Quick facts
- An automated zone reader measures the diameter of the growth-inhibition zone around an antimicrobial disc on an inoculated agar plate.
- In the United States it files under 21 CFR 866.2850, Automated zone reader, at Class I, product code KZK.
- In Canada it is an in vitro diagnostic device. The reader is an analyzer used with another IVDD, so Rule 7 of Schedule 1, Part 2 gives it the class of the device it works with — Class II for a susceptibility disc or panel.
- The near miss is Rule 8, the residual Class I default for IVDDs that no other rule reaches; Rule 7 displaces it because the reader is not used on its own.
- The published disc-diffusion method the reader supports sets the quality-control limits; the class sets the licence path, not the zone limits.
- Entries, classes and product codes below were checked against the eCFR, the Regulations and the openFDA classification database on 2026-10-14.
An automated zone reader is the instrument that measures, to the tenth of a millimetre, the clear ring of inhibited growth around an antimicrobial disc on an inoculated plate. It replaces the ruler and the bench lamp, and it produces a number that goes into the susceptibility report. The United States keeps it at Class I as a mechanical measuring device; Canada reads it as an in vitro diagnostic analyzer and lifts it to Class II, and this article sets out why — with the graduated specimen cups that sit on the same bench.
What does an automated zone reader do?
It measures zone diameters of microbial growth inhibition, such as those observed on the surface of certain culture media used in disc-agar diffusion antimicrobial susceptibility tests. The plate is laid in the instrument, the optics or the camera find the disc and the surrounding ring, and the diameter is recorded in millimetres against the disc identity and the organism.
The measurement is only meaningful against the method the laboratory runs. The medium, the inoculum density, the incubation time and temperature and the disc potency all move the zone, which is why the laboratory verifies the reading against the published control limits for the reference strains it uses, and why the reader is treated as part of a method rather than as a free-standing measuring tool.
The instrument's own contribution is the edge it finds. A plate read by eye under a bench lamp depends on the reader's judgement at the margin of the inhibition ring; an automated reader fixes the edge by contrast and reports a number in millimetres. That reproducibility is what makes the reading usable in a trend, and it is also what ties the reading to the instrument serial that produced it.
What class is an automated zone reader in the United States?
It is a named entry at Class I. 21 CFR 866.2850 is titled Automated zone reader, and the identification reads: "An automated zone reader is a mechanical device intended for medical purposes to measure zone diameters of microbial growth inhibition (or exhibition), such as those observed on the surface of certain culture media used in disc-agar diffusion antimicrobial susceptibility tests. The device aids in decisionmaking respecting the treatment of disease."
The openFDA device classification API lists product code KZK against 866.2850. The placement is a reflection of how the United States writes this part of the code: a measuring instrument that supports a test but does not itself produce a diagnostic result is treated as a low-risk article.
| Item | Detail |
|---|---|
| US regulation | 21 CFR 866.2850 - Automated zone reader |
| US class | Class I (general controls) |
| Product code | KZK |
| Route to market | Registration and listing; general controls |
| Stated function | To measure zone diameters of microbial growth inhibition on disc-agar diffusion plates |
| Used with | Antimicrobial susceptibility discs and agar media |
How does Canada classify an automated zone reader?
Canada keeps no named entry. The reader is an in vitro diagnostic device — it is used in vitro on a clinical specimen to produce information that aids a treatment decision — so its class comes from Part 2 of Schedule 1 of the Medical Devices Regulations. For a device of this shape the governing rule is Rule 7: "In cases where an IVDD, including its analyzers, reagents and software, is intended to be used with another IVDD, the class of both IVDDs will be that of the IVDD in the class representing the higher risk."
The words including its analyzers are the operative ones. The reader is used with a susceptibility disc or an identification and susceptibility panel, and those devices sit at Class II under Rule 4, the general diagnostic default, because they are used in diagnosis or patient management. Read together, Rule 7 pulls the analyzer to the same class as the article it works with, which is why the Canadian answer is Class II rather than the Class I that Rule 8 would give a stand-alone device.
| Item | Canada | United States |
|---|---|---|
| Named entry | None; classified by rule | 866.2850 - Automated zone reader |
| Class | Class II (Rule 7, analyzer used with a Class II IVDD) | Class I (general controls) |
| Governing text | Schedule 1, Part 2, Rule 7 | 21 CFR 866.2850 |
| Residual rule | Rule 8 would give Class I to a stand-alone IVDD; Rule 7 displaces it here | Not applicable |
| Route to market | Device licence with quality-system evidence for the analyzer and the assay it runs | Registration and listing |
Why does an analyzer take the class of the test it runs?
