How to: the application and analysis of categorical agreement in antimicrobial susceptibility testing using European Committee on Antimicrobial Susceptibility Testing breakpoints

In 2019, the European Committee on Antimicrobial Susceptibility Testing (EUCAST) changed the definitions of susceptibility testing categories S, I, and R. The most notable change was the definition of the I category to susceptible-increased exposure. This change altered the assessment of categorical agreement when comparing susceptibility testing methods. Categorical agreement is defined as agreement of S, I, and R results between two sets of susceptibility test results, where one set is ideally, but not necessarily, generated with a reference method.

To outline principles for assessment of categorical agreement with the new SIR-classification system used by EUCAST.

Assessment of presently used principles, followed by modification of the principles as required to adapt to the new SIR system.

The new system addresses logical inconsistencies that have been present since the revision of the EUCAST SIR system. Several errors involving the I-group have been upgraded from minor discrepancies to major discrepancies, given the new meaning of the I-group. A discrepancy by which a resistant isolate is classified as susceptible increased exposure must be regarded as a very major discrepancy.

While assessment of categorical agreement is not routinely required in the development of susceptibility tests, there are still some situations where it can be helpful in assessing breakpoint or materials performance. The 'How To' provided here fills a gap created when EUCAST revised its SIR-classification system.