CLSI M100 is the annual performance standard that supplies the clinical breakpoints, quality control ranges and reporting tables used for antimicrobial susceptibility testing. It does not create a legal reporting obligation, and it does not replace the laboratory policy that decides which interpretive system the bench follows. Because M100 is revised every year and breakpoints move between editions, a susceptibility report that does not name the edition used cannot be reproduced.
What does CLSI M100 actually contain?
Three blocks of content matter to a working bench: interpretive breakpoints, quality control material, and the screening and confirmatory tests for specific resistance mechanisms.
Breakpoints are published as MIC values and as disk diffusion diameters for the organism and agent combinations in the tables. The same document lists quality control ranges with the reference strains used to verify each test, and organises interpretation tables by organism group and agent class.
How often is M100 updated, and why does the edition matter?
M100 is revised once a year, and the edition code identifies both the standard and its year. Each revision adds agents, adjusts quality control ranges and revises interpretive breakpoints for at least some organism and agent combinations.
That is why the edition belongs in the record. If a result is reviewed against a later edition, an interpretation can change without the isolate having changed at all, and the reviewer has no way to tell whether the difference came from the organism or from the document. Naming the document and edition on the report is the control that makes a result reproducible.
How do CLSI and EUCAST breakpoints differ?
They are separate interpretive systems. Each sets breakpoints from its own data, uses its own dosing assumptions for the agent in question, and groups organisms differently in places, so the same MIC can land in a different category under each system.
A laboratory works to one system by policy and states which one it used, because mixing them across reports produces results that cannot be compared with each other. FDA breakpoints are a third reference point, and for some agents they are the only breakpoints published, which is another reason the source belongs on the report rather than in a conversation.
Which quality control strains are required?
The quality control tables name the reference strains to be run with each agent and method, and the specific numbers live in the tables rather than in memory. A typical panel includes an enteric reference strain and a Pseudomonas strain, plus a fungal strain when antifungal agents are tested, alongside the strains specified for each resistance mechanism screen.
Quality control is a release gate rather than a documentation exercise. If a control result falls outside its published range, the results read in that run are not reportable until the cause is identified and the run is repeated.
What supplies does a susceptibility bench need?
The consumables behind the standard are the ordinary ones: Mueller-Hinton plates, inoculum standards and sterile tubes, swabs and loops, plus the disks or MIC panels for the agents on your formulary.
Two procurement details matter more than the brand. Plate depth and lot consistency affect reading, and the swab used to lawn the plate should be specified in the procedure so the result is not a function of whichever applicator was on the bench. Record the lot with the run so an out-of-range control can be traced to the material or to the technique.
| M100 provides | M100 does not provide |
|---|---|
| Clinical breakpoints as MIC values and disk diffusion diameters | Legal reporting obligations, which come from your regulator or authority |
| Quality control ranges and reference strains | Accreditation decisions, which belong to the accreditation body |
| Screening and confirmatory tests for specific resistance mechanisms | Instrument calibration and maintenance schedules |
| Organism-specific interpretation tables organised by agent class | Clinical judgement about whether a result fits the patient |
The division in that table is the reason a laboratory documents its own policy alongside the standard. M100 supplies the interpretive values; the laboratory supplies the decisions about what it reports, to whom, and under which quality system.
Related Reading
- Clinical supply learning hub
- Diagnostic laboratory supply checklist
- Agar Petri dishes: types, prepared plates and storage
- QC laboratory supplies and sterility testing consumables
Ordering for a clinic, lab or care home? Wholesale and multi-site ordering covers case pricing and account setup, and the B2B wholesale collection lists the lines stocked for institutional buyers.
wholesale account to trial the bench consumables that sit behind a susceptibility run.
Related Reading
- Diagnostic laboratory supply checklist
- Agar Petri dishes: types, prepared plates and storage
- QC laboratory supplies and sterility testing consumables
- Clinical supply learning hub
Frequently Asked Questions
How often is CLSI M100 updated?
Annually. Each edition revises breakpoints, adds new agents and adjusts quality control ranges, which is why a report that does not state the edition used cannot be reproduced against a later document.
What is the difference between CLSI and EUCAST breakpoints?
They are separate interpretive systems with different methods for setting breakpoints, different dosing assumptions and some different organism groupings. A laboratory works to one system by policy and states which one it used, because mixing them produces results that cannot be compared.
Does a Canadian laboratory have to use CLSI M100?
No law requires it. Adoption comes through accreditation requirements, provincial laboratory programmes and hospital policy. Some laboratories run EUCAST instead, and some report both for specific organisms.
Why can our breakpoints differ from a published guideline?
Almost always because of edition or system. M100 values change between annual editions, and FDA breakpoints for the same agent can differ from CLSI. The fix is procedural: record the document and edition used on the report so the number can be traced.
CliniEco Medical is a licensed medical device establishment (MDEL #35334).
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