Because an analyzer without an assay is not a diagnostic device. The reader produces a zone diameter; the interpretation of that diameter depends on the organism, the agent and the breakpoint table the laboratory is using. The risk sits in the reported susceptibility result, not in the optics that measured the ring, so the rule that places the analyzer looks through to the assay it serves.
That is a deliberate design choice on the Canadian side, and it is mirrored in the way a laboratory's own quality system treats the reader. The instrument is verified with the assay, its readings are checked against the control limits for the reference strains, and the record ties the instrument to the method. The class and the quality routine point at the same unit of work.
What should a laboratory check before ordering one?
Four checks. First, confirm the plate formats and disc sizes the reader accepts, because a reader built for 90 mm plates is a different article from one built for large square plates. Second, confirm the maker's calibration and verification routine and how a reading is traced back to the instrument serial. Third, confirm the connectivity the laboratory needs, since the reading has to reach the laboratory information system with the plate identity attached. Fourth, confirm that the breakpoint rules the reader applies match the ones the laboratory reports against.
A diagnostic laboratory also restocks the bench consumables used through the susceptibility bench work-up. Clinics that stock for a whole practice can order through the wholesale programme and request a bulk quote for a standing or annual order; the graduated specimen cups sits in the same catalogue. Consumables for this line are listed under diagnostic laboratories. Reselling? become a distributor.
How does the class affect the laboratory's own validation?
The class sets who reviews the device before it reaches the market; it does not set the laboratory's validation. Under a quality-management framework such as the one described in the medical-laboratory standard, the laboratory verifies a new instrument before reporting patient results from it, and it keeps the verification record for as long as its own document control requires.
The verification holds whichever class the reader carries. What changes with the class is the market-entry evidence the maker has to hold — a device licence with quality-system evidence for Class II rather than registration and listing — and that evidence is part of what a laboratory can ask a supplier to produce when it adds the instrument to the bench.
Sources
- openFDA device classification API
- Medical Devices Regulations, SOR/98-282, full text and Schedule 1
- US FDA, medical device databases
- US FDA, overview of device regulation
- Health Canada, medical devices
- Health Canada, establishment licensing
- Standards Council of Canada
- CSA Group
- ISO 13485:2016, medical devices quality management
- ISO 14971:2019, application of risk management to medical devices
- ISO 15189:2022, medical laboratories, requirements for quality and competence
- Clinical and Laboratory Standards Institute
- Public Health Ontario, infection prevention and control
- World Health Organization
- US CDC, laboratory quality
- CCOHS, OSH answers
- ISO 20776-1:2019, susceptibility testing of infectious agents and evaluation of performance of antimicrobial susceptibility test devices
A printable bench sheet for the susceptibility run. A zone reading only means something with the plate identity, the medium lot and the reference-strain control beside it, and that is a single page. The compliance log centre holds the blank sheet — print it from the browser, no account and no sign-up, and run as many copies as the bench needs.
One set-up, a dated sheet back. Enter the laboratory name, the reader serial and the control strains once, and the sterilization log generator returns a dated layout you can print or file into the quality manual.
Ask a compliance specialist. A classification question usually turns on how the device is used, not only on its name. Ask a compliance specialist — you get a written answer specific to your equipment and province, with the regulation or standard it is based on cited.
Related reading
- Antimicrobial susceptibility test discs in Canada vs the US
- Antimicrobial susceptibility test media in Canada vs the US
- Culture media quality control kits in Canada vs the US
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 dental procurement professionals and is not legal advice; the class of a device and the records a practice keeps belong to the manufacturer and to the requirements that apply to it.
Frequently Asked Questions
What class is an automated zone reader in the United States?
Class I under 21 CFR 866.2850, product code KZK.
What class is it in Canada?
Class II. The reader is an in vitro diagnostic analyzer used with a Class II susceptibility device, so Rule 7 of Part 2 of Schedule 1 gives it the class of the device it reads.
Why not Class I under Rule 8?
Rule 8 is the residual rule for IVDDs that no other rule reaches. Rule 7 applies first, because the reader is used with another IVDD rather than on its own.
Does the class change the zone limits the laboratory uses?
No. The zone limits come from the published method and the control ranges for the reference strains. The class sets the licence path for the instrument.
Is the reader a stand-alone device?
No. The zone diameter it produces only becomes a susceptibility result once the organism, the agent and the breakpoint table are applied, which is why the instrument is classified with the assay it serves.
What does the laboratory keep as a record?
The plate identity, the medium lot, the disc or panel lot, the control-strain result and the instrument used, so a reported result can be traced back to the run that produced it.
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