Is a bacterial isolate considered susceptible to an antibiotic if its MIC is equal to the breakpoint ?
MICs less than or equal to the breakpoint concentrations are considered susceptible, MICs higher than the breakpoint indicate resistance. If two breakpoints are advocated for a particular bug/drug combination, isolates with MICs between the two values are considered to show intermediate susceptibility. Formally, therefore, answer is “Yes”. However, the distribution of susceptibilities for the particular species against the drug in question should be taken into account. If, for example, the modal MIC for a given bug/drug combination is 0.06 mg/L and the breakpoint is 1 mg/L then, clearly, an isolate with an MIC of 1 mg/L shows markedly elevated resistance compared with the mode. It may harbour an acquired resistance mechanisms even though it is still formally considered to retain susceptibility.
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