A reference set is defined as: a standard format for maintaining and distributing a set of references to SNOMED CT components.

Notes

• A reference set can be used to represent a subset of components (concepts, descriptions or relationships).

• A reference set may also associate referenced components with additional information such as:

  • Ordered lists of components

  • Sets of associations between components

  • Mapping between SNOMED CT concepts and other systems codes, classifications, or knowledge resources.

Hence, reference sets are a mechanism that can be used to represent subsets and value sets of SNOMED CT components. However, they can also be used for many other purposes, including those summarized in the section. The following table lists the attributes used in the standard reference set format, and their purpose.

Table: Standard reference set format

SNOMED CT is designed to allow the International Edition to be extended to meet national or local requirements. A SNOMED CT Extension may contain components of various types including concepts, descriptions, relationships and derivatives including reference sets. Since the International Release and extensions share a common structure, the same application software can be used to enter, store and process information from different extensions. Extensions can also be shared without requiring additional software procurement or development. Most Extensions are managed by Members or Affiliates of SNOMED International.

Reference sets authored in an extension use the same common format as those in the International Edition. This makes it easier to use the same software to create, maintain and share extension reference sets, as used by international reference sets. The reference set format can also be customized in an extension using a standardized customization approach, as explained in section .

A subset is defined as a set of members all of which are members of another set (from set theory in mathematics).

Notes

• In SNOMED CT, the definition of subset applies to SNOMED CT components as follows:

  • A subset of SNOMED CT concepts is a set of concepts taken from a wider set of concepts.

  • A subset of SNOMED CT descriptions is a set of descriptions taken from a wider set of descriptions.

• The members of a subset can be defined in one of two ways:

  • Extensionally, by enumeration, with a simple reference set as the standard distribution format.

  • Intensionally, using rules to determine inclusion, with a query reference set as the standard distribution format.

The diagram below shows an example of a subset. The English vowels are a subset of the set of alphabet characters included in the English alphabet

There are two distinct ways to define the membership of a subset. These are known as extensional and intensional subset definitions. These terms are defined and illustrated below.

A subset in which the members are represented by enumeration.

A subset definition in which the membership is represented by a set of rules specifying the conditions for inclusion.

Extensionally defined subsets of SNOMED CT components consist of a list of SNOMED CT component identifiers, which are typically either concept or description identifiers. Extensionally defined SNOMED CT subsets are formally represented using the simple reference set type.

The design of SNOMED CT lends itself to intensional subset definitions, because rules can be formulated to specify the conditions for member inclusion. The subtype hierarchy and formal definitions of SNOMED CT concepts enable constraints to be specified, such as:

• Concepts that are descendants of a particular concept.

  • For example: < 118794001 | Procedure on lung| (i.e. all descendants of the concept 118794001 | Procedure on lung| )

• Concepts that share a set of defining characteristics.

Intensionally defined SNOMED CT subsets are formally represented using a . The query string in this reference set represents the intensional definition of the subset. The standard way of representing the query is using the SNOMED CT (ECL).

The substrate is the superset of members to which an intensional subset definition is applied. Related to subsets of SNOMED CT components the 'Substrate' is the SNOMED CT content against which an intensional query is executed. Typically, the substrate is a specified version of a particular SNOMED CT Edition. Regardless of whether the subset members are intentionally defined or extensionally defined it is important to be specific about the substrate used to generate the subset, because the result of running a query, or manually selecting the subset members, may vary depending on the substrate used.

The expansion is the result of applying an intensional definition to a given substrate. The Expansion may differ depending on the substrate that the query is run against. The expansion that results from running the query against a specific SNOMED CT substrate can be represented extensionally as a simple reference set. Alternatively, the expansion may be computed dynamically or represented in an internal format within a software application.

For more information about the process and methods for developing subsets and reference sets, see the section about refset development.

In this section, we define some important terms that are used throughout this guide. In particular, subset, value set and reference set.

Subset and value set are general terms that are not specific to SNOMED CT. However, it is important to understand what they mean, and how they relate to SNOMED CT reference sets.

In summary:

• A subset is a set of members all of which are members of another set (from set theory in mathematics).

• A value set is a uniquely identifiable set of valid concept representations, where any concept representation can be tested to determine whether or not it is a member of the value set.

• A reference set is a standard format for maintaining and distributing a set of references to SNOMED CT components.

In the following subsections, we describe each of these terms in more detail.

A value set is a uniquely identifiable set of valid concept representations, where any concept representation can be tested to determine whether or not it is a member of the value set.

A value set is typically used to represent the possible values of a coded data element in an information model. The members of a value set may represent concepts using either simple codes or postcoordinated expressions.

There are a number of use cases for value sets, including constraining the permitted values for elements in a communication specification, specifying the values in a pick list on a user interface and defining the required values to use for reporting. Value sets may range from a simple flat list of codes from a single code system, to an unbounded hierarchical set of post-coordinated expressions drawn from multiple code systems. Value sets containing only SNOMED CT components may be represented as SNOMED CT reference sets.

For example, a message or reporting specification might define a single value set for a problem list, which includes:

• SNOMED CT 64572001 | Disorder| concepts

• SNOMED CT expressions that are subtypes of 64572001 | Disorder|

• ICD-10 classification codes representing diseases

The diagram below illustrates an example of an Observation model, which may be used to support diagnosis, monitor progress, determine patterns in clinical data, etc. Each data element in the information model is linked to a value set, which represents the value values for that element. As shown below, the value sets used in this information model may be selected from different code systems. In some cases, a single value set may also include concepts from different code systems.