Recommended resources
The freely available resources this lesson will direct you to.
Hello! This final lesson turns the course’s central question—how to use CLSI and EUCAST responsibly—into a concise postgraduate report. The challenge is not simply to list differences. A strong report explains why those differences matter for a laboratory result, supports its claims with appropriate sources, and ends with guidance a laboratory could actually follow.
Allow about 40 minutes: roughly 12 minutes with the sources below, 15 minutes drafting, and the remainder checking the report against the criteria at the end. The model report is a guide to structure and level of detail, not a substitute for checking the current documents before making a laboratory recommendation.
1. Establish what each source can support
CLSI and EUCAST provide frameworks for antimicrobial susceptibility testing (AST): standardized ways to obtain results and criteria for interpreting them. Their publications have practical authority because laboratories and health systems adopt them, but neither name alone overrides applicable local policy or regulation. Your report should distinguish an organization’s standards-setting role from a requirement imposed in a particular jurisdiction or laboratory.
Begin with CLSI’s own account of its role and publications:
Using Breakpoints | Rationale & Guidelines | CLSI
Read CLSI’s “Rationale for Using CLSI Breakpoints” for a primary-source description of its standards-setting role and the distinct purposes of M100 and M23.
At the start of the page, read the introductory paragraph beginning “CLSI is a not-for-profit standards development organization”; the introduction explains its consensus-based role. In the following paragraphs beginning “On an annual basis,” read the document descriptions: note that M100 supplies current AST interpretation and quality-control information, whereas M23 describes how criteria are established or revised. Then, in the discussion beginning “In the United States,” read the accreditation passage. Treat accreditation as evidence about CLSI’s standards-development process, not as proof that every laboratory must use CLSI.
The distinction between M100 as a source of interpretive information and M23 as a source about developing criteria is useful in a referenced report. Cite the document that supports the claim you are making; do not cite M23 as though it were the table from which you interpreted an isolate.
2. Define the categories before comparing them
An AST category is an interpretation of a measured result under specified conditions, not an intrinsic label permanently attached to an organism. It depends on such matters as the organism–drug pairing, test method, breakpoint, and—in clinically framed interpretations—drug exposure. This is why identical measurements need not receive identical labels under two systems.
EUCAST’s definition of I is especially important. It means susceptible, increased exposure, not “halfway resistant.” Increased exposure may depend on the dosing regimen or the drug concentration achieved at the infection site. Read EUCAST’s wording rather than paraphrasing from memory:
EUCAST: Definition of S, I and R
Read EUCAST’s official “Definition of S, I and R” so the report uses its category names precisely and does not confuse clinical categories with epidemiological cutoffs.
Before the “Publications” heading, read the three opening category bullets, beginning with “S - Susceptible” and ending with the definition of “R - Resistant.” The definitions distinguish standard from increased exposure. Continue through the paragraph warning against combining I and R for surveillance. Below “Publications,” find the bullets beginning “Susceptible categories: S and I”; the reporting guidance states how to retain the categories. Finally, continue to the wild-type and ECOFF bullets just above “See also”; the cutoff explanation distinguishes a wild-type distribution from a clinical prediction.
In a comparison, write “EUCAST I: susceptible, increased exposure” in full at first mention. CLSI’s intermediate terminology must not be silently translated into that definition; CLSI also has a susceptible-dose dependent category for some organism–drug combinations. Check the applicable current table and its notes rather than imposing one system’s meaning on the other.
Likewise, a clinical breakpoint supports an interpretation relevant to treatment under its stated assumptions. An epidemiological cutoff separates a wild-type distribution from isolates that may have a detectable resistance mechanism. Wild-type status alone does not establish that treatment will succeed. This distinction belongs in the report’s definitions section because confusing the two can turn a surveillance observation into an unjustified treatment recommendation.
3. Explain differences through their consequences
For a concise report, select differences that change what a laboratory does: governance and local adoption, access to current documents, revision practices, category language, and the assumptions behind breakpoints. Both systems revise guidance as evidence develops; neither is inherently “stricter” for every organism–drug pair.
The following video excerpts are useful for two practical points: how EUCAST presents updates and dose assumptions, and why consulting another system is not the same as mixing its criteria into an existing test.
Novedades CLSI, EUCAST & BrCast 2025 en la interpretación del Antibiograma
Watch selected excerpts of “Novedades CLSI, EUCAST & BrCast 2025 en la interpretación del Antibiograma” from redEMC - Educación Médica. Use the discussion to understand document maintenance and method compatibility; use current official tables, rather than a 2025 presentation, for any present-day breakpoint claim.
