MDR Annex VIII Rule 3 covers non-invasive devices intended to modify the biological or chemical composition of human tissues, cells, body liquids, or other liquids intended for infusion into the body. Its default class is IIb, dropping to IIa only where the treatment consists of filtration, centrifugation, or exchange of gas or heat. Rule 4 covers non-invasive devices that come into contact with injured skin or mucous membrane: Class I for mechanical barrier, compression, or exudate absorption; Class IIb for wounds that have breached the dermis and can only heal by secondary intent; Class IIa for everything else in this rule, including devices principally intended to manage the microenvironment of the wound. Both rules sit under Article 51 of Regulation (EU) 2017/745, and MDCG 2021-24 (October 2021) is the authoritative interpretation guide.

By Tibor Zechmeister and Felix Lenhard. Last updated 10 April 2026.

TL;DR

  • Rule 3 catches non-invasive devices that change what is inside tissue, cells, body fluids, or liquids on their way into the body. Default Class IIb, down to Class IIa only for filtration, centrifugation, or exchange of gas or heat.
  • Rule 4 catches non-invasive contact with injured skin or mucous membrane. Three tiers: Class I for barrier, compression, or exudate absorption; Class IIa for the general case including microenvironment management; Class IIb for wounds that have breached the dermis and can only heal by secondary intent.
  • The hinge word in Rule 3 is "modify." A device that only moves a fluid without changing it is Rule 2, not Rule 3. A device that changes the fluid's composition is Rule 3.
  • The hinge phrase in Rule 4 is "injured skin or mucous membrane." Intact skin is not Rule 4. The severity of the injury then drives the sub-class.
  • Both rules are driven by the intended purpose under Article 2(12) of the Regulation, not by the raw technology on the bench.

Why these two rules travel together

Rules 3 and 4 sit next to each other in Annex VIII because both cover non-invasive devices with a physiological effect beyond simple containment or transport. Rule 3 handles the chemistry-changing family. Rule 4 handles the wound-contact family. Both were written to close a gap that Rule 1 (the non-invasive default) and Rule 2 (channelling and storing) leave open: a device can be entirely external to the body and still do something serious to biology. When that happens, Rule 1 is the wrong place to land.

Startups walking Annex VIII in order tend to accept Rule 1 too quickly and never read Rules 3 and 4 carefully enough to notice their device is caught. This post is the line-by-line walk for both rules, written so a founder can apply them without guessing.

Rule 3 text decoded

Rule 3 of Annex VIII applies to non-invasive devices intended for modifying the biological or chemical composition of human tissues or cells, body liquids, or other liquids intended for infusion into the body. The default class under Rule 3 is Class IIb. There is one drop to Class IIa: where the treatment for which the device is used consists of filtration, centrifugation, or exchanges of gas or of heat.

Three words in Rule 3 do most of the work.

Non-invasive. If the device enters the body, Rule 3 does not apply. The invasive rules take over. Rule 3 devices sit outside the body; what flows through them comes back into the body through a separate invasive device or infusion line.

Modifying. This is the hinge. A device that only holds or channels a fluid without altering it is Rule 2. A device that changes the biological or chemical composition of the fluid is Rule 3. "Modifying" means an actual change to what is in the fluid: separating components, removing substances, adding substances, changing temperature in a way that changes composition, altering gas content.

Intended for infusion into the body. Rule 3 covers tissues, cells, and body liquids taken from the patient, and also other liquids that are on their way into the body. A device that modifies a liquid that is not going into a patient at all is not a medical device under the same logic, and not a Rule 3 device.

The Class IIa drop is narrow and specific. It applies only when the treatment consists of filtration, centrifugation, or exchange of gas or heat. Everything else modifying composition. Chemical exchange, active separation by means other than filtration or centrifugation, addition of substances, selective depletion. Stays at Class IIb. Founders sometimes read the IIa drop as a general escape hatch. It is not. Read the words.

