Antimicrobial Chemotherapy
Review
1 December 2023

Wild-type distributions of minimum inhibitory concentrations and epidemiological cut-off values—laboratory and clinical utility

SUMMARY

The characterization of wild-type minimum inhibitory concentration (MIC) and zone diameter distributions with the setting of epidemiological cut-off values (ECOFFs or ECVs) provides a reference for the otherwise relative MIC values in the international system for antimicrobial susceptibility testing. Distributions of MIC values for a species and an agent follow a log-normal distribution, which in the absence of resistance mechanisms is monomodal and designated wild type (WT). The upper end of the WT distribution, the ECOFF, can be identified with statistical methods. In the presence of phenotypically detectable resistance, the distribution has at least one more mode (the non-WT), but despite this, the WT is most often identifiable using the same methods. The ECOFF provides the most sensitive measure of resistance development in a species against an agent. The WT and non-WT modes are independent of the organism´s response to treatment, but when the European Committee on Antimicrobial Susceptibility Testing (EUCAST) determines the clinical breakpoints, the committee avoids breakpoints that split WT distributions of target species. This is to avoid the poorer reproducibility of susceptibility categorization when breakpoints split major populations but also because the EUCAST has failed to identify different clinical outcomes for isolates with different MIC values inside the wild-type distribution. In laboratory practice, the ECOFF is used to screen for and exclude resistance and allows the comparison of resistance between systems with different breakpoints from different breakpoint organizations, breakpoints evolving over time, and different breakpoints between human and animal medicine. The EUCAST actively encourages colleagues to question MIC distributions as presented on the website (https://www.eucast.org/mic_and_zone_distributions_and_ecoffs) and to contribute MIC and inhibition zone diameter data.

Get full access to this article

View all available purchase options and get full access to this article.

