AST of bacteria

EUCAST warnings concerning antimicrobial susceptibility testing products or procedures.

The EUCAST disk diffusion development laboratories, a network of laboratories coordinated from the EUCAST development laboratory in Växjö, Sweden, from time to time discover products (disks, media batches, gradient tests 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.


1. Problems with piperacillin-tazobactam gradient tests from two manufacturers - the warning issued 2015 will be removed when problem is resolved.

2. Wide variation in disk quality in 16 disks from nine manufacturers - this warning was issued in 2015 and reiterated 2016. There is new information on improved quality November 23, 2017 (see below). 

3. Problems with colistin susceptibility testing and several commercially available products - the warning initially issued July 2016 was updated 26 June, 2017.

1. Problems with piperacillin-tazobactam gradient tests from two manufacturers.

This warning, issued 2015, will be removed when problem is solved.

Following reports of problems related to piperacillin-tazobactam gradient tests, several batches of tests from bioMérieux and Liofilchem (from both single packs and multipacks) were evaluated at the EUCAST Development Laboratory, Växjö, Sweden (January 2016). Enterobacteriaceae and Pseudomonas aeruginosa (type strains and strains with resistance mechanisms) and broth micro dilution were used.

Gradient tests from both manufacturers gave variable and unreliable results. The difference between gradient tests and broth micro dilution was not systematic, some values were two dilutions too high, others too low. There was also considerable variation between batches from the same manufacturer.

We urge manufacturers to seriously consider these problems and users to introduce internal quality control of each procedure. We have detected no reason not to trust the disk diffusion (30+6 µg disk) results but currently you can not confirm your disk diffusion results with a gradient test.


2. Wide variation in disk quality in 16 disks from 9 manufacturers.

The warning was issued 2015 and reiterated in 2016. It will be removed when problem is solved.

During 2014 and 2015, EUCAST has evaluated the disk potency of 16 strategically important antibiotic disks from nine manufacturers of disks for antimicrobial susceptibility testing. Results from the first study were released at ECCMID 2014. Manufacturers were offered an opportunity to supply new disk lots to possibly improve their results. The second round of testing was presented at ECCMID in Amsterdam. Some manufacturers have asked that EUCAST performs further checks on new lots produced to try to right poor quality in previous lots. The current presentation (released 25 June, 2016) contains both old data and the new data when available. 


The study disclosed some good and some poor quality among disks and manufacturers. Disks from a few of them were consistently of a high quality whereas the opposite was true for some manufacturers. Laboratories are warned against the use of some disks and manufacturers are urged to improve their manufacturing process.

2017 (November 23): Follow-up studies have been performed and the results from autumn 2017 will be included in the presentation. A short summary is that disks from manufacturers with problematic disks have improved quality in the 2017 trials. Disks from HiMedia Labs have also improved but are still not acceptable. Several producers have problems related to the clavulanic acid component of amoxicillinclavulanic acid. Further testing is ongoing.  


Publication of this study is expected in 2018.


3. Antimicrobial susceptibility testing of colistin - problems detected with several commercially available products.


This warning was issued July 2016 and modified and extended 26 August  and 28 November, 2016. The current text was updated February 2018 and is a summary of the previous warnings and includes the results following further testing of more broth microdilution tests and more organisms. Warnings will be modified or removed when issues have been resolved.


Antimicrobial susceptibility testing of colistin has been fraught with difficulties. A joint EUCAST and CLSI subcommittee recently issued recommendations confirming that broth microdilution (BMD) is so far the only valid method and that disk diffusion does not work because of the poor diffusion of the large colistin molecule. The report did not evaluate gradient tests of colistin, but reports in the literature have questioned the validity of MICs obtained with gradient tests.

We used a collection of clinical isolates (n=75) obtained from international contacts (acknowledge: SENTRY collection (P. Rhomberg, JMI Laboratories, USA), S. Gatermann, Bochum, Germany, R. Henriksen, DTU, Copenhagen, Denmark, Ö. Samuelsen, Tromsö, Norway, J. Vila, Barcelona and L. Martinez-Martinez, Santander, Spain) of Escherichia coli (n= 14), Klebsiella pneumoniae, (n=18 )  Pseudomonas aeruginosa (n=21) and Acinetobacter spp (n=22) without and with various resistance mechanisms and with different levels of resistance to colistin. Several Enterobacteriaceae had the mcr-1 gene. Broth microdilution MIC values of the four species were 0.25 – 128 mg/L.


Summary and conclusion:

  • The broth microdilution tests we have evaluated all seem to give correct results both for susceptible and non-susceptible isolates (see below) although some of them may benefit from further improvements. 
  • Disk diffusion cannot be used for susceptibility testing of colistin. It does not discriminate between susceptible and resistant isolates.
  • Currently available gradient tests underestimate colistin MIC values and undercall resistance, and should be avoided, even when quality control results are within range.
  • We could not systematically evaluate semi-automated colistin methods but by sending isolates with MIC values in the non-susceptible range to colleagues around the world we have disclosed the frequent occurrence of Very Major Errors.
    Users of semi-automated devices should apply rigorous QC (see below) and check with the manufacturer whether or not they are confident that their method for colistin AST gives correct results.
  • Quality control of colistin must be performed with both a susceptible QC strain (E. coli ATCC 25922 or P. aeruginosa ATCC 27853) and the colistin resistant E. coli NCTC 13846 (mcr-1 positive). For E. coli NCTC 13846, the colistin MIC target value is 4 mg/L and should only occasionally be 2 or 8 mg/L.


Summary of results for all commercial BMD methods

Essential agreement (MICs within ± 1 dilution of reference MIC):

  • Sensititre (Thermo Fisher Scientific): 96%
  • MICRONAUT-S (Merlin Diagnostika): 96%
  • MICRONAUT MIC-Strip (Merlin Diagnostika): 99%
  • SensiTest (Liofilchem): 88%
  • UMIC (Biocentric): 82%

Major Errors (false resistance), No of tests of a total of 75

  • Sensititre (Thermo Fisher Scientific): 4
  • MICRONAUT-S (Merlin Diagnostika): 6
  • MICRONAT MIC-Strip (Merlin Diagnostika): 5
  • SensiTest (Liofilchem): 7
  • UMIC (Biocentric): 3

Very Major Errors (false susceptibility), No of tests of a total of 75

  • Sensititre (Thermo Fisher Scientific): 0
  • MICRONAUT-S (Merlin Diagnostika): 2
  • MICRONAT MIC-Strip (Merlin Diagnostika): 2
  • SensiTest (Liofilchem): 1
  • UMIC (Biocentric): 3


The study, now published in CMI, evaluates seven commercial MIC determination products against standard broth microdilution for Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter spp.




Warnings will be removed when the issues have been solved.