Rules 1 to 12 classify devices according to general criteria, particularly invasiveness, duration of continuous contact, nature of the tissue contact, and distinction between non-active and active devices.

The duration of continuous contact is either transient (intended use < 60 minutes), short term (intended use £ 30 days) or long term (intended use > 30 days). Note: This schedule differs from the regimen defined in ISO 10993-1 for the biological evaluation of medical devices, where limited exposure means £ 24 hours, prolonged exposure means £ 30 days and permanent contact means > 30 days. Further ISO 10993-1 considers a cumulative exposure whereas the classification under the MDD considers only one single continuous use.

Devices applied in the oral or nasal cavity or the ear canal up to the ear drum are generally in lower classes. Devices in contact with the central nervous system, heart, or central circulatory system are in higher classes than devices in contact with other tissues.

Reusable surgical instruments which are not connected to an active device are in class I.

Implantable and long-term invasive devices which are intended to have a biological effect or be mainly absorbed or undergo chemical change in the body are in class III (rule 8).

For active devices, whose operation depends on a source of electrical energy or any source of power other than that directly generated by the human body or gravity and which acts by converting this energy, rules 9 to 12 apply.

The following special rules apply and override other rules if applicable:

• Rule 13: Devices incorporating as an integral part a medicinal product with ancillary action to the main intended action of the device are class III devices
  
• Rule 14: Devices for contraception or prevention of the transmission of sexually transmitted diseases are in class IIb, unless they are implantable or long-term invasive, in which case they are in class III
  
• Rule 15: Contact lens care products are in class IIb; other devices specifically intended for disinfecting medical devices are in class IIa

This rule does not apply for products intended to physically clean other products than contact lenses.

• Rule 16: Non-active devices specifically intended for recording X-ray diagnostic images are in class IIa
  
• Rule 17: Devices using animal tissues or derivatives rendered non-viable are in class III, except when in contact with intact skin only
  
• Rule 18: Blood bags are in class IIb

Detailed guidance on the classification of medical devices is provided in the document MEDDEV 2.4/1 Guidelines to the classification of medical devices.