EUCAST Breakpoint Tables v16.1 (2026): Key Changes for Anaerobic Bacteria

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EUCAST Breakpoint Tables: Breakpoint tables for interpretation of MICs and zone diameters, Version 16.1 (valid from 2026-06-24).

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About the update

EUCAST has released Breakpoint Tables v16.1, valid from 24 June 2026.

Version 16.1 is a focused update to version 16.0. The main changes concern anaerobic bacteria, including restructuring of the breakpoint tables, addition of new organism-specific breakpoints, revision of existing breakpoints and introduction of new interpretive comments.

A further clarification has been introduced for carbapenems and carbapenemase-producing Enterobacterales.

Note for laboratories: Local AST interpretation rules, LIS middleware, decision-support systems and internal breakpoint tables should be reviewed whenever a new EUCAST version is implemented. Laboratories maintaining interpretation logic outside ABioGram should ensure that local configurations are updated to version 16.1.

What has changed

1) Anaerobic bacteria are now separated into Gram-negative and Gram-positive sections

EUCAST v16.1 introduces separate breakpoint tables for:

Gram-negative anaerobic bacteria
Gram-positive anaerobic bacteria

General information on the interpretation and use of breakpoints for anaerobic bacteria has also been added to both sections.

EUCAST emphasizes that clinical outcome data remain limited for many anaerobic species and that several breakpoints are therefore based primarily on MIC distributions.

The document also specifies that breakpoints for anaerobic bacteria should not be applied mechanically to polymicrobial infections. The clinical relevance of each isolated species should be assessed before susceptibility testing is used to guide antimicrobial therapy.

Practical impact: laboratories should review the structure of local AST panels, organism-specific interpretation rules and reporting logic for anaerobic isolates.

2) New breakpoints for Fusobacterium nucleatum

Version 16.1 introduces breakpoints for Fusobacterium nucleatum.

The organism is now included together with Fusobacterium necrophorum for several antimicrobial agents, including:

benzylpenicillin
ampicillin
carbapenems
clindamycin
metronidazole

For some beta-lactam/beta-lactamase inhibitor combinations, EUCAST does not provide MIC breakpoints for F. nucleatum. This is related to the antimicrobial activity of the fixed inhibitor concentration used during MIC determination, which may produce artificially low MIC values.

This limitation does not apply in the same way to disk diffusion because the concentrations of the antimicrobial agent and inhibitor decrease proportionally.

Practical impact: laboratories should ensure that F. nucleatum is not interpreted using historical or non-species-specific rules when EUCAST v16.1 criteria are available.

3) Revised breakpoints for Fusobacterium necrophorum

Several breakpoints for Fusobacterium necrophorum have been revised.

Changes include:

benzylpenicillin – MIC breakpoint revised
piperacillin–tazobactam – MIC and zone diameter breakpoints revised
meropenem – MIC breakpoint revised
metronidazole – MIC breakpoint revised

These changes are particularly relevant for laboratories that have already implemented susceptibility testing of clinically significant Fusobacterium isolates.

Practical impact: historical interpretation rules for F. necrophorum should be reviewed before v16.1 is introduced into routine reporting.

4) New organism-specific tables for Gram-positive anaerobes

EUCAST v16.1 substantially expands the number of Gram-positive anaerobic bacteria for which organism-specific breakpoints are available.

New tables have been introduced for:

Clostridium innocuum
Clostridium ramosum
Clostridium septicum
Clostridium tertium
Cutibacterium avidum
Finegoldia spp.
Parvimonas micra
Peptostreptococcus anaerobius
Peptoniphilus spp.

The breakpoints and accompanying comments for these organisms are new in version 16.1.

Importantly, EUCAST does not use a single universal breakpoint scheme for all Gram-positive anaerobes. Available agents and interpretive criteria differ between species.

Practical impact: species identification becomes increasingly important for correct AST interpretation. Generic “anaerobe” breakpoint rules should not replace organism-specific EUCAST criteria where these are available.

5) Revised breakpoints for Cutibacterium acnes

Several criteria for Cutibacterium acnes have been revised, including those for:

benzylpenicillin
cefotaxime
ceftriaxone
imipenem
meropenem

For cefotaxime and imipenem, some numerical MIC breakpoint entries have been replaced by interpretive notes.

