Clinical and Laboratory Standards Institute M02—Performance Standards for Antimicrobial Disk Susceptibility Tests includes a series of procedures to standardize the way disk diffusion tests are performed. The performance, applications, and limitations of the current CLSI-recommended methods are also described. Antimicrobial susceptibility testing is indicated for any organism that contributes to an infectious process warranting antimicrobial chemotherapy, if its susceptibility cannot be reliably predicted from knowledge of the organism’s identity. Susceptibility tests are most often indicated when the causative organism is thought to belong to a species capable of exhibiting resistance to commonly used antimicrobial agents. Various laboratory methods can be used to measure the in vitro susceptibility of bacteria to antimicrobial agents. In many medical microbiology laboratories, a disk diffusion method is used routinely for testing common, rapidly growing, and certain fastidious bacterial pathogens. The supplemental information (CLSI M1001 tables) used with this standard represents the most current information for drug selection, interpretation, and QC using the procedures standardized in CLSI M02.

CLSI M100 replaces M02-Ed13 (2018) with updates including revised nomenclature, new definitions, and updated guidelines for selecting and testing antimicrobial agents. Key changes include added information on FDA disk clearance, cefiderocol testing, and resistance detection methods for staphylococci and enterococci. The standard also introduces new tables and procedures for colony counts and fastidious organisms.

This standard describes the reference disk diffusion method used to determine the in vitro antimicrobial susceptibility of bacteria that grow aerobically and includes: 

• Agar plate preparation 

• Testing conditions, including inoculum preparation and standardization, incubation time, and incubation temperature 

• Results interpretation 

• QC procedures 

• Disk diffusion method limitations To assist the medical laboratory, recommendations are provided for selecting antimicrobial agents for routine testing and reporting. 

Standards for testing the in vitro antimicrobial susceptibility of bacteria that grow aerobically using dilution methods are found in CLSI M07.2 Standards for testing the in vitro antimicrobial susceptibility of bacteria that grow anaerobically are found in CLSI M11.3 Guidelines for standardized antimicrobial susceptibility testing (AST) of infrequently isolated or fastidious bacteria that are not included in CLSI M02, M07,2 or M113 are available in CLSI M45.5 The AST methods provided in this standard can be used in laboratories around the world, including but not limited to:

 • Medical laboratories 

• Public health laboratories 

• Research laboratories 

• Food laboratories 

• Environmental laboratories