28.02.2025

Colistin gradient tests and disks have no place in susceptibility testing

For many years EUCAST has warnedusers that colistin disk diffusion and gradient tests (Etest^TM from bioMerieuxand MTS^TM from Liofilchem) fail to predict colistin resistance in all relevant bacteria.

bioMerieux has has now (2025) informed EUCAST that they have stopped the production and all sales of colistin Etest.

Liofilchemon the other hand still produces colistin MTS^TM, but have since more than a year, removed the IVD label and introduced an RoU (Research Only) label which means it should not be used in clinical laboratories for predicting colistin susceptibility and resistance in bacteria.

So, whatever use there is for colistin gradient tests, it is certainly not related to clinical microbiology. Likewise, colistin disks have no place in disk diffusion susceptibility testing.