EUCAST also retains an important inference rule for beta-lactams. Isolates susceptible to benzylpenicillin may be reported susceptible to beta-lactam agents for which appropriate breakpoints or notes are provided.

Resistance to beta-lactams in C. acnes is considered uncommon. Therefore, unusual resistant phenotypes should prompt confirmation of both organism identification and the susceptibility result.

Practical impact: laboratories using automated expert rules for C. acnes should review both numerical breakpoints and susceptibility inference rules.

6) Metronidazole resistance in anaerobes requires particular attention

EUCAST v16.1 adds or expands comments concerning metronidazole susceptibility for several anaerobic organisms, including:

Bacteroides spp.
Prevotella spp.
Fusobacterium necrophorum
Fusobacterium nucleatum
Clostridium perfringens
Clostridioides difficile

EUCAST notes that metronidazole resistance is uncommon in several of these organisms and that inadequate anaerobic conditions may result in false resistance.

When an unexpected resistant result is obtained, the laboratory should verify:

adequate anaerobic conditions
organism identification
the antimicrobial susceptibility test result

Practical impact: an unusual metronidazole-resistant phenotype should not automatically be released without technical review and, where appropriate, confirmatory testing.

7) Clarification for carbapenems in carbapenemase-producing Enterobacterales

The numerical carbapenem breakpoints for Enterobacterales were not listed among the revised breakpoints in v16.1. However, EUCAST has modified the accompanying clinical comment.

If a carbapenemase is detected, clinical response to carbapenem treatment may be impaired even when the isolate is categorized as S or I, when the carbapenem is used without a beta-lactamase inhibitor active against the carbapenemase in question.

EUCAST states that alternative active antimicrobials are preferred, particularly in complicated infections.

If no suitable alternatives are available, a carbapenem may be considered at high exposure in combination with a second active agent.

The recommended meropenem screening cut-off for carbapenemase detection remains:

MIC >0.125 mg/L
zone diameter <28 mm

Practical impact: the presence of a carbapenemase remains clinically relevant even when the isolate falls within an S or I category. Interpretation should not be based on the S/I/R category alone.

What laboratories should review

Before implementing EUCAST v16.1, laboratories should consider the following:

Update the active EUCAST version in LIS, middleware and AST interpretation systems.
Add the new Gram-positive anaerobic species and groups for which breakpoints are now available.
Review rules for F. nucleatum and F. necrophorum.
Update breakpoint and inference rules for C. acnes.
Review metronidazole-resistant anaerobic isolates before reporting unusual phenotypes.
Update carbapenemase-related comments for Enterobacterales.
Review historical analyses if susceptibility categories are compared across periods interpreted using different EUCAST versions.
Store the EUCAST version used for interpretation together with the AST result whenever possible.

The last point is particularly important for antimicrobial resistance surveillance. The same MIC or inhibition zone diameter may not always produce the same categorical interpretation after breakpoint revisions. Therefore, longitudinal datasets should retain information on the breakpoint standard and version used for each interpretation.

Summary

EUCAST v16.1 is a relatively focused update, but its impact on anaerobic susceptibility testing is substantial.

The main changes include:

separation of Gram-negative and Gram-positive anaerobic breakpoint tables
new breakpoints for Fusobacterium nucleatum
revised breakpoints for Fusobacterium necrophorum
new organism-specific tables for several Gram-positive anaerobes
revised criteria for Cutibacterium acnes
additional guidance on unexpected metronidazole resistance
clarification of carbapenem use in carbapenemase-producing Enterobacterales

For laboratories performing AST of anaerobic bacteria, version 16.1 should therefore be considered more than a minor technical revision. It expands the range of organisms that can be interpreted using standardized EUCAST criteria and introduces additional requirements for species-specific interpretation and quality control of unusual resistance phenotypes.

How to cite

The European Committee on Antimicrobial Susceptibility Testing. Breakpoint tables for interpretation of MICs and zone diameters. Version 16.1, 2026. https://www.eucast.org.

Source

Official EUCAST website: eucast.org