The concentration of antibiotic used to define whether an infection by a particular bacterial strain/isolate is likely to be treatable in a patient. Typically, these are defined as susceptible or resistant to an antibiotic. The definition of the term ‘intermediate’ depends on the reference method and should be checked before using.

Clinical microbiology laboratories use susceptibility testing and clinical breakpoint concentration data to provide guidance to clinicians so they can choose the most appropriate antibacterial therapy and dosing. Clinical breakpoint concentrations are different from antimicrobial susceptibility tests, since they take into account what happens to an antibiotic within the body (pharmacokinetics), which will affect an antibiotic’s clinical efficacy.

Breakpoint concentrations are established by reference groups such as the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Clinical and Laboratory Standards Institute (CLSI). However, for some drugs, EUCAST and CLSI recommend different breakpoint concentrations; this affects whether clinical isolates are considered to be susceptible or resistant.