Colistin susceptibility testing

A few weeks ago, we informed EUCAST users, that the Warning against bioMerieux AST material for colistin, still stands. The item referred (by a link) to the EUCAST Warnings page where it was made clear exactly which material was malfuntioning and should have been withdrawn by bioMerieux. However, the news item itself was not sufficiently specific. Following contact with bioMerieux and Liofilchem and also with colleagues asking questions, we would like to modify the item as follows:

Malfunctioning colistin gradient tests

Colistin gradient tests sold by bioMerieux (Etest) and Liofilchem (MTS) for susceptibility testing (AST) have been malfunctioning since the very beginning. It is 7 years since it was conclusively shown that both colistin gradient tests completely fail to meet acceptable accuracy, especially on values close to the breakpoint. The warning was first published in 2016 and has been reviewed and retained several times by EUCAST in the period 2016 – 2023.Liofilchem informed us in the spring of 2023 that their gradient test was for ROU (research use only) and should not be used for clinical susceptibility testing.EUCAST has now been informed by bioMerieux that their product will no longer be available after the 1st quarter 2024.

A new colistin test on VITEK2 cards has been developed by bioMerieux and we are now frequently asked by colleagues about the performance. EUCAST never evaluated Vitek2 or any other semiautomated AST system - it is beyond our capacity to systematically review material and methods other than that which is part of EUCAST standard methods. We encourage users to ask to see the full evaluations performed by the companies (species, broth microdilution MIC-values, Vitek2 results) to make sure that sufficient and relevant species and strains/isolates with reference MIC values on or close to the breakpoints (2 mg/L for all Enterobacterales, 4 mg/L for Pseudomonas aeruginosa) were used in the validation. We also encourage colleagues to report problems both to the manufacturer and to EUCAST.

We apologize for any misunderstanding which could have occurred if readers did not go to the full text.