REFERENCES

1.
Ringertz S, Olsson-Liljequist B, Kahlmeter G, Kronvall G. 1997. Antimicrobial susceptibility testing in Sweden. II. species-related zone diameter breakpoints to avoid interpretive errors and guard against unrecognized evolution of resistance. Scand J Infect Dis Suppl 105:8–12.
2.
Kahlmeter G, Brown DFJ, Goldstein FW, MacGowan AP, Mouton JW, Osterlund A, Rodloff A, Steinbakk M, Urbaskova P, Vatopoulos A. 2003. European harmonization of MIC breakpoints for antimicrobial susceptibility testing of bacteria. J Antimicrob Chemother 52:145–148.
3.
Kahlmeter G. 2015. The 2014 garrod lecture: EUCAST - are we heading towards international agreement? J Antimicrob Chemother 70:2427–2439.
4.
International Organization for Standardization. 2019. Susceptibility testing of infectious agents and evaluation of performance of antimicrobial susceptibility devices —part 1: reference methods for testing the in vitro activity of antimicrobial agents against bacteria involved in infectious diseases (ISO 20776-1:2019). Geneva ISO
5.
European Committee on Antimicrobial Susceptibility Testing – Standard Operating Procedure 10.2 (SOP 10.2). 2021. MIC distributions and the setting of epidemiological cut-off (ECOFF) values. Available from: https://www.eucast.org/fileadmin/src/media/PDFs/EUCAST_files/EUCAST_SOPs/2021/EUCAST_SOP_10.2_MIC_distributions_and_epidemiological_cut-off_value__ECOFF__setting_20211202.pdf
6.
Clinical and Laboratory Standards Institute. 2023. Development of in vitro susceptibility testing criteria and quality control parameters. In CLSI guideline M23, 6th ed. Clinical and Laboratory Standards Institute, Wayne, PA.
7.
Mouton JW, Meletiadis J, Voss A, Turnidge J. 2018. Variation of MIC measurements: the contribution of strain and laboratory variability to measurement precision. J Antimicrob Chemother 73:2374–2379.
8.
Hombach M, Ochoa C, Maurer FP, Pfiffner T, Böttger EC, Furrer R. 2016. Relative contribution of biological variation and technical variables to zone diameter variations of disc diffusion susceptibility testing. J Antimicrob Chemother 71:141–151.
9.
Annis DH, Craig BA. 2005. The effect of interlaboratory variability on antimicrobial susceptibility determination. Diagn Microbiol Infect Dis 53:61–64.
10.
Clinical and Laboratory Standards Institute. 2016. Principles and procedures for the development of epidemiological cutoff values for antifungal susceptibility testing. In CLSI guideline M57, 1st. Clinical and Laboratory Standards Institute, Wayne, PA.
11.
International Organization for Standardization. 2021. Clinical laboratory testing and in vitro diagnostic test systems — broth micro-dilution reference method for testing the in vitro activity of antimicrobial agents against yeast fungi involved in infectious diseases (ISO 16256-1:2021). Geneva ISO
12.
Schön T, Werngren J, Machado D, Borroni E, Wijkander M, Lina G, Mouton J, Matuschek E, Kahlmeter G, Giske C, Santin M, Cirillo DM, Viveiros M, Cambau E. 2020. Antimicrobial susceptibility testing of Mycobacterium tuberculosis complex isolates – the EUCAST broth Microdilution reference method for MIC determination. Clin Microbiol Infect 26:1488–1492.
13.
Pfaller MA, Diekema DJ, Ghannoum MA, Rex JH, Alexander BD, Andes D, Brown SD, Chaturvedi V, Espinel-Ingroff A, Fowler CL, Johnson EM, Knapp CC, Motyl MR, Ostrosky-Zeichner L, Sheehan DJ, Walsh TJ, Clinical and Laboratory Standards Institute Antifungal Testing Subcommittee. 2009. Wild-type MIC distribution and epidemiological cutoff values for Aspergillus fumigatus and three triazoles as determined by the clinical and laboratory standards Institute broth microdilution methods. J Clin Microbiol 47:3142–3146.
14.
Kronvall G. 2010. Normalized resistance interpretation as a tool for establishing epidemiological MIC susceptibility breakpoints. J Clin Microbiol 48:4445–4452.
15.
Turnidge J, Kahlmeter G, Kronvall G. 2006. Statistical characterisation of bacterial wild-type MIC value distributions and the determination of epidemiological cut-off values. Clin Microbiol Infect 12:418–425.
16.
Meletiadis J, Mavridou E, Melchers WJG, Mouton JW, Verweij PE. 2012. Epidemiological cutoff values for azoles and Aspergillus fumigatus based on a novel mathematical approach incorporating cyp51A sequence analysis. Antimicrob Agents Chemother 56:2524–2529.
17.
Jaspers S, Aerts M, Verbeke G, Beloeil PA. 2014. Estimation of the wild-type minimum inhibitory concentration value distribution. Stat Med 33:289–303.
18.
Meletiadis J, Curfs-Breuker I, Meis JF, Mouton JW. 2017. In vitro antifungal susceptibility testing of Candida isolates with the EUCAST methodology, a new method for ECOFF determination. Antimicrob Agents Chemother 61:e02372-16.
