Recommended resources
The freely available resources this lesson will direct you to.
Hello. This lesson turns the CLSI–EUCAST comparison into a laboratory decision. The question is not which system is better in the abstract, but which one a particular laboratory can use correctly, maintain, and defend for the organisms and antimicrobials it reports.
You will build a recommendation from four kinds of evidence: local policy, regulatory requirements, method compatibility, and access to current guidance. A recommendation is sound only if it survives all four checks. Allow about 40 minutes, including the readings.
Begin with the laboratory’s obligations
Suppose two laboratories can perform the same disk-diffusion test. One participates in a coordinated national EUCAST programme; the other reports into a network that uses CLSI and depends on comparable surveillance data. Even before examining disks or agar, their circumstances differ. A switch could improve consistency for the first laboratory while disrupting it for the second.
A useful first distinction is between a requirement and a preference. A regulator’s rules, an accreditation condition, or a binding national policy may constrain the choice. Familiarity with one system, lower document costs, and comparability with neighbouring laboratories matter, but they do not override such requirements. If no policy specifies a system, ask which guidance the laboratory’s clinical partners and surveillance programme use—and whether a change can be coordinated rather than made by one laboratory alone.
Time to switch from CLSI to EUCAST? A Southeast Asian perspective
Read this article as a historical decision case. It shows why attractive features of a standard do not, by themselves, settle whether a laboratory should adopt it.
In the Introduction, find the paragraph describing a laboratory network already using CLSI disk diffusion. Read the initial tradeoff from the perceived benefits to the end of that paragraph. Then find the paragraph in the closing discussion beginning “Despite compelling reasons to adopt EUCAST AST guidelines.” Read the decision through its final sentence. Focus on national coordination and data comparability; do not treat the article’s dated plans or prices as current policy.
The case illustrates a principle that travels well beyond its setting: an isolated change has consequences outside the laboratory. Clinicians must understand the reporting terminology; surveillance partners must know which interpretive criteria produced the data; and staff need a controlled transition. This does not mean “keep CLSI whenever you already use CLSI.” It means that any proposed switch needs a stronger justification than document access alone.
Regulation adds a different constraint. In the United States, for example, an automated antimicrobial susceptibility testing (AST) device can lag behind revised breakpoints. The laboratory must know what its device reports, what its manufacturer supports, and what verification or validation is required before changing interpretation. Those details cannot be inferred simply from the word “CLSI” or “EUCAST” on a proposed policy.
Updating Breakpoints in Antimicrobial Susceptibility Testing
This American Society for Microbiology article explains why choosing a breakpoint source and implementing it on a commercial device are separate decisions.
In the opening paragraph, before “New College of American Pathologists (CAP) Checklist Requirement for AST Breakpoints,” start at the discussion of automated AST devices. Read the device problem. Pay attention to the possible delay between a revised breakpoint and a device that can support it. Treat the article’s regulatory discussion as a United States example, not a rule for every jurisdiction.
At this stage, a responsible recommendation might still be conditional: Local policy favours EUCAST, provided the laboratory can implement its specified methods and interpretations. The next check tests that condition.
Check the method, not just the breakpoint table
A disk-diffusion zone diameter has meaning only in the setting for which its interpretation was established. The organism identification, disk content, medium, inoculum, incubation, reading procedure, and quality-control (QC) scheme all matter. A laboratory therefore cannot choose its preferred technique from one system and its preferred zone breakpoint from the other as if the values were interchangeable.
The differences are practical, not merely editorial. For some tests, CLSI and EUCAST specify different disk contents, supplemented media, incubation conditions, or QC strains. Coverage can differ too: an organism–agent combination that can be interpreted by disk diffusion under one system may require another method under the other. Check the current, applicable tables and notes before relying on any example from a published comparison.
Time to switch from CLSI to EUCAST? A Southeast Asian perspective
Return to the case study to examine what a switch would require at the bench, beyond obtaining different breakpoint tables.
Find the paragraph beginning “Recommendations for inoculum preparation” immediately before Table 1. Read the method overview, then inspect Table 1 through its final QC-strain row. Continue into the paragraph immediately after the table for the QC comparison. In the following cost discussion, read the testing tradeoff. Finally, read the media constraint. Focus on which requirements the laboratory could actually meet; the listed products, costs, and coverage are historical examples.
This reading reveals a common false economy. Free access to guidance may save a subscription cost, yet the chosen method might require different disks, reliable access to a specified medium, or an additional MIC-testing method. Conversely, remaining with an established method is not justified if the laboratory cannot meet its media, QC, or update requirements either.
