Clinical and Laboratory Standards Institute (CLSI)

Clinical and Laboratory Standards Institute (CLSI) breakpoints are determined by the CLSI Subcommittee on Antimicrobial Susceptibility Testing by assessing commentary, resources and studies (including in vitro data, pharmacokinetics–pharmacodynamics and clinical studies). The published CLSI standards include current consensus for the pathogen–drug combinations and rationale documents for breakpoint changes. The CLSI breakpoints and interpretative categories are available for purchase from the CLSI website. The rationale documents are freely available.

CLSI has slightly differing definitions to EUCAST for the interpretation of susceptible and resistant. Dosing information may vary between pathogen–drug and site of infection. Generally, susceptible and resistant determinations are based on the achievable serum concentrations of the antimicrobial (or achievable urinary concentration where relevant).

The CLSI results are reported as:

• susceptible (S): there is a high probability of a favourable treatment outcome
• susceptible, dose dependent (SDD): efficacy is dependent on the dosing regimen used. It is necessary to use a regimen that results in higher drug exposure (eg higher dose, more frequent doses) than the regimen that was used to establish the susceptible breakpoint. Consider the maximum approved dosage regimen as higher exposure gives the highest probability of adequate coverage
• resistant (R): there is a low probability of a favourable treatment outcome
• intermediate (I): usually results in adequate blood and tissue concentration but response rates may be lower than response rates seen in susceptible isolates. The agent may be effective if it has the potential to concentrate at the anatomical site (eg urinary tract).