Expected Phenotypes

Expected phenotypes refer to the expected results of susceptibility tests without testing. For a species to be included in the “expected resistant phenotype”, 90% or more should be considered resistant (e.g. Klebsiella pneumoniae vs. ampicillin). 

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 (e.g Streptococcus pyogenes vs. benzylpenicillin).  

Laboratories should avoid testing and report the isolate as resistant/susceptible according to the expected phenotype. If susceptibility testing is performed, a result which contradicts the expected phenotype should be viewed with suspicion. 

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