19.
CRyPTIC Consortium. 2022. Epidemiological cut-off values for a 96-well broth microdilution plate for high-throughput research antibiotic susceptibility testing of M. tuberculosis. Eur Respir J 60:2200239.
20.
Ellington MJ, Ekelund O, Aarestrup FM, Canton R, Doumith M, Giske C, Grundman H, Hasman H, Holden MTG, Hopkins KL, Iredell J, Kahlmeter G, Köser CU, MacGowan A, Mevius D, Mulvey M, Naas T, Peto T, Rolain JM, Samuelsen Ø, Woodford N. 2017. The role of whole genome sequencing in antimicrobial susceptibility testing of bacteria: report from the EUCAST subcommittee. Clin Microbiol Infect 23:2–22.
21.
Skov R, Matuschek E, Sjölund-Karlsson M, Åhman J, Petersen A, Stegger M, Torpdahl M, Kahlmeter G. 2015. Development of a pefloxacin disk diffusion method for detection of fluoroquinolone-resistant Salmonella enterica. J Clin Microbiol 53:3411–3417.
22.
Skaare D, Lia A, Hannisdal A, Tveten Y, Matuschek E, Kahlmeter G, Kristiansen B-E. 2015. Haemophilus influenzae with non-beta-lactamase-mediated beta-lactam resistance: easy to find but hard to categorize. J Clin Microbiol 53:3589–3595.
23.
de Jong A, Thomas V, Klein U, Marion H, Moyaert H, Simjee S, Vallé M. 2013. Pan-European resistance monitoring programmes encompassing food-borne bacteria and target pathogens of food-producing and companion animals. Int J Antimicrob Agents 41:403–409.
24.
Karatuna O, Dance DAB, Matuschek E, Åhman J, Turner P, Hopkins J, Amornchai P, Wuthiekanun V, Cusack T-P, Baird R, Hennessy J, Norton R, Armstrong M, Zange S, Zoeller L, Wahab T, Jacob D, Grunow R, Kahlmeter G. 2021. Burkholderia pseudomallei multi-centre study to establish EUCAST MIC and zone diameter distributions and epidemiological cut-off values. Clin Microbiol Infect 27:736–741.
25.
Arendrup MC, Kahlmeter G, Rodriguez-Tudela JL, Donnelly JP. 2009. Breakpoints for susceptibility testing should not divide wild-type distributions of important target species. Antimicrob Agents Chemother 53:1628–1629.
26.
Angeby KA, Jureen P, Giske CG, Chryssanthou E, Sturegård E, Nordvall M, Johansson AG, Werngren J, Kahlmeter G, Hoffner SE, Schön T. 2010. Wild-type MIC distributions of four fluoroquinolones active against Mycobacterium tuberculosis in relation to current critical concentrations and available pharmacokinetic and pharmacodynamic data. J Antimicrob Chemother 65:946–952.
27.
Avershina E, Khezri A, Ahmad R. 2023. Clinical diagnostics of bacterial infections and their resistance to antibiotics-current state and whole genome sequencing implementation perspectives. Antibiotics (Basel) 12:781.
28.
Kahlmeter G, Turnidge J. 2022. How to: ECOFFs-the why, the how, and the Don’ts of EUCAST epidemiological cutoff values. Clin Microbiol Infect 28:952–954.
29.
Yang Q, Li X, Jia P, Giske C, Kahlmeter G, Turnidge J, Yu Y, Lv Y, Wang M, Sun Z, Lin J, Li Y, Zheng B, Hu F, Guo Y, Chen Z, Li H, Zhang G, Zhang J, Kang W, Duan S, Wang T, Jing R, Xu Y, Chinese Committee on Antimicrobial Susceptibility Testing (ChiCAST). 2021. Determination of norvancomycin epidemiological cut-off values (ECOFFs) for Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus haemolyticus and Staphylococcus hominis. J Antimicrob Chemother 76:152–159.
30.
Li X, Jia P, Zhu Y, Xu Y, Yu Y, Lv Y, Wang M, Sun Z, Lin J, Li Y, Zheng B, Hu F, Guo Y, Chen Z, Li H, Zhang G, Zhang J, Kang W, Duan S, Wang T, Jing R, Yang Q. 2021. Establishment of epidemiological cut-off values for cefoselis, a new fourth-generation cephalosporin, against Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Proteus mirabilis and Pseudomonas aeruginosa. J Antimicrob Chemother 76:2593–2599.
31.
European Committee on Antimicrobial Susceptibility Testing. 2022. EUCAST Phenotypic screening tests to detect / exclude resistance of clinical relevance. Available from: https://www.eucast.org/fileadmin/src/media/PDFs/EUCAST_files/Guidance_documents/Screening_to_detect_and_exclude_resistance_2022_08_22.pdf
32.
European Centres for Disease Control and Prevention. 2020. EARS-net-reporting-protocol. Available from: https://www.ecdc.europa.eu/sites/default/files/documents/EARS-Net-reporting-protocol-2020.pdf
33.
European Committee on Antimicrobial Susceptibility Testing. 2023. EUCAST guidance on ”what to do when there are no breakpoints” Available from: https://www.eucast.org/fileadmin/src/media/PDFs/EUCAST_files/Guidance_documents/When_there_are_no_breakpoints_20230630_Final.pdf
34.
Mouton JW, Muller AE, Canton R, Giske CG, Kahlmeter G, Turnidge J. 2018. MIC-based dose adjustment: facts and fables. J Antimicrob Chemother 73:564–568.

