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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 IHTSDO 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.

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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:

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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:

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The expression will be represented in the representational from with the smallest number of refinements which full represents the meaning of the code in the other code system. This form which is derived as follows is sometimes referred to as the "author normal form".

Starting point

The stated representation as authored to most accurately reflect the meaning of the source code using the SNOMED CT concept model.

Steps applied to derive the distributed expression

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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|

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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 |

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