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The classes of the national extension model are, with the exception of the PCD, shown on the right hand side: two of the classes of the national extension model 'mirror' classes in the international model; their concepts represent the real world medicinal products available within a nation that are types of the concepts in the international release:

• the optional Real Medicinal Product (RMP) mirrors the Medicinal Product Only class and represents products marketed by a single organisation (supplier) under a single name (which may be a trade or brand name) and which contain the same set of active ingredient substances
• the Real Clinical Drug (RCD) mirrors the Clinical Drug class and represents a product marketed by a single organisation (supplier) under a single name (which may be a trade or brand name) which contains the same set of active ingredient substances in the same strength and which is formulated within a single dose form form; this class, like its partner in the international edition content, is at the core of the specification 

Then there are two classes in the national extension model represent that represent classes that are more specific than the classes of the international model in that they represent real medicinal products as they are presented into the in the supply chain in packages for clinical/patient use  in a particular country.  Any sub-packaging used inside a single package (for example, blister strips) and the aggregate packaging used in wholesaling and delivery such as shrink-wraps and pallets are excluded. The two additional package classes are

• the optional Real Packaged Clinical Drug (RPCD) which is a representation of a packaged product marketed by a single organisation (manufacturer or supplier) under a single name (which may be a trade or brand name) which contains one or more Real Clinical Drugs within it, in set amounts
• the optional Packaged Clinical Drug (PCD) which is an abstract representation of the Real Packaged Clinical Drug, in that it has no manufacturer or supplier information and therefore represents a package containing one or more Clinical Drugs within it, in set amounts

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Definitions and detailed descriptions of the extension classes are given in the sections below this model introduction.

Implementation

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Options

Extensions may wish to populate and use all of the classes described in the model, or they may wish to use only a subset; it is envisaged that the Clinical Drug clinical drug class from the international core will be foundational, associated with the Real Clinical Drug as will the real clinical drug in a national extension, but all others may be considered optional for implementation.

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• some nations may not require Packaged Clinical Drug or Real Packaged Clinical Drug packaged clinical drug or real packaged clinical drug concepts, if all products are licensed and used in healthcare at the Real Clinical Drug real clinical drug level, whereas
• if a nation licenses all its products at the Real Packaged Clinical Drug real packaged clinical drug level and uses those concepts in their healthcare culture, the Real Clinical Drug class may not be required if using the Clinical Drug description for the content of the package if considered sufficient. real clinical drug class should be populated as it acts as a grouper concept for all the packages associated with it.  This grouper concept is important particularly if extensions require additional product characteristic information such as for excipients of concern 

Similarly, some nations may require the MP Precisely concept for some classes of medicines where the precise ingredient substance can affect the clinical characteristics such as potency (e.g. glucocorticosteroids) if these concepts need to be available to support "dose based prescribing" (i.e. prescribing that specifies a medicine concept plus a route of administration, a dose quantity and a dose frequency, but does not specify a dose form or a strength so therefore not a clinical drug with its precise ingredient substance).  MP Precisely concepts can be authored in a national extension using the principles given in the international model specification (see section 3.1.3 of the International Specification).

Extensions may wish to author additional Clinical Drug clinical drug concepts using a presentation strength description for liquid products for which the international release has only a concentration strength representation (see section 3.3 of the International Specification and further detail below).

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The model uses the standard generalisation/specialisation relationship between the mirrored "real" classes of the national extension and their abstract classes in the international core.   It also uses a partitive relationship, shown in the diagram as the specialised composition relationship (the 'live together, die together' relationship), between the Clinical Drug and Packaged Clinical Drug classes, indicating that the Packaged Drug classes are "composed of" concepts that are themselves Clinical Drugs.  This is reflected in the "contains clinical drug" attribute that is part of the logical definition of a Packaged Clinical Drug.   The composition relationship is particularly appropriate for those packaged medicinal products that contain more than one clinical drug (often referred to as "kit products"). The combination of the usual SNOMED CT generalisation relationship and partitive relationship is manageable within the SNOMED CT tooling and will be applicable for description of other types of product concepts within the overall scope of SNOMED CT such as medical devices, which are often composed of more than one type of entity (e.g. drug eluting stents). The use of the composition relationship between Clinical Drugs and Packaged Clinical Drugs means that if implementations wish to display Packaged Clinical Drugs with a direct relationship to the Clinical Drugs that they contain (as in "under" them in a hierarchical display) nd and they wish to transfer information such as the therapeutic role from the Clinical Drug to the Packaged Clinical Drug that they are related to, this composition relationship and the 'contains clinical drug' attribute must to be used to facilitate that.  

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The relationships in the diagram are shown only in terms of their semantics; no cardinality is given. National extensions may populate those classes for which they have use cases; for example, if a nation has no requirement for the Real Medicinal Product class, it does not have to be populated.

Relationship between

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International and

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Extension Content

The international core content of SNOMED CT will provide all the attribute relationships (see detail below) needed to define medicinal product concepts in a national extension. These will be suitably defined according to SNOMED machine-readable concept model (MRCM) principles and available for use in the tooling. The concepts to provide the values for the product name and supplier cannot be present in the International edition as these do not have international applicability or even international uniqueness (especially for product name).  This specification therefore cannot provide ranges for these attributes; the supertype concepts are available (774167006 | Product name (product name) | and 774164004 | Supplier (supplier) |) to support extension authoring.

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