Antimicrobial Susceptibility Reporting

Pathlab uses the European Committee on Antimicrobial Susceptibility Testing (EUCAST) system for testing and reporting antimicrobial susceptibilities. Until now EUCAST has used the following categories for the reporting of antimicrobial susceptibilities, “Susceptible”, “Intermediate”, and “Resistant”.

In 2019, EUCAST introduced changes that redefined the categories used for interpreting and reporting antimicrobial agents. (The full background and details about the changes are available on the EUCAST website.)

The new categories that antimicrobials will be reported under and their definitions are as follows:

• Susceptible, standard dosing regimen: A microorganism is categorised as Susceptible, standard dosing regimen*, when there is a high likelihood of therapeutic success using a standard dosing regimen of the agent.
• Susceptible, increased exposure: A microorganism is categorised as Susceptible, increased exposure* when there is a high likelihood of therapeutic success because exposure to the agent is increased by adjusting the dosing regimen or by its concentration at the site of infection.
• Resistant: A microorganism is categorised as Resistant when there is a high likelihood of therapeutic failure even when there is increased exposure*.
• * Exposure is a function of how the mode of administration, dose, dosing interval, infusion time, as well as distribution, metabolism and excretion of the antimicrobial agent will influence the infecting organism at the site of infection.

The new definitions reflect the need for correct exposure of the microorganism being treated. The dosing strategies relevant to the EUCAST breakpoints are available on the EUCAST website in the “Clinical Breakpoints – bacteria” excel file under the “Dosages” tab, (far right end tab), see EUCAST clinical breakpoints.

If you have questions or would like to discuss the changes further, please contact us.