The  705109006 | Code to expression type reference set|  is designed to enable associations between codes in another code system (other-codes) and SNOMED CT concepts, where the following constraints apply:

  1. Some of the other-codes cannot be mapped to an individual SNOMED CT concept.
  2. Licensing conditions (or other considerations) prevent addition of new SNOMED CT concepts to represent the same meaning as the other-codes.
  3. The other-codes can be logically defined using the SNOMED concept model to represent the same meaning (sufficiently defined) or a similar though less specific meaning (primitive).
  4. Other requirements similar for those applicable to mapping may also apply including:
    1. An indication of the degree of correlation between the  other-code and the SNOMED CT expression.
    2. An indication of whether the other-code was created before any single concept representation of that meaning in SNOMED CT or whether the single concept representation in SNOMED CT predated the creation of the association.

Data Structure

The general approach to the above requirements is to associate each of the other-codes with a representation of the same logic based definition as would have been applied to a SNOMED CT concept with that meaning. However, since the  other-code are not identified by an  SCTID, the logical definition cannot be represented using  defining relationships. There are two potential approaches to this, one would be to use a general purpose description logic language (e.g.  OWL) and the other is to use a SNOMED CT expression to represent each definition. The  | Code to expression type reference set|  is designed to support the expression-based approach.

Table Code to Expression Reference Set - Data structure


Data type


Part of Primary Key


A 128 bit unsigned Integer, uniquely identifying this reference set member.

Different versions of a reference set member share the same id but have different effectiveTime. This allows a reference set member to be modified or made inactive (i.e. removed from the active set) at a specified time.

NOYES (Full /Snapshot)

The inclusive date or time at which this version of the identified reference set member became the current version.

Note: In distribution files the effectiveTime should follow the short ISO date format (YYYYMMDD) and should not include the hours, minutes, seconds or timezone indicator.

The current version of this reference set member at time T is the version with the most recent effectiveTime prior to or equal to time T .


YES (Full)

Optional (Snapshot) 

The state of the identified reference set member as at the specified effectiveTime .

If active = 1 (true) the reference set member is part of the current version of the set, if active = 0 (false) the reference set member is not part of the current version of the set.


Identifies the SNOMED CT module that contains this reference set member as at the specified effectiveTime .

The value must be a subtype of 900000000000443000 | Module (core metadata concept)| within the metadata hierarchy.


Identifies the reference set to which this reference set member belongs.

In this case, a subtype descendant of: 705109006 | Code to expression type reference set|


A reference to the SNOMED CT component to be included in the reference set.

A subtype of 705113004 | Terminology system| identifying the code system from which the code in the mapTarget field is derived.


The other-code which is mapped to the SNOMED CT expression.


SNOMED CT expression that represents the meaning of the  other-code. This expression may be a  stated or  inferred view of the definition provided that documentation of each identified reference set specifies the view provided.

The expression must conform to the syntax defined in the SNOMED CT Compositional Grammar - Specification and Guide (


Indicates whether or not the expression contains a sufficient definition of the other-code in the mapSource field.
Possible values are the following subtypes of 900000000000444006 | Definition status| :


The correlation between the other-code and the SNOMED CT expression. Possible values are the following subtypes of 1193546000 | Map source to map target correlation (foundation metadata concept)| :

When these values are applied to this reference set type, the phrase "map source" refers to the other-code. and the "map target" refers to the SNOMED CT expression.


Indication of whether the concept was initially in one of the terminologies (SNOMED CT or other-codes) and added to the other as part of mapping or was in both terminologies at the outset. Values are subtypes of   705116007 | Original code system source for linked content value| .



The following metadata hierarchy supports this reference set:

Table Code to Expression Reference Set in the Metadata Hierarchy

Reference Set Descriptor and Example Data

Notes on the tables used to show descriptors and examples

The reference set example tables on this page have been revised as follows to aid clarity and understanding:

  • The first four columns which are present in all release files are not shown. The omitted columns (ideffectiveTimeactivemoduleId) are used in the same way in all referenced sets to support identification, versioning and packaging. They do not directly affect the specific features of a particular reference set or reference set type.
  • Reference set columns that contain SNOMED CT identifiers are expanded to show details of the concept or description referenced by that identifier. In some cases, the term is shown in the same column using the expression syntax, in other cases an additional column with a name suffix '_term' has been added. In the standard reference set files only the identifier is present in the column and there is no added column for the term. When using reference sets, the term and other details of the component are looked up from the relevant component release files.

Descriptor template and examples

The tables below show the descriptors that define examples of reference sets that follow the 705109006 | Code to expression type reference set (foundation metadata concept)| pattern.

Table Refset Descriptor rows for a Code to Expression Reference Set

Refset Examples

Table Sample Content from a Code to Expression Type Reference Set





705117003 | Originally in LOINC|

705117003 | Originally in LOINC|

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