Quick facts
- An antimicrobial susceptibility test disc is the small paper disc that carries a measured amount of an antimicrobial, placed on a seeded plate so that the drug diffuses into the agar.
- United States: the disc is reached as an antimicrobial susceptibility test disc at 21 CFR 866.1620, placed in Class II under performance standards.
- Canada: the disc is an in vitro diagnostic device used to guide treatment, so it reaches Class II through Rule 3 of Part 2 to Schedule 1 to the Medical Devices Regulations, SOR/98-282.
- The two countries land on the same class: Class II in the United States and Class II in Canada, by different machinery.
- The class can move up where an erroneous result would lead to a management decision with an imminent life-threatening outcome, which Rule 3 places in Class III.
- The entry and rule text below were checked against the current CFR text and the Regulations on 2026-10-09.
An antimicrobial susceptibility test disc is the paper carrier that turns a plate into a treatment decision. It holds a measured amount of a drug, it is placed on agar that has been seeded with the organism, and the zone of no growth that forms around it is measured against a standard range. The disc is a small object with a large duty, and it is filed as a device in both countries. This article sets out what the disc does, how each country files it, and what a laboratory should be able to point at when a box is opened and when a zone is read.
What does an antimicrobial susceptibility test disc do?
The disc carries a set quantity of an antimicrobial in a stable form. When it is placed on a seeded plate, moisture in the agar releases the drug and it diffuses outward at a rate the manufacturer controls. The organism grows everywhere except where the drug inhibits it, and the diameter of the clear zone is read against a range to give a category such as susceptible, intermediate or resistant. The drug load, the disc diameter and the storage of the disc are the three fields that decide how the zone behaves.
The disc guides treatment, so it is used for patient management rather than for identification. That use is the fact that fixes its class in Canada, and it is why the disc and a general differential medium do not share a class even when both are stored on the same shelf.
How does the United States file a susceptibility test disc?
In the United States the disc is reached by a named entry. 21 CFR 866.1620 covers an antimicrobial susceptibility test disc, and the section places it in Class II under performance standards. A Class II device of this kind reaches the market under general and special controls, and the performance standard is what sets the zone ranges the laboratory reads.
The entry names the disc and its use in the susceptibility test, so the disc and the powder of the same drug sit in two different entries. A laboratory comparing a disc with a dilution panel is comparing two entries rather than one device and its part.
Why does Canada reach Class II by rule?
Canada classifies the disc through the in vitro diagnostic device rules in Part 2 to Schedule 1. Rule 3 provides that an IVDD intended to be used for patient management is classified as Class II, unless it falls into one of the listed categories, in which case it is classified as Class III. A susceptibility disc guides treatment, so it is used for patient management and reaches Class II.
The class moves on what a wrong result could do. Where a disc is used in the management of a patient suffering from a life-threatening disease, or where an erroneous result would lead to a management decision with an imminent life-threatening outcome, Rule 3 lifts it to Class III. The need for accuracy in the reading is what separates the two classes.
| Item | Canada | United States |
|---|---|---|
| Class | Class II, by Rule 3 | Class II, performance standards |
| Named entry | No entry names the disc; it is an IVDD | 21 CFR 866.1620 |
| Basis of the class | The rule that fits an IVDD used for patient management | The named entry the disc is filed under |
| When it moves up | Class III where a wrong result could be immediately life-threatening | The named entry sets a single class |
| Premarket route | Device licence under section 32 for a Class II device | General and special controls |
| Quality system | CAN/CSA-ISO 13485 certificate behind the licence | Quality system regulation |
The two countries agree on the class and disagree on the machinery: the United States names the disc and Canada reaches it through the rule for an IVDD used in patient management. A laboratory that keeps the disc lot and the control organism with the reading has the record the class implies.
Which standards sit behind the disc?
The class says how the disc 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 disc produces.
| Element | What it covers | Why it matters on the bench |
|---|---|---|
| Drug load | How much antimicrobial the disc carries | A load off its value gives a zone that is too wide or too narrow |
| Disc diameter | The size of the carrier | The diameter is part of the zone the range expects |
| Agar depth | The thickness of the poured plate | A plate too shallow gives a zone that is too wide |
| Storage | How the disc box is kept | Discs stored damp or warm lose their drug load |
| Control organism | The reference strain run with the plate | The check that the disc is behaving as expected |
| Records | The lot and the run | The link between a reading and the disc it came from |
A disc box 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 a disc panel read against the control?
A susceptibility panel is read against a control organism run in the same batch. The control is the reference the zone is compared with, and it is the field that turns a measurement into a reading. A panel read without a control is a measurement with no yardstick, and a laboratory that records the zone without the control records half the result.
The disc box is checked at the same time. The lot, the storage and the expiry are the three fields that decide whether the drug load the zone depends on is still there, and a box of discs kept warm is a box whose zones shorten before the reading is taken.
The zone is also read against the ranges the laboratory works to rather than against the last reading it took, because a category carried over from a previous run is not a reading of the plate in front of the reader. A laboratory that records the range with the zone has the field that makes the reading reproducible from one run to the next.
What else sits beside the disc?
The susceptibility bench is stocked as a set. A sterile petri dish, 100 x 15 mm is the plate the disc sits on, and sterile centrifuge tubes, 50 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 866.1620, antimicrobial susceptibility test disc
- openFDA device classification API
- Medical Devices Regulations, SOR/98-282, full text and Schedule 1
- US FDA, medical devices
- US FDA, medical device databases
- US FDA, overview of device regulation
- 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 susceptibility bench is audited on the record that follows the plate. The printable disc and control log is a blank sheet with columns for the organism, the disc, the lot, the zone, the control and the operator, so a reading can be reconciled with the disc it came from.
The disc log generator on the same page sizes the sheet to the number of plates and shifts a laboratory reads, so a small bench and a regional laboratory print the form at the volumes they need.
Every laboratory and every disc 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, differential and selective, in Canada vs the US: which rules
- Laboratory quality control materials in Canada vs the US: which class
- 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 an antimicrobial susceptibility test disc in Canada?
It is an in vitro diagnostic device used for patient management, so it reaches Class II through Rule 3 of Part 2 to Schedule 1.
What class is the disc in the United States?
It is a named entry at 21 CFR 866.1620 in Class II under performance standards.
Do the two countries agree on the class?
Yes. The disc is Class II in the United States and Class II in Canada, reached by a named entry on one side and by the patient-management rule on the other.
When would the disc reach Class III in Canada?
Rule 3 lifts an IVDD used for patient management to Class III where it is used to manage a life-threatening disease, or where an erroneous result would lead to a management decision with an imminent life-threatening outcome.
What sets the zone range the laboratory reads?
The performance standard behind the US entry and the maker's stated ranges on the Canadian side, together with the control organism run alongside the plate.
Why does storage matter so much for a disc?
A disc holds a measured drug load, and a box stored warm or damp loses part of it, which changes the zone before the plate is ever seeded.
What should a laboratory record for a susceptibility test?
The organism, the disc, the lot, the zone and the control organism, kept together as a run record. That is the link between a reading and the disc it came from.
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