In the EUCAST presentation, watch tables and updates for the website’s update notices, annual breakpoint table, and the reason dose information accompanies the breakpoints. In the panel discussion, watch mixing methods for the warning about applying one system’s disk-diffusion criteria to disks specified by the other. Then watch local adoption for an example of why an explicitly chosen local standard matters to consistent reporting.
The supplied image makes the interpretive consequence visible:
A band crossing from one category to another is a reason to investigate the applicable methods, breakpoints, exposure assumptions, and table notes. It is not a reason to choose whichever label seems more favorable. Nor should the image be used to calculate a general disagreement rate: the panels represent different antibiotics, and the figure does not supply the methodological context needed for that claim.
One particularly consequential error is to perform disk diffusion under one system and interpret its zone using the other’s criteria without establishing compatibility. Disk content and associated zone criteria can differ. For either system, the coherent unit of practice is the applicable test procedure, quality-control criteria, breakpoint table, interpretive rules, and document version. Where guidance is missing, record that limitation and seek an approved resolution; an epidemiological cutoff is not an automatic replacement for an absent clinical breakpoint.
4. Draft the report as an argument, then audit it
A useful postgraduate structure is four short paragraphs: definitions and authority; importance; material differences; appropriate-use recommendation. State the scope near the beginning—here, routine bacterial AST unless otherwise specified—because guidance for other organisms or tests may require different documents.
Below is an illustrative report. Its references support general claims, but it deliberately gives no numerical breakpoint or assertion about a particular current table. A report for an actual laboratory must add the current document edition and the relevant organism–agent table and notes.
CLSI and EUCAST in bacterial antimicrobial susceptibility testing
CLSI is a consensus-based medical laboratory standards-development organization. Its M100 publication provides AST drug-selection, interpretive, and quality-control information, while M23 describes processes used to establish or revise interpretive criteria [1]. EUCAST likewise publishes criteria for interpreting AST results. Both frameworks give laboratories a disciplined basis for converting measurements into reportable categories; their use in a particular laboratory must also accord with applicable local policy and regulation.
Standardization matters because an interpretation is only as reliable as the measurement and criteria behind it. Consistent procedures and quality control support results that clinicians can act on and surveillance data that can be compared meaningfully. Clinical breakpoints should not be confused with epidemiological cutoffs: a result in a wild-type distribution does not, by itself, predict therapeutic success [2].
Differences between the systems include their standards-setting arrangements, local adoption, publication and update practices, category terminology, and breakpoint assumptions. In particular, EUCAST defines I as “susceptible, increased exposure” and advises against grouping I with R as “non-susceptible” in surveillance [2]. CLSI terminology should be interpreted from its own current guidance rather than treated as interchangeable with EUCAST’s. EUCAST also presents dose information alongside breakpoint tables because exposure assumptions are relevant to interpretation [3]. A categorical difference between systems therefore warrants examination of the underlying criteria, not an assumption that one result is erroneous.
A laboratory should first identify the system required or accepted locally, then verify the current document version, organism–drug coverage, method, quality-control requirements, breakpoint, and all relevant notes. It should confirm that instruments, disks, software, and reporting language implement that same compatible framework. Criteria from another system should not be substituted merely to fill a gap—particularly for disk diffusion—without a validated, locally approved approach [3]. Where no applicable clinical breakpoint exists, the laboratory should communicate the limitation rather than present an epidemiological cutoff as a susceptibility category. This approach makes the final report both clinically interpretable and traceable to its governing criteria.
References for the illustrative report
- Clinical and Laboratory Standards Institute. Rationale for Using CLSI Breakpoints (including descriptions of M100 and M23).
- European Committee on Antimicrobial Susceptibility Testing. Definition of S, I and R.
- redEMC - Educación Médica. Novedades CLSI, EUCAST & BrCast 2025 en la interpretación del Antibiograma; selected EUCAST presentation and panel discussion. Secondary explanation, not a substitute for a current official breakpoint table.
Before submitting your own version, check it against these questions:
- Scope: Have you named the organisms and kind of AST to which the recommendation applies?
- Source fit: Do official sources support the definitions, and have you identified the current relevant tables and notes for any specific interpretation?
- Precision: Have you kept EUCAST I distinct from CLSI terminology, and epidemiological cutoffs distinct from clinical breakpoints?
- Practicality: Does the recommendation identify local requirements and keep method, quality control, criteria, software, and version compatible?
- Restraint: Have you avoided presenting a conference update, an illustrative figure, or a missing breakpoint as more authoritative than it is?
The report’s governing idea is traceability: a susceptibility category should be traceable to a suitable measurement, a compatible interpretive system, an applicable current document, and its clinical assumptions. CLSI and EUCAST are both substantial AST frameworks, but careful use of either requires more than selecting a table. Your finished report should leave a reader able to see not only how the systems differ, but how a laboratory would make and defend an appropriate choice.