Typical Rule 3 devices include haemodialysis components that sit outside the body and treat blood before it returns, cell separation systems used in apheresis, blood component processing devices, and oxygenators used in extracorporeal circulation where they exchange gas with blood. Haemodialysis equipment that relies on exchange across a membrane is a borderline interpretation question between the Class IIa drop and the Class IIb default, and MDCG 2021-24 is the place to settle it for a specific device.

Rule 4 text decoded

Rule 4 applies to non-invasive devices which come into contact with injured skin or mucous membrane. Three sub-classes apply in parallel:

  • Class I if the device is intended to be used as a mechanical barrier, for compression, or for absorption of exudates.
  • Class IIb if the device is intended to be used principally with wounds which have breached the dermis and can only heal by secondary intent.
  • Class IIa in all other cases, including devices principally intended to manage the microenvironment of a wound.

Two phrases in Rule 4 do most of the work.

Injured skin or mucous membrane. Rule 4 does not apply to devices that only contact intact skin. A bandage applied to unbroken skin for support does not sit in Rule 4. A bandage applied to a wound does. If the device is designed to contact both, the design intent under the instructions for use is what counts. Ambiguity here is a common reason classification gets challenged during conformity assessment.

Principally. Rule 4 asks what the device is principally intended for. A dressing that absorbs exudate as its principal function is Class I. A dressing that manages the wound microenvironment as its principal function is Class IIa. A dressing intended principally for deep secondary-intent wounds is Class IIb. A device that does several things is classified by what it is principally intended to do. And the intended purpose under Article 2(12) of the Regulation governs that determination, not the raw technical capability.

The tiers matter commercially. Class I wound dressings can reach the market without Notified Body involvement in most pathways. Class IIa and Class IIb wound dressings cannot. Founders who inherit a Class I assumption from a legacy MDD certificate need to re-read Rule 4 against the current product. A claim added to the marketing page. "promotes healing," "manages the wound environment," "reduces infection risk". Can shift the principal intended purpose and move the device from Class I to Class IIa or higher without any change to the physical product.

What counts as "modification" or "injury"

The two words that cause the most disputes in Rules 3 and 4 are "modification" (Rule 3) and "injury" (Rule 4). Both are interpreted against the intended purpose, not against the physical mechanism alone.

For Rule 3, a modification is a change to the biological or chemical composition that the device is intended to produce. An incidental change that is not part of the intended treatment. For example, a trivial warming of a liquid passing through a tube that is not the point of the device. Does not pull the device into Rule 3. An intended change of temperature that shifts the composition or function of the fluid does. The written intended purpose decides.

For Rule 4, an injury is a breach of the skin or mucous membrane's integrity. A superficial abrasion is an injury. A deep ulcer is an injury. The sub-class then depends on how deep the injury goes and how it is expected to heal. "Breached the dermis and can only heal by secondary intent" is a specific clinical condition: the wound cannot be closed by primary intention (edges brought together) and must heal by granulation from the base upward. Pressure ulcers of advanced stages, deep burns, and chronic ulcers typically fall here. A paper cut does not.

MDCG 2021-24 contains worked examples and interpretation notes for both rules. For any device that sits near the boundary between Rule 3 and Rule 2, or between the Rule 4 sub-classes, MDCG 2021-24 is the first document to consult after the Regulation text itself.

Examples that land in each tier

A Rule 3 Class IIb device: a plasma exchange system that removes specific components from blood and returns the rest to the patient. The modification is clear, the treatment is not limited to filtration, centrifugation, or gas or heat exchange.

A Rule 3 Class IIa device: a blood-warming device that exchanges heat with blood on its way into the patient, where the warming is the entire point of the device. The Class IIa drop for heat exchange applies.

A Rule 4 Class I device: a sterile gauze pad intended for absorption of exudates from a wound, with no claim beyond absorption, barrier, or compression.

A Rule 4 Class IIa device: a hydrocolloid dressing principally intended to manage the microenvironment of a superficial wound. The principal intended purpose is microenvironment management, which is explicitly named in Rule 4 as Class IIa.

