinical decision-making, guiding the selection of effective antimicrobial treatment.