AST of bacteria

EUCAST warnings concerning antimicrobial susceptibility testing products or procedures.

The EUCAST Development Laboratories, from time to time discover products (disks, media batches, gradient tests, devices and/or procedures) which are not performing to the expected standard. When this is the case we inform the manufacturer and publish a warning on this page.


We do not systematically test all products so the lack of a warning does not imply that there is no problem with the product in question.


Laboratories which experience problems with a susceptibility test method, and suspect that this may be related to a particular product, may contact EUCAST for advice. Use the EUCAST subject related contact form.


1. Warnings against piperacillin-tazobactam gradient tests from two manufacturers removed. 

Warnings against Etest and MTS were removed November 2019 and April 2020, respectively.

Note. The new Etest and MTS for piperacillin-tazobactam are valid only for Enterobacterales, Pseudomonas aeruginosa and Acinetobacter - and should not be used with other bacteria, such as Bacteroides or Haemophilus without having confirmed with the manufacturers. 


2. Wide variation in disk quality in 16 disks from nine manufacturers - this warning was issued in 2015 and reiterated 2016 and 2017.

There is new information on improved quality November 23, 2017 (see document). 


3. Problems with colistin susceptibility testing and several commercially available products - the warning initially issued July 2016 was updated 26 June, 2017 and is extended and still valid November 2023. bioMerieux has failed to address the problem or to withdraw the malfunctioning product which generates VME because MIC-values around colistin breakpoints are too low by one or several dilutions.


4. Vancomycin susceptibility testing in Enterococcus faecalis and E. faecium 

using MIC gradient testsa modified warning; 21 May, 2019. The EUCAST Development Laboratory has had no reason to modify the warning (November, 2023)


5. A problem with meropenem in a Thermofisher Sensititre panel (DKMGN) discovered

(6 November, 2018)


6. Variable performance of the itraconazole agar in the azole resistance screening

plate from VipCheckTM  (13 December, 2018). The problem has now been solved, the affected batches are no longer available, and the warning withdrawn (11 March, 2021) For further clarification, contact the EUCAST AFST.  


7. Warning against the use of gradient tests for determining

benzylpenicillin MICs in S. pneumoniae 

(21 November, 2019; data and data analysis added 27 November; updated 4 May 2022).

bioMerieux has added a "Follow-up information to customers" on this issue. 

There is no further information from either company to indicate product development. The EDL is now conducting further tests (January, 2022).

Please observe, that the 8 mm cut-off mentioned in the document, since then has been changed to 9 mm (see breakpoint table v 12.0).


8. Mueller-Hinton media - EUCAST has evaluated 21 brands of commercially available MH powders (29 February, 2020).


9. Mueller-Hinton pre-poured media - EUCAST has now evaluated commercially available pre-poured MH plates.

Results are available at

Results: The best performance was observed for MHA from Becton Dickinson (BBL MHA II), bioMérieux (MHE agar) and Hardy Diagnostics, for which >97% of zone diameters were within QC ranges and >60% on target ±1 mm. The poorest performance was seen for plates from HiMedia (MHA and MHA No. 2), where 20 and 18% of readings were outside the QC ranges, respectively. 


10. Cefiderocol MIC by broth microdilution on freeze dried panels from Thermofisher - the manufacturer has issued a warning against all current batches of MH broth when used together with any freeze dried Sensititre BMD panel.

The following plates are listed as containing cefiderocol, MDRGNX2F, CAN2MSTF, GNARUM6F, THAMDR1F, CMP3QLN, DEURUB1, SWEEDL1, EUSHION8, FRCNRP4 and the manufacturer issues the following recommendation: "If you have bought any of these lots, then please ignore results for Cefiderocol".

EUCAST recommendation: If uncertain confirm directly with the manufacturer (January, 2022). There is as yet no news of a solution to the problem (November, 2023).


11. General warning against incomplete species validation and concentration range validation of commercially available susceptibility testing material and devices. 

In many cases manufacturers of susceptibility testing material and devices list species for which the product is validated for use. This is in the package insert and by necessity often in fine print. The user of AST products and material is adviced to insist on obtaining information and background data on the conditions (atmosphere, temperature etc) and for which species the product is intended for use. In the absence of specific information on the procedure for and extent of validation, the product should NOT be used for the species.

The user of products should also obtain information from the manufacturer on which range of MIC concentrations (and thus susceptibilities) the product was validated. This is not always clear. /June 2022, September 2023, November 2023.


12. Cefiderocol: EUCAST has evaluated (August 2022) commercially available tests and all have problems with accuracy, reproducibility, bias and/or for some, with skipped wells. For this reason, it is difficult to resolve the susceptibility interpretation of isolates in the ATU (area of technical uncertainty) in routine testing. Considering the current limitations and the need for testing in multiresistant bacteria, these are the recommendations of EUCAST until the issue of confirmatory MIC determination is solved.