A Rule 4 Class IIb device: an advanced dressing principally intended for deep pressure ulcers that can only heal by secondary intent. The clinical population and wound severity put the device in the IIb tier.

Each of these examples assumes a specific intended purpose written into the label, instructions for use, and promotional materials. Change the intended purpose, and the classification can change. This is why the Purpose Pass in the Subtract to Ship framework for MDR compliance runs before the Classification Pass.

Escalations and multi-rule situations

Rules 3 and 4 interact with the Annex VIII Section 2 implementing rules. Two of those implementing rules matter here.

First, when more than one rule applies, the highest class wins. A device that is caught by Rule 4 as Class IIa and by a special rule as Class IIb is Class IIb. Do not stop at the first rule that fits.

Second, a device connected to an active medical device in Class IIa or higher can be escalated under specific rules. Rule 3 and Rule 4 devices are often used together with active devices. Infusion pumps, dialysis machines, vacuum-assisted closure systems. And the relationship between the non-invasive device and the active device needs to be analysed against the implementing rules.

Rule 3 devices that also touch a special rule. For example, a device that processes cells or tissues of human or animal origin (Rule 18 territory). Can escalate to Class III under the stricter rule. Rule 4 devices that incorporate a medicinal substance with ancillary action move into Rule 14 and become Class III regardless of the Rule 4 analysis. These are not rare edge cases. The complete guide to the Annex VIII classification rules walks the full set.

Common misclassifications under Rules 3 and 4

Treating Rule 3 as "anything that touches blood." Rule 3 requires modification of composition, not just contact. A blood collection bag is Rule 2. A blood processing device that separates components is Rule 3.

Assuming all wound dressings are Class I. The Class I tier in Rule 4 is narrow: mechanical barrier, compression, or exudate absorption as the principal intended purpose. Microenvironment management claims alone push the device to Class IIa.

Ignoring the "principally" word in Rule 4. A dressing that does three things is classified by what it is principally intended to do. If the marketing emphasises healing promotion, the principal intended purpose is not simple absorption, and the Class I position becomes hard to defend.

Applying the Rule 3 Class IIa drop too broadly. Filtration, centrifugation, or exchange of gas or heat. That is the complete list. Any other kind of modification keeps the device at Class IIb.

Forgetting Article 2(12). Intended purpose is defined as "the use for which a device is intended according to the data supplied by the manufacturer on the label, in the instructions for use, or in promotional or sales materials or statements, and as specified by the manufacturer in the clinical evaluation." A website claim is part of the intended purpose. A scientific paper linked from the product page can become part of the intended purpose if the link is presented as a product claim.

Letting a legacy MDD classification set the MDR class. Rule 4 in particular was rewritten under the MDR, and some dressings that sat at Class IIa under the directive may sit at Class IIb now depending on the principal intended purpose. Re-read the rule against the current product.

The Subtract to Ship angle on Rules 3 and 4

Rules 3 and 4 are places where the Purpose Pass pays off directly. A dressing positioned principally as an absorbent barrier lives in Class I. The same physical product positioned principally as a microenvironment manager lives in Class IIa. The difference is not physical. It is in the intended purpose, and the intended purpose is a decision the founder makes deliberately. Not a label applied after the fact.

The Subtract to Ship discipline is to ask: what is the minimum intended purpose that sells the product and keeps the classification defensible at the lowest tier the evidence supports? The answer is often a tighter, more specific claim than the marketing team initially wants to make. The narrower claim wins a lower class; the lower class shortens the path to market; the shorter path preserves runway; the preserved runway funds the next version where broader claims can be added with matching evidence.

This is not gaming the rules. It is applying them to the product as it actually is, at the version that actually ships, with an intended purpose that is written down, stable, and defensible under Article 2(12). When a founder wants a broader claim, the framework does not block it. It just requires the evidence and the class that the broader claim demands.

Reality Check. How well do you know Rules 3 and 4 for your device?

