AST definitions and breakpoints

Antibiotic susceptibility testing is extremely important for determining which antimicrobials are most likely to be effective against different microorganisms. In this video, Professor Gunnar Kahlmeter gives us an insight into AST definitions and breakpoints.

Breakpoints are used to distinguish between treatable and non-treatable micro-organisms. A breakpoint is a concentration at which a therapeutic success can be expected. These allow for micro-organisms to be categorised into the following groups:

• S for susceptible

• I for susceptible at increased exposure

• R for resistant

Below is an example of a susceptibility report for Staphylococcus aureus: Methicillin R Ceftaroline S Ciprofloxacin I Vancomycin S

MIC distribution (mg/L) data is used to determine ranges for S, I and R. As breakpoints are expressed as: MIC: S ≤ X mg/L and R > Y mg/L, MIC values between X and Y are categorised as I.

To determine the breakpoints, there are few things that we need to know, such as:

• Which species are relevant

• MIC distributions

• Which infections are relevant

• Mode of administration – IV, oral or topical along with dosing and dosing intervals

• Pharmacokinetics and pharmacodynamics of agent

• Clinical evidence for effect including data for wildtype and non-wildtype (often resistant) organisms along with how efficacy is related to different MIC values

EUCAST has breakpoint tables which include MIC breakpoints. These are used as reference ranges which determine how to report AST of particular microorganisms which can be found here.