- 28 Feb 2025: Colistin gradient tests and disks have no place in susceptibility testing
For many years EUCAST has warned users that colistin disk diffusion and gradient tests (EtestTM from bioMerieux and MTSTM 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.
Liofilchem on the other hand still produces colistin MTSTM, 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.
- 23 Feb 2025: Streptococcus pneumoniae and benzylpenicillin susceptibility testing.
We are frequently asked questions regarding (1) EUCAST Warning No. 7: is this warning specifically directed at benzylpenicillin gradient tests (EtestTM, bioMérieux, and MTSTM, Liofilchem) when used for Streptococcus pneumoniae or does it apply to other bacteria, (2) is it still valid or have we forgotten to remove the warning and (3) since there is no warning directed at ampicillin or amoxicillin (and many others) – does this mean that EUCAST did not identify a problem with these gradient tests when used for S. pneumoniae or any other species?
Warning No. 7 states that both gradient tests underestimate benzylpenicillin MIC values by one to two dilutions, most prominently and with the most serious consequences, in the area around the R-breakpoint. The evaluation of the performance was repeated in 2022 with the same result - so the warning is still valid. As to whether gradient tests for ampicillin and amoxicillin can be trusted, EUCAST has not offered an opinion.
However, with breakpoint table 15 (1 January, 2025), a change in the benzylpenicillin breakpoints and the introduction of the benzylpenicillin 1U disk as part of the extended testing of oxacillin screen positive isolates of S. pneumoniae has obviated the need for MIC testing. This will simplify distinguishing between benzylpenicillin I and R.
Gradient tests (like other tests, including disk diffusion) need to be developed and validated (calibrated) for each agent and species for which their use is intended. Manufacturers and users are responsible for making sure this is the case.
EUCAST takes responsibility for standardised methods developed by EUCAST but does not have the resources to systematically investigate other tests. We react to “signals” from colleagues and then investigate and issue warnings when appropriate. We have investigated the quality of disks and media used for disk diffusion (see Warnings 2, 8 and 9) and we invest considerable efforts into identifying target values for QC strains for every relevant agent. This is to provide means to measure the accuracy in disk diffusion and to provide manufacturers and users a means to measure accuracy.
But, most tests, outside EUCAST reference broth microdilution and standardised EUCAST disk diffusion testing are not routinely investigated. The absence of a Warning cannot be interpreted to mean that EUCAST approves.
- 21 Feb 2025: Guidance on cephalosporins for Staphylococcus aureus infections
Updated guidance on the use of and dosing of cephalosporins for S. aureus infections.
Various cephalosporins have been used for many years for treatment of a variety of infections caused by MSSA. The third-generation cephalosporins, cefotaxime and ceftriaxone, have been used in selected instances of more serious MSSA infections, such as allergy to penicillins, mixed infections, and in the case of ceftriaxone, as stepdown outpatient parenteral antimicrobial therapy (OPAT). Their use in these settings is controversial given their ecological impact on the gut microbiome and their resistance selection pressure. Ceftriaxone has become widely used in some countries for outpatient intravenous therapy because it permits once-daily dosing.
EUCAST has reviewed the PK/PD of the intravenous agents and the data on clinical outcomes for both oral and intravenous agents to determine if the current recommendation that S. aureus susceptibility to these agents can be inferred provided that the isolates are phenotypically (cefoxitin susceptible) or genotypically negative for the presence of mecA or mecC.
- 20 Feb 2025: Reference MIC testing of M.tuberculosis released
The EUCAST AMST reference method for M. tuberculosis MIC determination has now been released in the Mycobacteria methods section. A summary is below:
- A 96-well U-bottom-shaped polystyrene microtiter plate with an untreated surface.
- Liquid Middlebrook 7H9 medium (7H9) made from the broth base with a final concentration of 0.2% glycerol (no Tween 80), sterilized and then enriched with 10% Oleic Albumin Dextrose Catalase (OADC) growth supplement.
- A final inoculum of 104-106 CFU/mL of a MTBC isolate.
- A range of log2 concentration values of the ATTB to be tested, calculated from 1 mg/L and encompassing the targeted MIC value.
- At least two positive growth controls: one with 100% of the inoculum (GC100%) and one with 1% (100-fold dilution, GC1%).
- A negative control: a well with 200 µL of uninoculated broth.
- For water-insoluble agents, the final proportion of the solvent should not exceeding 1% and should be the same concentration in all ATTB-containing and growth controls wells (no ATTB).
- Incubation of the microtiter plate should be at 36°C ± 2 °C in ambient air (no CO2).
- The MIC value defined as the lowest concentration in mg/L, where no visible growth is observed at the time point when both GC100% and GC1% are showing visible growth.
- Mycobacterium tuberculosis H37Rv ATCC 27294 as the quality control (QC) strain.
- 19 Feb 2025: Company Rationale Document template revised
Companies are requested to complete a "company rationale document" to aid the EUCAST process of setting breakpoints for new agents.
A template has been available for many years. It is now revised.
- 05 Feb 2025: ECOFFinder software removed from the EUCAST website
ECOFFinder. The previously available ECOFFinder software is no longer accessible on the EUCAST website because, according to EUCAST procedures (SOP 10.2), setting ECOFFs or TECOFFs on single or already aggregated distributions is not acceptable according to EUCAST recommendations. Software and procedures to set individual ECOFFs on 5 or preferably more accepted distributions result in mean ECOFFs with confidence limits and an analysis of the quality of the procedure leading to the establishment of a formal ECOFF recognized by EUCAST in the processes for setting breakpoints, screening for resistance etc.
Questions about this can be directed to the curators via the question form EUCAST wild type distributions.
All changes to the EUCAST website and to AST recommendations and guidance are listed in a table on the EUCAST website!
