Quick facts
- A susceptibility test medium is the agar that carries the antibiotic in a diffusion or dilution test and that lets a laboratory read whether an organism is likely to respond to it.
- United States: the medium is reached as a culture medium for antimicrobial susceptibility tests at 21 CFR 866.1700, placed in Class II; the openFDA classification API lists it under product code LKA.
- Canada: a microbiological medium used to identify or infer the identity of a microorganism is named at Class I in the table to Rule 9 of Part 2 to Schedule 1, and a medium not caught there reaches Class I through Rule 8 of the Medical Devices Regulations, SOR/98-282.
- The two countries land on different classes for the same medium: Class II in the United States and Class I in Canada, which is the clearest example in the laboratory range of a device whose class follows the rule path rather than the object.
- A general differential medium is a separate entry again, so a laboratory comparing one agar with another is comparing entries rather than names.
- The entry and rule text below were checked against the current CFR text and the Regulations on 2026-10-08.
A susceptibility test medium is the plate that turns an organism into an answer. A disc diffuses into the agar and the zone around it is measured, or a dilution series gives a minimum inhibitory concentration, and the laboratory reads the medium to decide whether a drug is likely to work. The medium is bought as a poured plate or as a powder, and it is filed as a device in both countries at two different classes. This article sets out what the medium does, how each country files it, and what a laboratory should be able to point at when the box is opened and when the plate is read.
What does a susceptibility test medium do?
The medium carries the nutrients the organism needs and sets the conditions under which the antibiotic behaves predictably. In a diffusion test the drug spreads through the agar and the zone of no growth is the measurement; in a dilution test the drug is held at a series of concentrations and the point at which growth stops is the result. Either way the depth of the agar, its pH and its cation content are what make the reading reproducible from one run to the next.
Because the medium is the thing that produces the diagnostic reading, it is not an ordinary nutrient plate. A medium with the wrong depth gives a zone that is too wide or too narrow, and a laboratory that reads it without checking the depth reads a number that belongs to the plate rather than to the organism.
How does the United States file a susceptibility test medium?
In the United States the medium is reached by a named entry. 21 CFR 866.1700 covers a culture medium for antimicrobial susceptibility tests, and the section places it in Class II. The openFDA classification API lists it under product code LKA at Class II. A Class II device of this kind reaches the market under general and special controls.
A general differential medium sits in its own entry at Class I, so the same agar poured for identification and poured for a susceptibility panel is two different articles to the rules even when the formula is the same. The claim the medium is sold under is what decides which entry applies.
Why does Canada reach Class I by rule?
Canada classifies the medium through the in vitro diagnostic device rules in Part 2 to Schedule 1. Rule 9 carries a table, and item 3 names microbiological media used to identify or infer the identity of a microorganism at Class I; item 4 names an IVDD used to identify or infer the identity of a cultured microorganism at Class I. A medium not caught by that table and not caught by Rules 1 to 7 reaches Class I through Rule 8, which catches every other IVDD.
The result is a real difference between the two countries for the same plate. The United States files a susceptibility medium at Class II, and Canada reaches Class I, so a laboratory comparing a supplier's claim across the border is comparing two classes and two rule paths rather than one device.
| Item | Canada | United States |
|---|---|---|
| Class | Class I, by the table to Rule 9 or by Rule 8 | Class II |
| Named entry | No entry names the medium; it is an IVDD | 21 CFR 866.1700 |
| Basis of the class | The IVDD rule that catches the medium | The named entry the medium is filed under |
| Product code | Not applicable | LKA |
| A general differential medium | Class I, like other IVD media | Its own entry at Class I |
| Premarket route | Establishment licence; a Class I IVDD does not get a licence of its own | General and special controls |
| Quality system | The manufacturer keeps the records the Regulations require | The manufacturer keeps the records the regulation requires |
The difference in class is a difference in the rule path rather than a disagreement about the agar. A laboratory that orders to a specification rather than to a name keeps the two classes separate on the shelf.
Which standards sit behind the medium?
