1.2.2 Clinical breakpoints

AST provides prescribers with information that can be used to determine which antibiotic therapy is appropriate. Clinical breakpoints are used for this.

The clinical breakpoint is based on the MIC and the safely achievable tissue concentrations expected when a patient or animal is given a standard antibiotic dose. It is a cut-off value that defines isolates as susceptible or resistant in clinical practice given a standard treatment regimen. Remember that if the MIC of an isolate is less than or equal to the clinical breakpoint for a given antibiotic, then the bacteria are considered susceptible (S) to treatment with that antibiotic, following a standard dosing regimen. Isolates with MICs higher than this breakpoint are considered as having resistance (R) to treatment with the antibiotic in question.

For some pathogen-antibiotic combinations, there is an ‘intermediate’ zone between definite susceptibility and definite resistance. Isolates that have an MIC in this ‘intermediate zone’ are susceptible to increased antibiotic exposure (I) (Kahlmeter and the EUCAST Steering Committee, 2019). The benzylpenicillin MIC distribution for S. pneumoniae that we looked at earlier is a good example of this type of pathogen-antibiotic combination (Figure 4). The antibiotic can still potentially be used, depending on the site of the infection, but it must be at a higher dose. It may for example be effective in sites where high concentration can easily be achieved, such as in the lung (pneumonia), but not in sites such as the central nervous system (meningitis) due to lower levels crossing the blood-brain barrier. This should be made clear in the laboratory report.

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Figure 4 Interpreting MICs in the ‘intermediate’ zone (Kahlmeter and the EUCAST Steering Committee, 2019).

1.2.1 Epidemiological cut-off (ECOFF)

1.2.3 How breakpoints are set