# Exchanging Expressions

The main options for sharing postcoordinated expressions are:

* To exchange the terse expression as the ‘code’ of the relevant field, with the displayed or auto-generated term.
  * For example, the HL7 version 3 CD data type (Release 2) allows the 'code' property to include SNOMED CT expressions (see [HL7 TermInfo guide](http://wiki.hl7.org/index.php?title=File:V3_IG_SNOMED_R1_D5_2014JAN.docx)).
  * Within the coding also include a precoordinated concept, for systems that can not process expressions. This should be the proximal inferred parent concept of the expression.
  * *This option is shown in the example below.*
* To decompose the expression into separate fields - one for a focus concept and then one for each of the attribute values in the expression.
  * In each field, the concept identifier may be used as the code, and one of the synonyms as the display text.
  * This approach assumes alignment between the message structure and the relevant parts of the SNOMED CT concept model.
* To use an expression identifier from your expression repository as the ‘code’, with the display term.
  * This approach requires the use of a shared expression repository enabling the recipient system to look up and resolve this identified to the relevant expression.

**Example: A FHIR Resource including a postcoordinated expression**

```json
[code]
{
"resourceType": "Condition",
"id": "f205",
"text": {
"status": "generated",
"div": "<div xmlns="http://www.w3.org/1999/xhtml">..."
},
"clinicalStatus": {},
"verificationStatus": {},
"code": {
"coding": [
{
"system": "http://snomed.info/sct",
"code": "301354004 |Pain of ear| : 272741003 |Laterality| = 7771000 |Left|",
"display": "Left ear pain"
},
{
"system": "http://snomed.info/sct",
"code": "301354004 |Pain of ear|",
"display": "Pain of ear"
}
  ]
},
"subject": {
"reference": "Patient/f201",
"display": "PP"
},
"recordedDate": "2013-04-04",
"asserter": {
"reference": "Practitioner/f201"
}
}
[/code]
```

**Terminology services**

Enabling expressions to be included in communication messages requires:

* Lookup Expression. This will be used to get either the CTU form of the expression or the expression identifier to be included as the code for the coded data element in the message
* Get Display Term. This is required in cases where a single display term is required for the expression, and this approach is acceptable in the given setting.

{% hint style="info" %}
More information about the Terminology Services requirements please see: [terminology-services-requirements](https://docs.snomed.org/snomed-ct-practical-guides/snomed-ct-postcoordination-guide/expressions-in-a-terminology-server/requirements/terminology-services-requirements "mention")
{% endhint %}

{% hint style="warning" %}
*Note that additional terminology services will be required to decompose expressions to enable the separation of values into specific fields of the message structure.*
{% endhint %}

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