This page summarizes the implementation of an Expression Extension using the HL7 FHIR Terminology Services API. 

This approach is in the process of being implemented and supported by the Snowstorm FHIR Terminology Services API.


This section is still under active development, and the API examples will change as reference implementations mature and this is discussed with the community of practice.

Solution Overview

To represent an expression repository using FHIR terminology services, the required solution will depend on what approach is taken to uniquely refer to the expressions:


Services

With this implementation, the defined operations for CodeSystems and ConceptMaps can be utilized to support the services required for an expression repository. Please refer to the HL7 FHIR documentation for further details:

Create expression repository

Creating an expression repository using a FHIR Terminology Server involves the creation of the following structures:


The workflow for populating an expression repository using a FHIR Terminology Server is illustrated in 



ServiceDescriptionInput
Create Code System Supplement

POST /CodeSystem
Create the CodeSystem supplement.
This operation must specify a version URI that uses the snomed.info xsct URI with a specific module but no version.
This resource must supplement a specific version of a SNOMED CT edition.
The example supplements the International Edition of SNOMED CT (900000000000207008), version Jan 2023 (20230131). 
{
    "resourceType": "CodeSystem",
    "url": "http://snomed.info/info",
    "version": "http://snomed.info/xsct/11000003104",
    "content": "supplement",
    "supplements" : "http://snomed.info/sct|http://snomed.info/sct/900000000000207008/version/20230131"
}

Create SNOMED Exp IDs Code System

POST  /CodeSystem
(Only needed if EHR cannot store expressions)
If needed, create a CodeSystem for expression identifiers.


{

    "resourceType": "CodeSystem",
    "url": "http://snomed.info/snomed/exp-id/1000003",
    "status": "active",
    "content": "complete"
}
Create Concept Map for Exp Ids

POST /ConceptMap
(Only needed if EHR cannot store expressions)
Create the ConceptMap to map expressions in the code system supplement to expression identifiers in the Exp. Ids code system. This will be populated manually.
{

    "resourceType": "ConceptMap",
    "url": "http://snomed.info/snomed/exp-id-map",
    "status": "active",
    "source": {
        "sourceUri": "http://snomed.info/xsct/1234007?fhir_vs"},
    "target": {
        "targetUri": "http://snomed.info/snomed/exp-id/1101234?fhir_vs"}

}

Get repository details
ServiceDescriptionInput
GET /CodeSystem/{id}Get the details of the Code System using the provided ID; it is the same operation for the supplement or the Exp. IDs code system.
Code System ID, provided in the response of the code system creation
Add expression
ServiceDescriptionInput
PATCH /CodeSystem/{id}
Adds an expression in the Code System Supplement using a patch operation.
The display value can be generated from the expression, or the expression itself could be used in this field.
This operation should run validation on the expression being inserted into the supplement.
[
  { 
    "op": "add", 
    "path": "/concept", 
    "value": {
      "code": "359817006 |Closed fracture of hip (disorder)| : 
       272741003 |Laterality (attribute)| = 182353008 |Side (qualifier value)|",
      "display": "Right closed fracture of hip"
    } 
  }
]
(Source: https://fhirblog.com/2019/08/13/updating-a-resource-using-patch/)
Lookup expression
ServiceDescriptionInputOutput
GET /CodeSystem/{supplementId}/$lookup?code={CTU}
GET /CodeSystem//$lookup?system={versionUri}&code={CTU}
Lookup Expression in the supplement
  • versionUri
  • CTU: close-to-user expression

GET /ConceptMap/{snomed-exp-id-map}/$translate?system={versionUri}&code={CTU}
Example
GET /ConceptMap/snomed-exp-id-map/$translate?system=http://snomed.info/xsct/1234007
&code=87971000:272741003=7771000
Lookup Expression Identifier from Expression
  • versionUri
  • CTU: close-to-user expression


GET /ConceptMap/{snomed-exp-id-map}/$translate?
reverse=true&system={versionUri}&code={expId}
Example
GET /ConceptMap/snomed-exp-id-map/$translate?
reverse=true&system=http://snomed.info/snomed/exp-id/1101234&code=101101234165
Lookup Expression from Expression Identifier 
  • versionUri
  • expId: expression identifier as defined pr. implementation


Search for expression
Use an implicit value set with ECL.
Get display term
A display term should be returned in the response of the lookup operation. The display term may be generated in a very simple way by just concatenating.
Create Classifiable Form 
This should happen when the expression is inserted into the code system.
Create Necessary Normal Form
This should happen when the expression is inserted into the code system.


Subsumption
ServiceDescriptionInput
Expression subsumption test
GET /CodeSystem/$subsumes
Test the subsumption between a pair of expressions or an expression and a code.
{
    "resourceType": "CodeSystem",
    "url": "http://snomed.info/snomed/exp-id/1234007",
    "status": "active",
    "content": "supplement",
    "supplements": "http://snomed.info/snomed/exp-id/900000000000207008",
    "valueSet": "http://snomed.info/snomed/exp-id/1234007?fhir_vs" 
}

ECL
ServiceInputOutput
  • GET /ValueSet/$expand?url={url}?fhir_vs=ecl/{ecl}
  • url: The URI of the expression repository
  • ecl: A valid expression constraint defining the constraints of the query


Example
A hospital department would like to report on the number of closed reduction procedures performed. All codes representing this type of procedure must be fetched so that they can be matched against the EHR records.