  • Disk diffusion. Laboratories are recommended to start testing cefiderocol with disk diffusion. Resistance to beta-lactam agents is increasing and therapeutic alternatives are few. Disk diffusion, when correctly performed and calibrated using quality material and recommended quality control guidelines, is predictive of susceptibility and resistance outside the ATU. Template zone diameter distributions for relevant species against which to calibrate in-house results, are available (
  • Inside the ATU, and as long as there is no alternative method to resolve interpretative uncertainties (eg MIC-testing in the routine laboratory or assistance from a reference laboratory), EUCAST recommends colleagues to ignore the ATU and interpret using the zone diameter breakpoints in the breakpoint table.
  • However, as with all AST methods and for all antimicrobial agents, results depend on the quality of materials used. EUCAST does not have the resources to systematically investigate all products from all manufacturers. Below a few commonly used manufacturers of disks and MH agars are mentioned but others are not. Assume that those that are not mentioned may have undisclosed problems.


For cefiderocol, using a cross-over protocol, disks from three manufacturers and Mueller-Hinton media from five manufacturers were evaluated.

If care is taken to ensure that the EUCAST QC criteria for the two strains, E. coli ATCC 25922 and P. aeruginosa ATCC 27853, are met, i.e. with the mean of at least 5 repeated tests is within +/-1 mm of the target values, disk diffusion using EUCAST breakpoints performed well. When the mean value was more than +/-1 mm from the target, an increasing proportion of results were erroneous. This was particularly problematic for Pseudomonas aeruginosa.


Among the evaluated cefiderocol 30 µg disks (Liofilchem, Mast and Oxoid) and Muller-Hinton agars (BBL, bioMérieux, Bio-Rad, Liofilchem and Oxoid), disks from Oxoid and MH agar from Bio-Rad produced larger than acceptable zone diameters for both QC strains and clinical isolates. Combining Oxoid disks with Bio-Rad MH agar increased the problem further.


Further work to elucidate the problems surrounding cefiderocol susceptibility testing is ongoing. Two commercial products currently under investigation, are UMIC  (Bruker) and ComASP (Liofilchem). 

The issue has been discussed by the EUCAST Steering Committe, January 2023, and it was decided that issues have not yet been resolved and that the warning should remain.

A EUCAST evaluation is ongoing and results will be made available as soon as testing is completed and an analysis by the committee performed.


13. VIPcheck. In May, 2022, the EUCAST development laboratory for fungi, noticed variable performance with several batches of VIPcheck (Mediaproducts BV) azole screening agar plates (see E.Def 10.1) for the detection of azole resistance in Aspergillus fumigatus. Use of these and future batches require extensive QC to ensure adequate separation of azole susceptible and resistant isolates until the issues is solved by the manufacturer. More detail is found in the pdf file.


Warnings will be modified or removed when the issues have been solved.   




14. EUCAST disk diffusion of anaerobe bacteria and the FAA-HB medium.


EUCAST disk diffusion of anaerobe bacteria (Bacteroides spp., Prevotella spp., Fusobacterium necrophorum, Clostridium perfringens and Cutibacterium acnes) is performed on Fastidious Anaerobe Agar. FAA is available from several manufacturers, both as a powder for in-house production in laboratory media-kitchens, and as commercially available pre-poured plates. The EUCAST FAA-HB consists of FAA with 5% mechanically defibrinated horse blood without other additives. The agar depth is important and when buying pre-poured plates, make sure to check. The variation (±0.5 mm) around the prescribed agar depth (4 mm) is to allow random variation within and between plates, but the average of repeat measurements should meet the prescribed depth of 4±0.5 mm and all individual measurements inside the range 3.5 - 4.5 mm. This meets the EUCAST general rule for all plates for disk diffusion. QC criteria are available from EUCAST ( – be sure to check your plates before introducing them for disk diffusion in the lab. The addition of 5% mechanically defibrinated horse blood is identical to the procedure used for the production of MH-F plates.


Note, plates for susceptibility testing with FAA media may not be interchangeable with FAA plates already used in the lab for culturing anaerobic bacteria.


15. Thermo Fisher Scientific Sensititre broth external contamination.

Several laboratories, including the EUCAST Development Laboratory, have over the last several months experienced contamination of packages of Sensititre broth. The contamination occurs on the outside of vials and the inside of the packaging material. It consists of "powdery" nonbiological material laden with Bacillus spp. The broths themselves appear unaffected but unless carefully handled before use, the broth may become contaminated in the handling procedure. Thermo Fisher is now investigating the issue. Until further notice, be very observant when using the broth.