Optimal Use of the Polymyxins

R1: The joint European Committee on Antimicrobial Susceptibility Testing (EUCAST) and Clinical and Laboratory Standards Institute (CLSI) polymyxin breakpoint working group recommended that standard broth microdilution ISO-74 20776 be used as the reference method for the MIC testing of colistin and be performed with cation-adjusted Mueller Hinton broth, with sulfate salts of colistin in plain polystyrene trays without additives such as polysorbate-80. Sulphate salts of polymyxins must be used (the methanesulfonate derivative of colistin must not be used - it is an inactive pro-drug that breaks down slowly in solution). Agar dilution, disk diffusion, and gradient diffusion are not currently recommended by CLSI-EUCAST as these methods yield unacceptably high error rates compared to broth microdilution. We recommend that the CLSI/EUCAST Joint Working Group clinical breakpoints be used for colistin. (, )