Author Bios

Technical Data Coordinator of the European Committee on Antimicrobial Susceptibility Testing (EUCAST), Växjö, Sweden
Head of the Swedish Reference Laboratory for phenotypic susceptibility testing, Växjö, Sweden
Head of the EUCAST Development Laboratory, Växjö, Sweden
Gunnar Kahlmeter is a clinical microbiologist, senior professor, and lecturer of the medical faculties of Lund and Uppsala Universities. He was the head of clinical microbiology in Kronoberg county 1985 – 2016, Kalmar county 1989 – 1996, and Blekinge county 2010 – 2016. He was asked by ESCMID to rejuvenate EUCAST in 2001 and he chaired EUCAST between 2001 – 2012. He was the president of ESCMID 2012 – 14 and has been the advisor to many national and international organizations involved in antimicrobial resistance surveillance, antimicrobial susceptibility testing and external quality control. He recently chaired the WHO AMR STAG committee. He is currently the EUCAST Technical Data Coordinator and webmaster and the head of the EUCAST Development Laboratory, and the Swedish Reference Laboratory for phenotypic susceptibility testing of bacteria.
Document and Technical Support to the European Committee on Antimicrobial Susceptibility Testing (EUCAST), Växjö, Sweden
School of Biological Sciences, University of Adelaide, Adelaide, Australia
John Turnidge is an infectious diseases physician and microbiologist, currently holding professorships in the School of Biological Sciences at the University of Adelaide, South Australia. He holds the Document and Technical Support role in the European Committee on Antimicrobial Susceptibility Testing and is a Senior Medical Advisor to the Australian Commission on Safety and Quality in Health Care. He obtained his medical degree from the University of Sydney and subsequently received fellowship degrees from the Royal Australasian College of Physicians and the Royal College of Pathologists of Australasia. He held senior positions in Microbiology and Infectious Diseases in Melbourne and Adelaide, Australia. His current interests include the science of phenotypic antimicrobial susceptibility testing, surveillance of antimicrobial resistance and One Health policy and management of antimicrobial resistance.

Information & Contributors

Information

Published In

cover image Clinical Microbiology Reviews
Clinical Microbiology Reviews
Volume 36Number 420 December 2023
eLocator: e00100-22
Editor: Graeme N. Forrest, Rush University, Chicago, Illinois, USA
PubMed: 38038445

History

Published online: 1 December 2023

Permissions

Request permissions for this article.

Keywords

  1. ECOFF
  2. epidemiological cut-off value
  3. antimicrobial susceptibility testing
  4. AST
  5. phenotypic wild-type distribution
  6. ECV

Contributors

Authors

Technical Data Coordinator of the European Committee on Antimicrobial Susceptibility Testing (EUCAST), Växjö, Sweden
Head of the Swedish Reference Laboratory for phenotypic susceptibility testing, Växjö, Sweden
Head of the EUCAST Development Laboratory, Växjö, Sweden
Author Contributions: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Software, Supervision, Validation, Visualization, Writing – original draft, and Writing – review and editing.
Document and Technical Support to the European Committee on Antimicrobial Susceptibility Testing (EUCAST), Växjö, Sweden
School of Biological Sciences, University of Adelaide, Adelaide, Australia

Editor

Graeme N. Forrest
Editor
Rush University, Chicago, Illinois, USA

Notes

The authors declare no conflict of interest.

Metrics & Citations

Metrics

Note:

  • For recently published articles, the TOTAL download count will appear as zero until a new month starts.
  • There is a 3- to 4-day delay in article usage, so article usage will not appear immediately after publication.
  • Citation counts come from the Crossref Cited by service.

Citations

If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. For an editable text file, please select Medlars format which will download as a .txt file. Simply select your manager software from the list below and click Download.

View Options

Get Access

LOGIN OPTIONS
Non-Member Login
Buy Article
Clinical Microbiology Reviews Vol.36 • Issue 4 • ASM Journals Pay Per View, PPV 25
Journal Subscription
Clinical Microbiology Reviews
ASM members can purchase subscriptions to journals.
Join or renew

Restore your content access

Enter your email address to restore your content access:

Note: This functionality works only for purchases done as a guest. If you already have an account, log in to access the content to which you are entitled.

Get Access

LOGIN OPTIONS
Non-Member Login
Buy Article
Clinical Microbiology Reviews Vol.36 • Issue 4 • ASM Journals Pay Per View, PPV 25
Journal Subscription
Clinical Microbiology Reviews
ASM members can purchase subscriptions to journals.
Join or renew

Restore your content access

Enter your email address to restore your content access:

Note: This functionality works only for purchases done as a guest. If you already have an account, log in to access the content to which you are entitled.

Figures and Media

Figures

Media

Tables

Share

Share

Share the article link

Share with email

Email a colleague

Share on social media

American Society for Microbiology ("ASM") is committed to maintaining your confidence and trust with respect to the information we collect from you on websites owned and operated by ASM ("ASM Web Sites") and other sources. This Privacy Policy sets forth the information we collect about you, how we use this information and the choices you have about how we use such information.
FIND OUT MORE about the privacy policy