The class says how the medium is regulated; the standards say how it has to behave. ISO 13485:2016 is the quality management basis behind the manufacturer's records, and ISO 14971:2019 is the risk management basis behind the design file. A laboratory that asks for those two documents is asking for the evidence behind the zone the medium produces.
| Element | What it covers | Why it matters on the bench |
|---|---|---|
| Agar depth | The thickness of the poured plate | A plate too shallow gives a zone that is too wide |
| pH | The acidity of the medium | An out-of-range pH shifts how the drug diffuses |
| Cations | Calcium and magnesium content | The cation level changes the result for some drugs |
| Lot | The batch the plate came from | A shift between lots is a shift in the readings |
| Control organism | The reference strain run with the plate | The check that the medium is behaving as expected |
| Records | The lot and the run | The link between a reading and the plate it came from |
A plate that cannot show its lot and a control organism that cannot be traced to a run are the two gaps that meet on the same bench.
How is the plate read against the control?
A susceptibility plate is read against a control organism run in the same batch. The control is the reference the zone or the dilution is compared with, and it is the field that turns a measurement into a reading. A plate read without a control is a measurement with no yardstick, and a laboratory that records the reading without the control records half the result.
The medium itself is checked at the same time. The lot, the depth and the date are the three fields that decide whether the plate is behaving as the maker intends, and a shift in any of them is a shift in the readings rather than in the organism. The record that ties the two together is the document the class implies.
What else sits beside the medium?
The microbiology bench is stocked as a set. A sterile petri dish, 100 x 15 mm is the vessel the medium sets in, and sterile centrifuge tubes, 15 mL sit with the sample-prep lines. Clinics that monitor their own reprocessing pair the sterilization monitoring collection with the 24-hour biological indicator 50-pack. Multi-site groups ordering at case level use the wholesale ordering page, and the B2B wholesale collection lists the lines held for institutional buyers. Reselling? become a distributor.
Sources
- 21 CFR 866.1700, culture medium for antimicrobial susceptibility tests
- openFDA device classification API
- Medical Devices Regulations, SOR/98-282, full text and Schedule 1
- US FDA, medical devices
- Health Canada, medical devices
- Standards Council of Canada
- CSA Group
- ISO 13485:2016, medical devices quality management
- ISO 14971:2019, risk management of medical devices
- Clinical and Laboratory Standards Institute
- Public Health Ontario, infection prevention and control
- World Health Organization
- US CDC, laboratory quality
- CCOHS, OSH answers
A microbiology bench is audited on the record that follows the plate. The printable media and control log is a blank sheet with columns for the medium, the lot, the control organism, the reading and the operator, so a plate and its reading can be reconciled on one form.
The media log generator on the same page sizes the sheet to the number of benches and shifts a laboratory runs, so a small plate room and a regional laboratory print the form at the volumes they need.
Every laboratory and every medium differs. Ask a compliance specialist and you will get a written answer specific to your equipment and province, with the regulation or standard it is based on cited.
Related reading
- Culture media in Canada vs the US: which rules apply to differential and transport types
- Cell and tissue culture flasks in Canada vs the US: which rules
- Specimen acceptance criteria in Canada vs the US: which rules
CliniEco Medical supplies eye care, laboratory and dental consumables of the kind described in this article. MDEL #35334. This article is written for optometry, laboratory and procurement professionals and is not legal advice; the class of a device and the records kept by a practice belong to the manufacturer and to the requirements that apply to it.
Frequently Asked Questions
What class is susceptibility test media in Canada?
A microbiological medium used to identify or infer the identity of a microorganism is named at Class I in the table to Rule 9 of Part 2 to Schedule 1, and a medium not caught there reaches Class I through Rule 8.
What class is susceptibility test media in the United States?
It is a named entry at 21 CFR 866.1700 in Class II, and the openFDA classification API lists it under product code LKA.
Why do the two countries differ on the class?
Because the United States names the medium for susceptibility testing at Class II, while Canada reaches Class I through the IVDD media table and Rule 8. The class follows the rule path rather than the agar itself.
Is a general differential medium the same device?
No. In the United States a general differential medium sits in its own Class I entry, and in Canada it is caught by the same IVD media rules, so it is a different article from a susceptibility medium.
Does agar depth really change the result?
Yes. A plate that is too shallow or too deep changes how far the drug diffuses, so a measured zone belongs to the plate as well as to the organism.
What should a laboratory check with each lot?
That the lot number is recorded and that a control organism was run alongside the plate. The control is the check that the medium is behaving as expected.
Does a Class I IVD medium need its own licence?
No. A Class I in vitro diagnostic device does not get a licence of its own; the manufacturer keeps the records the Regulations require and the establishment holds the establishment licence.
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