Antimicrobial Susceptibility Test Methods: Dilution and Disk Diffusion Methods

Abstract The performance of antimicrobial susceptibility tests (ASTs) to help guide patient therapy is one of the most important functions of a clinical microbiology laboratory. The most commonly used AST methods to date can be categorized into dilution methods that generate minimum inhibitory concentration values and disk diffusion methods that generate a zone diameter result. Most clinical microbiology laboratories in the United States and in many developed countries utilize manual or automated commercial dilution products that provide AST results which are demonstrated to correspond to the International Organization for Standardization/Clinical and Laboratory Standards Institute broth microdilution reference method. This chapter provides descriptions, context, advantages, and disadvantages for each of the methods: broth microdilution, broth macrodilution, agar dilution, and disk diffusion. The convenience afforded by the availability of sterile microdilution trays with round or conical bottoms for use in dilution susceptibility testing led to the widespread use of broth microdilution methods.