Create a post-coordinated expression library

A postcoordinated expression is a SNOMED-CT notion that is relevant to FHIR resources that use SNOMED CT.

By creating a Post Coordination Expression (PCE) Library, you are making prepared PCEs available for inclusion in a ValueSet alongside individual SNOMED-CT Concepts.

In this example, we walk through creating a PCE library, and then including a PCE in a ValueSet.

1. Create a new Code System.  Here we have called it “PCE Library Supplement”.  Make sure you add in a Code System URI so the Code System can be successfully uploaded to Ontoserver. Then change the Content to “Supplement (R4)” – the Code System to supplement field will then be revealed.

We wish to create a PCE that can be used with any SNOMED-CT based FHIR ValueSet.  So we create a FHIR CodeSystem supplement for the SNOMED-CT CodeSystem. The appropriate edition of SNOMED CT to supplement should be chosen.

2. Click on Find existing FHIR Resources, then change the Type of the search to “CodeSystem”.  Change the Value to “snomed” (to search for all “snomed” CodeSystems) and click on the + button to get a local copy of the latest available UK edition of the SNOMED-CT CodeSystem.

3. We will need to copy the Code System URI and the Version field values into the Code System to supplement field of our newly created supplement. The field accepts canonicals, which means it accepts the URI and the version as one string separated by a “|” character. So we will be entering “http://snomed.info/sct|http://snomed.info/sct/999000031000000106/version/20200204” into the field.

The supplement now looks like this:

The next step is to add the PCEs to the supplement.  Then will be added to the table.  You can resize the column widths to give yourself more room to see the values of the “code” and “display” columns.

4. Add your PCEs to the Code column of the table – FHIR always considers SNOMED CT codes to be expressions, where a single code is the simplest form of an expression. You can optionally include a value for the Display column. For more information about creating PCEs, please refer to the SNOMED-CT documentation.

5. Upload your supplement to Ontoserver to make it available for use. Click on the Upload to FHIR Server tab, then click on Validate Code System to validate before uploading, then click on Upload Code system. This operation will take a few minutes while the supplement is classified and indexed for subsequent high performance querying.

You will know your code system supplement was successfully uploaded when you see this message:

You have now created a PCE Library. Continue on to Using a postcoordinated expression library to see how to use a PCE expression in a FHIR ValueSet.