For each major organism–agent combination in a proposed service, the compatibility check should establish:
- Coverage: Does the current system provide an interpretation for this organism, drug, and test method?
- Bench conditions: Can the laboratory meet the specified disks or panels, media, incubation, reading rules, and QC?
- Reporting conditions: Does the result have the intended clinical meaning under the dosing and reporting assumptions in the applicable guidance?
A laboratory may have a documented reason to use different systems for separate, clearly defined testing workflows. That is not permission to combine a method from one system with QC limits or interpretive breakpoints from the other within a single workflow. Any exception needs applicable guidance, local authorization, validation where required, and unambiguous documentation of how each result was produced.
Confirm that the choice can be kept current
“Use CLSI” or “use EUCAST” is incomplete unless someone can identify the edition or version in use and maintain it. Access means more than being able to download a table today. The laboratory needs a workable process for checking updates, assessing affected organism–agent combinations, updating procedures and software, training staff, and retaining the version used to interpret past results.
Automated systems make this especially important. A device may contain an older interpretation even when newer guidance is available; its test panel may not measure a range that accommodates a revised breakpoint. A recommendation must therefore distinguish the desired interpretive criteria from what the instrument currently performs and reports. Depending on applicable regulation, using manufacturer-cleared criteria may call for verification, while modifying device interpretation may require more extensive validation. Neither step should be assumed to be automatic.
Updating Breakpoints in Antimicrobial Susceptibility Testing
Use the article’s implementation discussion to identify questions a laboratory must resolve with its device manufacturer before recommending a change.
Under “Validation and Verification Steps for Updating Breakpoints,” go to “Step 2: Engage Industry Partners.” Begin where the text asks laboratories using commercial AST systems to contact the manufacturer. Read the device checklist, stopping before the paragraph about manufacturers’ supportive tools. Focus on clearance status, the difference between verification and validation, and whether the panel or card can measure around the proposed breakpoint. Check current local rules rather than applying the article’s dated deadlines universally.
Before signing off on either system, ask who will check the current CLSI or EUCAST documents, how changes reach the laboratory information system, and how staff will recognize a device–guidance mismatch. If the laboratory cannot fund current CLSI guidance, that is a genuine obstacle to a CLSI recommendation. If it cannot obtain supplies needed to follow EUCAST methods, free EUCAST tables do not remove that obstacle.
Turn the evidence into a recommendation
Consider three illustrative contexts.
A coordinated EUCAST service. Local policy calls for EUCAST; the laboratory can obtain the specified materials, perform the relevant QC, cover its routinely reported organism–agent combinations, and access current guidance. Recommend EUCAST, with a documented implementation plan for method setup, reporting terminology, staff training, and updates. The reason is not simply that EUCAST is freely available: policy alignment and feasible implementation make it the defensible choice.
An established CLSI service with no directive to switch. Its surveillance partners use CLSI; its current workflow meets the applicable CLSI method and QC requirements; it can obtain current documents. A proposed EUCAST switch would leave an important routine test without a feasible method. Recommend continuing CLSI for now, while reviewing the unresolved test and reassessing if policy or resources change. Do not silently substitute EUCAST breakpoints for its CLSI disk method—or assume an older published comparison still describes today’s coverage.
A regulated laboratory with an automated device carrying older criteria. Local policy permits CLSI, but some current CLSI-relevant breakpoint values are not yet implemented on the device. Do not recommend simply retaining the older values because the device produces them. Recommend a CLSI-based service only alongside a documented assessment of the affected results, manufacturer capabilities, applicable regulatory requirements, and a permitted verification, validation, or alternative testing plan. If those conditions cannot be met, the proposed workflow is not yet ready for that use. Choosing EUCAST instead would require the same scrutiny of policy, method, device, and regulatory compatibility; it is not a shortcut around the mismatch.
Notice how each answer includes both a choice and its conditions. A concise recommendation for a real laboratory can use this form:
Recommend [CLSI/EUCAST] for [defined testing scope] because [local policy and regulatory position] and [method and QC compatibility]. Current guidance is accessible through [maintenance process]. Before implementation, resolve [specific coverage, device, validation, or reporting issue].
That final sentence prevents a preference from masquerading as a fully implemented service. It also makes the recommendation auditable: another reader can see which facts would change the conclusion.
Conclusion
Recommend the system a laboratory is permitted and able to apply as a complete, current testing workflow. Start with local policy and regulation; test the choice against organism–agent coverage, methods, QC, and device capability; then confirm a sustainable route to current guidance and updates. Where evidence is missing, state a condition rather than making an unconditional choice.
The next lesson moves from recommending a system to checking its source documents: how to verify that a particular version, table, and note are current and applicable to the result you intend to report.