How many medical device classes are there?

Four under the MDR: class I (low risk), IIa (moderate risk), IIb (high risk) and III (highest risk). In vitro diagnostic devices follow a separate classification under the IVDR, in classes A, B, C and D.

How do you determine a medical device's class?

By applying the rules of MDR Annex VIII to the device's intended purpose, duration of use (transient, short term, long term), invasive or active nature and the part of the body concerned. If several rules apply, the strictest prevails. MDCG 2021-24 illustrates each rule with examples.

What is the difference between transient, short-term and long-term use?

Under Annex VIII, transient use normally lasts less than 60 minutes continuously, short-term use between 60 minutes and 30 days, and long-term use more than 30 days.

Does a class I device need a notified body?

No, unless it is sterile (Is), has a measuring function (Im) or is a reusable surgical instrument (Ir): the notified body then assesses those aspects only. Other class I devices rely on the manufacturer's self-declaration.

Medical device classification: the MDR rules

A medical device's class drives almost everything else: the conformity assessment procedure, whether a notified body is involved, the depth of clinical evidence, the PSUR frequency. A classification error is costly, sometimes at audit time. This guide presents the rules of MDR Annex VIII and how to apply them.

The principle

MDR Article 51 divides devices into four classes according to their intended purpose and inherent risks:

Class	Risk level	Common examples
I	Low	Dressings, wheelchairs, corrective glasses
IIa	Moderate	Hypodermic needles, ultrasound scanners, most decision-support software
IIb	High	Infusion pumps, external defibrillators, X-ray equipment
III	Highest	Cardiac implants, hip replacements, resorbable devices

It is the intended purpose declared by the manufacturer, not how some users use the device, that determines the class. If the manufacturer and the notified body disagree, the manufacturer's competent authority decides (Article 51(2)).

The implementing rules

Before the 22 rules, Chapter II of Annex VIII sets reading rules:

Duration of use matters: transient (normally less than 60 minutes continuously), short term (60 minutes to 30 days), long term (more than 30 days).
A device used with another is classified separately; an accessory is classified in its own right.
Software that drives a device or influences its use falls within the same class as that device; standalone software is classified in its own right.
If several rules apply, the strictest determines the class.
A device intended for several parts of the body is classified on its most critical specified use.

The 22 rules, by group

Non-invasive devices (Rules 1 to 4)

Rule 1: all non-invasive devices are class I, unless another rule applies.
Rule 2: devices intended for channelling or storing blood, liquids, cells or tissues for infusion or administration: generally IIa, IIb for blood bags.
Rule 3: devices modifying the biological or chemical composition of blood, liquids or tissues intended for reintroduction: IIb, or IIa if the treatment consists of filtration, centrifugation or exchanges of gas or heat.
Rule 4: devices in contact with injured skin or mucous membrane: from class I (mechanical barrier, compression, absorption of exudates) to class IIb (wounds breaching the dermis that can only heal by secondary intent).

Invasive devices (Rules 5 to 8)

Rule 5: devices invasive through a body orifice: I for transient use, IIa for short term, IIb for long term, with exceptions for the oral cavity, ear canal and nasal cavity.
Rule 6: transient surgically invasive devices: generally IIa, I for reusable surgical instruments, III in direct contact with the heart, central circulatory system or central nervous system.
Rule 7: short-term surgically invasive devices: generally IIa, III in contact with the heart, central circulatory or nervous system, or if they have a biological effect or are absorbed.
Rule 8: implantable and long-term surgically invasive devices: generally IIb, IIa if placed in the teeth, III for implants in contact with the heart or central nervous system, active implantable devices, breast implants and surgical meshes, total and partial hip, knee and shoulder replacements, and spinal disc replacement implants.

Active devices (Rules 9 to 13)

Rule 9: active therapeutic devices intended to administer or exchange energy: IIa, IIb if potentially hazardous; III for those that control or directly influence an active implantable device.
Rule 10: active devices for diagnosis and monitoring: generally IIa, IIb for monitoring vital parameters whose variations could result in immediate danger, and for emitting ionising radiation for diagnostic radiology.
Rule 11: software. IIa by default for decision support, IIb or III depending on the seriousness of an error's consequences. Our guide on Rule 11 covers it in detail.
Rule 12: active devices intended to administer or remove medicinal products or body liquids: IIa, IIb if potentially hazardous.
Rule 13: all other active devices are class I.

Special rules (Rules 14 to 22)

Rule 14: devices incorporating a medicinal substance with ancillary action: III.
Rule 15: devices for contraception or prevention of sexually transmitted infections: IIb, III if implantable or long-term invasive.
Rule 16: disinfecting or sterilising devices: IIa, IIb for disinfecting or cleaning contact lenses and for disinfecting invasive devices as the end point of processing.
Rule 17: devices recording diagnostic X-ray images: IIa.
Rule 18: devices made from non-viable tissues or cells of human or animal origin: III, except for contact with intact skin.
Rule 19: devices incorporating nanomaterials: from IIa to III depending on the potential for internal exposure.
Rule 20: devices invasive through a body orifice intended to administer medicinal products by inhalation: IIa, or IIb depending on mode of action and condition treated.
Rule 21: devices composed of substances introduced into the body and absorbed or locally dispersed: from IIa to III depending on site and degree of absorption.
Rule 22: active therapeutic devices with an integrated diagnostic function that significantly determines patient management, such as closed-loop systems or automated external defibrillators: III.

These summaries give the logic of each rule; their exact wording, with all exceptions, is in Annex VIII of Regulation (EU) 2017/745.

What the class changes

Class	Conformity assessment
I	Manufacturer's self-declaration, except sterile (Is), measuring (Im) or reusable surgical instruments (Ir), for which a notified body assesses those aspects
IIa	Notified body: quality system (Annex IX) or product verification (Annex XI), with technical documentation assessed on a sampling basis
IIb	Notified body: Annex IX, or type examination (Annex X) combined with Annex XI; technical documentation assessed for each implantable device
III	Notified body with technical documentation assessed for each device, clinical investigation generally required, expert panel consultation for certain implants

The class also sets the PSUR frequency (at least annual for IIb and III, at least every two years for IIa) and summary obligations (SSCP for class III and implantable devices).

Common mistakes

Classifying by actual use rather than declared intended purpose: instructions and labelling must match the class claimed.
Missing the strictest rule when several apply.
Ignoring the special rules (nanomaterials, absorbed substances, tissues of animal origin).
Underestimating software: Rule 11 places most decision-support software in IIa or above.
Not documenting the reasoning: the justification of the class, rule by rule, is part of the technical documentation (Annex II).

Useful references

MDR Annex VIII (Regulation (EU) 2017/745), on EUR-Lex
MDCG 2021-24: guidance on classification of medical devices, with examples for each rule
MDCG 2019-11: qualification and classification of software
The European Commission's manual on borderline and classification

Classification evolves with MDCG guidance and borderline decisions: a device correctly classified yesterday may be questioned tomorrow. It is a topic medical device regulatory monitoring must cover.

Medical device classification: the 22 MDR rules

Key takeaways