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1.12.6.1 Extensibility Examples

In order to use an extension, there is a three step process:

1. Define the extension
2. Register the extension
3. Use it in the instance

This page contains examples of how this process executes.

1.12.6.1.1 Patient Consent for Record Sharing

The basic patient resource contains no information relating to patient consent, and/or the policy under which the patient to their registration details. A social web provider of personal healthcare record (PHR) services might be obliged to to keep track of the particular policy under which a patient has created their relationship with the PHR provider, and share this with their partipcants via their FHIR api. If they wish, they can extend the patient resource to represent the patient's participation agreeement. Note that other approaches to this problem are possible, but it suffices to demonstrate the extension process.

For the purposes of this example, we assume that the patient agrees to a participation policy as part of their sign up, and that as the provider has to change their policy, they ask patients to agree to new participation details. Each participation agreement has a URI by which it is identified, and the patient resource will carry this URI for each policy agreement that the patient has agreed to.

1.12.6.1.1.1 Define the Extension

For each extension, the first thing to do is to fill out the definitional properties of the extension:

1.12.6.1.1.2 Write the Profile

From this table, we can build an actual profile.

This is all then represented formally in a profile. Such profiles do not need to include constraint statements of resources; instead, they include just extension declarations and their associated bindings. In this case, the profile looks like this:

<Profile xmlns="http://hl7.org/fhir">
  <name value="Example Extension Definition" />
  <!-- snip other metadata -->
  <extensionDefn>
    <code value="participation-agreement" />
    <contextType value="resource" />
    <context value="Patient" />
    <definition>
      <short value="Agreed agreement/policy" />
      <formal value="A URI that identifies a participation agreement/policy 
        to which the patient has agreed" />
      <comments value="URI is a literal reference to agreement text (html). 
        Systems SHALL conform to the policies as indicated. 
        For further information, see the partnership agreement..." />
      <min value="1" />
      <max value="*" />
      <type>
         <code value="uri" />
      </type>
      <mustSupport value="true" />
      <isModifier value="false" />
    </definition>
  </extensionDefn>

  <!-- snip narrative -->
</Profile>

Note that usually you would build the actual profile using some tool. This example was built by hand for this example.

1.12.6.1.1.3 Register the Extension

This means the profile shown above that defines the extension is placed on the web somewhere. By preference, it will be hosted in a FHIR Profile endpoint, and the best location of all is the HL7 Profile registry (yet to be implemented).

For this example, we assume that it has been uploaded to the PHR provider's own website at http://example.org/phr/documents/fhir/extensions.

1.12.6.1.1.4 Use it in the instance

To using the extension in an instance, the extension is placed in the root of the resource. Note that the url of the extension refers to the registered location, with the id of the extension as a fragment identifier.

<Patient xmlns="http://hl7.org/fhir">
  <extension url="http://example.org/phr/documents/fhir/extensions#participation-agreement" >
    <valueUri value="http://example.org/phr/documents/patient/general/v1" />
  </extension>
      
</Patient>

1.12.6.1.1.5 Adding it to Patient Profile

The profile definition above simply defines the extension "participation-agreement", and says that it is used with patient. But the profile above doesn't say that the server actually uses it. For the PHR provider to indicate that all the patients resources will use this resource, a profile on the patient resource is used:

<Profile xmlns="http://hl7.org/fhir">
  <name value="iso-21090" />
  <!-- snip other metadata, narrative -->
  <structure>

    <!-- first, the patient root element 
      - can be copy/paste from the base patient profile -->    
    <element>
      <path value="Patient"/>
      <!-- snip definition -->
    </element>
    
    <!-- now, the general definition for extensions
      - can be copy/paste from the base patient profile,
      with changes for slicing  -->
    <element>
      <path value="Patient.extension"/>
      <name value="base extension"/>
      <!-- we're going to slice the extension element, and 
        one of the extensions is one we have defined -->
      <slicing>
        <!-- extension is always sliced on url -->
        <discriminator value="url"/> 
        <!-- we don't care what the order of any extensions is -->
        <ordered value="false"/>
        <!-- Other extensions are allowed in addition to this one -->
        <rules value="open"/>
      </slicing>
      <!-- snip definition -->
    </element>
    
