Modification of antimicrobial susceptibility testing methods

The development of new antimicrobial agents is essential to combat antimicrobial resistance. Reliable antimicrobial susceptibility testing (AST) methods should be established early and thoughtfully to ensure timely patient access to these agents. The standard reference method for AST is broth microdilution (BMD) in cation-adjusted Mueller-Hinton broth (CAMHB), as defined by CLSI M07 (CLSI. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically. 12th ed. CLSI Standard M07. Clinical and Laboratory Standards Institute, 2024) and ISO 20776-1 (ISO. Susceptibility Testing of Infectious Agents and Evaluation of Performance of Antimicrobial Susceptibility Test Devices-Part 1: Broth Micro-dilution Reference Method for Testing the In Vitro Activity of Antimicrobial Agents Against Rapidly Growing Aerobic Bacteria Involved in Infectious Diseases. ISO 20776-1. International Organization for Standardization, 2019). While some agents may require modifications to this method to better reflect clinical activity, such changes must be scientifically justified. Modifications aimed solely at producing lower minimal inhibitory concentration (MIC) values-or to make one antimicrobial agent appear superior to others-are not scientifically valid and are strongly discouraged. Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) caution developers that unnecessary deviations from reference AST methods can lead to increased costs, regulatory hurdles, delays in test availability and reduced clinical adoption. Until joint guidance under development by CLSI and EUCAST is finalized, early and rigorous evaluation using the reference method is essential.