SNOMED Documentation Search

 Other Documents
Skip to end of metadata
Go to start of metadata

You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 3 Next »


The Code to Expression type reference set is used meet the requirements for representation of associations between codes in another code system and SNOMED CT expressions, where the following requirements apply:

  1. Some or all of the codes in the other code system cannot be mapped to an individual SNOMED CT concept but can be associated with (i.e. mapped to) a postcoordinated SNOMED CT expression which convey the same meaning, or at least a similar meaning.
  2. A requirement that SNOMED International shall not systematically add missing content to enable a single SNOMED CT identifier to represent the meaning represented by the codes in the other code system.
  3. A requirement to indicate the degree of correlation between the code in the other code system and the SNOMED CT expression.
  4. A requirement to indicate the code in the other code system was created before any single concept representation in SNOMED CT or whether the single concept representation in SNOMED CT predated the creation of the association.
  5. No requirements for mapping rules or advice to be included with each association or map.


The Code to Expression type reference set is designed to meet the requirements as they apply to the associations between LOINC Term Codes and SNOMED CT expression.

The cooperation agreement between IHTSDO and Regenstrief Institute as signed in July 2013 does not permit systematic addition of SNOMED CT concepts based on LOINC Terms. Instead, LOINC Term Codes are associated with SNOMED CT expressions that have the same meaning. If a LOINC Term is recognized as having the same meaning as an existing SNOMED CT concept the associated expression will be precoordinated (e.g. an expression containing a single conceptId).

This requirement cannot be met by simply expanding an existing reference set pattern since there is a significant change to the required functionality. The reason for this is the SNOMED CT end of the association is an expression rather than a single component identifier. An expression is a variable length string and cannot be represented using a 64-integer (the datatype of the referencedComponentId).

With that exception the requirements are similar to those of the Map correlation and origin type pattern. Therefore, this is used as the foundation from which modifications have been made to meet these requirements.

After detailed discussion of a range of options, the decision has been made that for the Technology Preview release the referencedComponentId will be included and will be populated by a SNOMED CT concept identifier referring to a new concept 705114005 | LOINC Code System (qualifier value)|. This value can be ignored, as it does not convey any relevant information. The advantage of this approach is that it retains the Reference Set file structure, though it does so at the expense of redundancy.

The two specific columns added to the Code to Expression type reference set are:

  • expression attribute – added to meet requirement (a).
    • This attribute has not been used before but is also included in the design of the "Expression Association Reference Set" specified in this document.
  • definitionStatusId attribute – added for alignment with SNOMED CT concept definitions
    • This attribute indicates whether the expression is sufficient to define the idea represented by the LOINC Term Code.

The mapTarget field is retained as is to contain the code in the other codes system (the LOINC Term Code). It is arguable that in this pattern this is not really a "target" because these associations have the ability to be considered as bidirectional. Thus the expression could return a LOINC code or the LOINC code could be rendered as a SNOMED CT expression. In practice this is also true for the Map correlation and origin type pattern. Arguably, in terms of flexibility of use a direction independent name is needed but for the preview release it seems wiser to go for alignment with previous work.

The three additional component references included in the Map correlation and origin type pattern are also included in this pattern with the same functionality:

  • correlationId attribute – retained to meet requirement (c).
  • contentOriginId attribute – retained to meet requirement (d).

Expression Format

Each expression conforms to the SNOMED CT Composition Grammar. For further details of the syntax please see the Compositional Grammar - Specification and Guide ( The expression represents the SNOMED CT definition of the LOINC Term based in the recently developed concept model for the |Observable entity| domain. Note that the expression that represents a LOINC Term does not include any inferred relationships with other LOINC Terms. Furthermore, since the new |Observable entity| has not yet been applied to many SNOMED CT concepts, the LOINC Terms are defined as subtypes |Observable entity| with an appropriate set of attribute relationships.  Thus the expressions released in the beta release on 2017-03-31 include only stated relationships.

LOINC Term Code Status Changes

Deprecated statuses will cause the expression associations to be marked as inactive but trial use status will not be represented. LOINC Terms that were already deprecated at the time of initial mapping will not be included in maps and associations. However, dependent on demand and priority to assessments, it is possible these may be added later.

Code to Expression Reference Set Data Structure


Data type



A 128 bit unsigned integer, uniquely identifying the reference set member.

Specifies the inclusive date at which this change becomes effective.

Specifies whether the member's state was active or inactive from the nominal release date specified by the effectiveTime field.

Identifies the member version's module. Set to a child of |Module| within the metadata hierarchy.

Identifies the reference set.

A reference to a SNOMED CT metadata concept referring to the 705114005 |LOINC Code System (qualifier value)|.

The value of the LOINC Term Code associated with the expression.

A reference to the SNOMED CT concept being mapped to/from the LOINC Term Code.

Indicates whether or not the expression contains a sufficient definition of the LOINC Term Code in the mapTarget field.
Values are children of:
900000000000444006 |Definition status (core metadata concept)|

  • Necessary but not sufficient concept definition status
  • Sufficiently defined concept definition status

The correlation between the SNOMED CT expression and the LOINC Term Code.
Values are selected children of:
447247004 |SNOMED CT source code to target map code correlation value|

  • Exact match
  • Broad (SNOMED CT) to narrow (LOINC)
  • Narrow (SNOMED CT) to broad (LOINC)
  • Partial overlap

Indication of whether concept was initially in one of the terminologies (LOINC or SNOMED CT) and added to the other as part of mapping or was in both terminologies at the outset. Values are subtype children  

  705116007 |Original code system source for linked content value|

  • Originally in LOINC (true if expression was postcoordinated when the association was first made)
  • Originally in SNOMED CT
  • Originally in both LOINC and SNOMED CT


Refset Name

The following metadata in the "Foundation metadata concept" hierarchy supports this reference set:

  900000000000454005 |Foundation metadata concept|
900000000000455006 |Reference set|
705109006 |Code to expression type reference set|
705110001 |LOINC Term to Expression reference set|

Values for definitionStatusId

  900000000000444006 |Definition status|
900000000000074008 |Necessary but not sufficient concept definition status|
900000000000073002 |Sufficiently defined concept definition status|

Values for CorrelationId

Suggestion is use of current values set. However, an argument can be made in favor of duplication of the hierarchy with "source code" replaced by "expression".

  447247004 |SNOMED CT source code to target map code correlation value|
447559001 |Broad to narrow map from SNOMED CT source code to target code|
447557004 |Exact match map from SNOMED CT source code to target code|
447558009 |Narrow to broad map from SNOMED CT source code to target code|
447560006 |Partial overlap between SNOMED CT source code and target code|

Values for ContentOriginId

  705116007 |Original code system source for linked content value|
705117003 |Originally in LOINC|
705118008 |Originally in SNOMED CT|
705119000 |Originally in both LOINC and SNOMED CT|

  • No labels