Expert rules and expected phenotypes

Expected resistant and susceptible phenotypes

Expected resistant phenotypes v 1.2 (13 January, 2023); Expected resistant phenotypes v 1.1 (25 March, 2022)


Expected susceptible phenotypes v 1.1 (25 March, 2022)


For many years EUCAST and other committees have struggled with the term “intrinsic resistance”.

There is no agreed definition and since breakpoints are always “exposure dependent” it is hard to agree on a definition which will survive changes in dosing, modes of administration and a sudden willingness to accept a new and higher level of toxicity because of a lack of alternatives.

One advantage of defining a species as an "expected resistant phenotype" or "expected susceptible phenotype" in relation to an agent or class of agents, is that susceptibility testing becomes unnecessary and it allows colleagues to report the isolate as resistant and susceptible, respectively, without having performed a test. Also it informs us of important characteristics, shortcomings and assets, of agents.

EUCAST has decided to replace the term “intrinsic” with the terms “expected susceptible phenotype” and “expected resistant phenotype”. For a species to be included in the “expected resistant phenotype”, 90% or more should be considered resistant (Klebsiella pneumoniae vs. ampicillin is an example). For a species to be included in the “expected susceptible phenotype” the wild type should be considered susceptible (S or I) to the agent and a very high proportion (99%) of isolates should be devoid of acquired resistance to the agent (Streptococcus pyogenes vs. benzylpenicillin is one example).


In both cases, susceptibility testing is best avoided. A result which goes against the expected phenotype should be viewed with suspicion.


For questions and advice open the EUCAST subject related contact form and choose a relevant subject.


Archived, previous versions: 


EUCAST advice on expected (resistant and susceptible) phenotypes v 1.0 (4 - 25 March, 2022). 


EUCAST advice on intrinsic resistance and exceptional phenotypes v 3.3 (4 January, 2022).