  1. Does your device contact a body fluid, tissue, or cell population outside the body? If yes, does it modify the composition or only channel it?
  2. If it modifies composition, is the modification limited to filtration, centrifugation, or exchange of gas or heat. Or is it something broader?
  3. Is your device intended to contact injured skin or mucous membrane, intact skin, or both?
  4. For a Rule 4 device, what is the principal intended purpose written into the instructions for use: barrier, compression, absorption, microenvironment management, or deep secondary-intent wound healing?
  5. Does any claim on your product page or marketing material change the principal intended purpose in a way that pushes the device into a higher tier?
  6. Have you cross-checked your tentative classification against MDCG 2021-24 (October 2021)?
  7. Have you walked the Section 2 implementing rules and confirmed no other rule produces a higher class?
  8. Could you state, in one paragraph, which specific Annex VIII rule applies to your device, which sub-rule or tier within that rule governs, and what the intended-purpose statement is that anchors the decision?

Frequently Asked Questions

What is the difference between MDR Rule 2 and Rule 3? Rule 2 covers non-invasive devices that channel or store blood, body liquids, cells, tissues, liquids, or gases for eventual infusion, administration, or introduction into the body. Rule 3 covers non-invasive devices that modify the biological or chemical composition of those fluids. The hinge is modification. If the device only moves the fluid, it is Rule 2. If the device changes what is in the fluid, it is Rule 3.

When does Rule 3 drop from Class IIb to Class IIa? Rule 3 drops to Class IIa only when the treatment for which the device is used consists of filtration, centrifugation, or exchanges of gas or of heat. Every other kind of modification keeps the device at the Class IIb default. The drop is a specific list, not a general exception for "less intensive" treatments.

Are all wound dressings Class I under MDR Rule 4? No. Class I under Rule 4 is limited to devices principally intended as a mechanical barrier, for compression, or for absorption of exudates. Dressings principally intended to manage the microenvironment of a wound are Class IIa. Dressings principally intended for wounds that have breached the dermis and can only heal by secondary intent are Class IIb. The principal intended purpose under Article 2(12) of the Regulation decides the tier.

Does Rule 4 apply to devices that contact intact skin? No. Rule 4 applies to non-invasive devices that contact injured skin or mucous membrane. Devices that only contact intact skin typically fall to Rule 1 (the non-invasive default) unless a different rule catches them. The transition from intact to injured skin is a clinical boundary that needs to be stated in the intended purpose.

Can a Rule 4 device be escalated to Class III? Yes, when another rule with a higher class also applies. A wound dressing incorporating a medicinal substance with ancillary action moves under Rule 14 and becomes Class III. A device incorporating non-viable tissues or cells of human or animal origin can escalate under Rule 18. The highest applicable class wins under the implementing rules in Section 2 of Annex VIII.

Where does MDCG 2021-24 fit in the application of Rules 3 and 4? MDCG 2021-24 (October 2021) is the authoritative interpretation guide for the Annex VIII classification rules. It contains worked examples and interpretation notes for each rule, including Rules 3 and 4. When the Regulation text alone does not settle a borderline question. And it frequently does not for wound dressings or extracorporeal treatment devices. MDCG 2021-24 is the first document to consult after the Regulation itself.

Sources

  1. Regulation (EU) 2017/745 of the European Parliament and of the Council of 5 April 2017 on medical devices, Article 2(12) (intended purpose), Article 51 (classification), Annex VIII Section 2 (implementing rules) and Section 3 Rules 3 and 4. Official Journal L 117, 5.5.2017.
  2. MDCG 2021-24. Guidance on classification of medical devices, October 2021.

This post is part of the Device Classification & Conformity Assessment series in the Subtract to Ship: MDR blog. Authored by Felix Lenhard and Tibor Zechmeister. The decoded text above is an orientation layer. The actual classification of a specific device must be made against the full Annex VIII text, cross-checked with MDCG 2021-24, and documented with a specific rule citation tied to the intended purpose under Article 2(12).