    <!-- now, the slice that contains our extension -->    
    <element>
      <path value="Patient.extension"/>
      <definition>
        <!-- clone information from the extension definition. 
          duplicative, but this duplication makes it over all simpler -->
        <short value="Agreed agreement/policy"/>
        <formal value="A URI that identifies a participation agreement/policy 
        to which the patient has agreed"/>
        <!--  min has to be 1, since the extension itself has min = 1 -->
        <min value="1"/>
        <max value="*"/>
        <type>
          <!-- obviously it has to be an extension -->
          <code value="Extension"/>
          <!-- and here is the link to the extension definition:
            this extension has to conform to the rules laid down in it's definition -->
          <profile value="http://example.org/phr/documents/fhir/extensions#participation-agreement"/>
        </type>
        <isModifier value="false"/>
      </definition>
    </element>
    
  <!-- snip the rest of the profile -->
  
  </structure>
</Profile>

Note - this step is optional.

1.12.6.1.2 Patient Name Parts

ISO 21090 (Healthcare Data Types) defines a concept called a "name part qualifier" that contains extra information about how a particular name part should be used or interpreted. In practice, this field is used rarely, except in particular cultural contexts, where certain part qualifiers are used as a matter of practice. Following the FHIR design policy, such a field is not included in the overall definition of the core name data type, instead is it added as an extension.

In practice, for cases such as these in ISO 21090, HL7 provides common extensions, and these are defined at [location still to be finalized].

1.12.6.1.2.1 Define the Extension

For each extension, the first thing to do is to fill out the definitional properties of the extension:

Not all the codes of the EntityNamePartQualifierR2 are required in this context, because prefix and suffix are explicitly part of the name types. Rather than simply refer to the OID for EntityNamePartQualifierR2 (2.16.840.1.113883.5.1122), in this case we enumerate the available codes, and set the type of the extension to code. The type of "code" is only allowed if the profile itself defines the codes that can be used. Here is a table of the codes (see the EntityNamePartQualifierR2 reference for the full definitions):

This is all then represented formally in a profile. Such profiles do not need to include constraint statements of resources; instead, they include just extension declarations and their associated bindings. In this case, the profile looks like this:

<Profile xmlns="http://hl7.org/fhir">
  <name value="iso-21090" />
  <!-- snip other metadata, including definition of RIM Mapping -->
  <extensionDefn>
    <code value="name-qualifier" />
    <contextType value="datatype" />
    <context value="HumanName.given" />
    <context value="HumanName.prefix" />
    <context value="HumanName.family" />
    <context value="HumanName.suffix" />
    <definition>
      <short value="LS | AC | NB | PR | HON | BR | AD | SP | MID | CL | IN" />
      <formal value="A set of codes each of which specifies a certain subcategory
          of the name part in addition to the main name part type" />
      <comments value="Used to indicate additional information about the
             name part and how it should be used" />
      <min value="0" />
      <max value="*" />
      <type>
         <code value="code" />
      </type>
      <mustSupport value="false" />
      <isModifier value="false" />
      <binding>
        <name value="EntityNamePartQualifier" />
        <isExtensible value="codelist" />
        <conformance value="required" />
        <description value="A set of codes each of which specifies a certain subcategory
            of the name part in addition to the main name part type" />
        <referenceResource>
          <reference value="ValueSet/name-part-qualifier" />
        </referenceResource>
      </binding>
      <mapping>
         <identity value="RIM" />
         <map value="ENXP.qualifier" />
      </mapping>
    </definition>
  </extensionDefn>

  <!-- snip narrative -->
</Profile>

Note that usually you would build the actual profile using some tool. This example was built from a spreadsheet definition by the FHIR build tooling.

1.12.6.1.2.2 Register the Extension

For this example, it is registered at http://hl7.org/fhir/Profile/iso-21090. This is the url that will appear in the definition element when the extension is used.

1.12.6.1.2.3 Use it in the instance

To using the extension in an instance, the extension is nested within the attribute that is extended. Note that the url of the extension refers to the registered location, with the id of the extension as a fragment identifier.

<name>
  <use value="official" />
  <given value="Östlund">
     <extension url="http://hl7.org/fhir/Profile/iso-21090#name-qualifier" >
        <valueCode value="MID" />
     </extension>
  </given>
</name>

This particular example is a Scandinavian mellannamn. See Datatypes examples for additional examples.