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Extension Design

It is common for specific implementations to have valid requirements that are not part of the agreed common requirements modelled in the FHIR standard. Incorporating all specific use cases would make the standard too cumbersome. Therefore, additional valid requirements can be implemented as extensions to the agreed common resources. Further information about extensibility is available.

There are two basic types of extensions used by UK Core:

Simple extensions: A simple extension has a single Extension.urlelement and a single Extension.value[x] element, which may have many datatypes associated with it. See Extension Element for more details.
Complex extensions: A complex extension has a single Extension.urlelement and a single Extension.extension element. The latter contains a nested structure and are used for one of two reason:
- The extension has complex content, which is presented as a nested tree of values and defined locally within the extension.
- The extension is extended with an additional extension defined separately.

Currently the UK Core does not use modifierExtensions, so no guidance is given here on this type. More information on modifierExtensions is available in the FHIR specification.

Each Extension SHALL have an associated example to describe its usage, and the example SHALL be added to the example tab within the relevant profile and the extension page.

Any binding in an Extension SHALL only be done to CodeableConcept value's - cvode and coding values SHALL NOT be used.

Referencing to resources SHOULD be to the base resource rather than a UK Core Profile, unless agreed.

Metadata Usage

The list below contains the element differences between the UKCore and HL7.

Base Resource Definitions

View further information about FHIR base resource definitions.

Element name	Base Cardinality	UK Core Cardinality	Type	Definition, Constraints and Notes
id	0..1	1..1	id	In the UK Core the format in the following, with the FHIRAssetName and Specialism in PascalCase: `UKCore-[FHIRAssetName][Specialism]`.

StructureDefinition Content

View further information about FHIR Resource StructureDefinition - Content.

Element name	Base Cardinality	UK Core Cardinality	Type	Definition, Constraints and Notes
url	0..1	1..1	uri	In the UK Core the format is the following, with the FHIRAssetName and Specialism in PascalCase: `https://fhir.hl7.org.uk/StructureDefinition/UKCore-[FHIRAssetName][Specialism]`.
identifier	0..*	0..0	Identifier	identifier SHALL NOT be used within the UKCore
version	0..1	1..1	string	This will follow the Semantic Versioning standard `[major.minor.patch]`. Further details about internal versioning for UK Core assets are available.
name	0..1	1..1	string	In the UK Core the format is PascalCase: `UKCore[FHIRAssetName][Specialism]` .
title	0..1	1..1	string	In the UK Core the format is Proper Case: `UK Core [FHIRAssetName] [Specialism].`
status	1..1	1..1	code	See NamingSystem.status for more information.
date	0..1	1..1	dateTime	Only the date, without time, is populated in the UK Core.
publisher	0..1	1..1	string	For all UK Core Profiles, where the base URL is `https://fhir.hl7.org.uk`, the name of the publisher will be `HL7 UK`.
contact	0..*	1..*	ContactDetail	See Publisher, Contact & Copyright for details of how this SHALL be populated for all UK Core Profiles, where the base URL is `https://fhir.hl7.org.uk/`.
description	0..1	1..1	markdown	This SHALL be in the following format: `This profile defines the UK constraints and extensions on the International FHIR resource [FHIRAssetName]([link to HL7 Resource]).`
copyright	0..1	1..1	markdown	All UK Core Profiles SHALL contain the the copyright as listed in Publisher, Contact & Copyright
fhirVersion	0..1	1..1	code	The version of the FHIR specification on which this StructureDefinition is based - this is the formal version of the specification, without the revision number, e.g. [publication].[major].[minor]. The UKCore mandates the use of this element.
purpose	0..1	1..1	markdown	This provides traceability of ''why'' the resource is either needed or ''why'' it is defined as it is. This SHOULD contain the correct usage of the profile, for instance when it is similar to another profile examples of the expected use cases and examples which would be a misuse of the profile. The purpose is not to record the scope if these are subject to change, these SHALL be entered into the IG 'example use cases' section instead.
baseDefinition	0..1	1..1	canonical(StructureDefinition)	The baseDefinition SHALL be a complete URI referencing the relevant HL7 FHIR resource for UK Core base profiles, and the relevant UK Core base profile for any UK Core derived profiles.
snapshot	0..1	1..1	BackboneElement	Snapshots SHALL be present within the UK Core package and only rendered when creation of the package. Snapshots SHALL NOT be present within the UKCore GitHub Repo.
differential .element	1..*	1..*	ElementDefinition	When used for extensions, the element ID SHALL be `[Profile].extension:[Extension name without the ExtensionUKCore prefix, in camelCase]`. E.g, for ExtensionUKCoreBirthSex this will be `Patient.extension:birthSex`
differential .element .sliceName	0..1	0..1	string	When used for extensions, the element ID SHALL be `[Extension name without the ExtensionUKCore prefix, in camelCase]`. E.g, for ExtensionUKCoreBirthSex this will be `birthSex`
differential .element .defaultValue[x]	0..1	0..0	*	Defining default values creates many difficulties in implementation (e.g. when is a value missing?). For these reasons, default values SHALL NOT be used within the UKCore.

The Extension Root Element

Extensions are optional within the UK Core (minimum cardinality of 0), but certain use cases may mandate their use. Currently, all UK Core extensions are modelled as being optional at the extension root element. Where the use case requires an extension to be mandated, the Implementation Guide for that use case will specify the requirements.

Extension Bindings When the Value is a Coded type

If any of the extension.value elements is constrained to a coded type e.g. one of Code, Coding, or CodeableConcept, the extension.value SHALL be bound to a ValueSet.

Further information about FHIR datatypes is available.

Cardinality of a value[x] element

Simple extension - The UK Core mandates the use of Extension.value[x] for a simple extension, that is, a cardinality of 1..1.

Complex extension - The UK Core mandates the use of Extension.extension:[Element].value[x] for a complex extension, that is, a cardinality of 1..1.

Extension Examples

Simple Extension Illustration

Snapshot View

ExtensionUKCoreEthnicCategory (Extension)	I		Extension	Element id Extension Short description Ethnic category Definition The ethnicity of the subject. Data type Extension Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` ext-1: Must have either extensions or value[x], not both `extension.exists() != value.exists()` Mappings rim: n/a rim: N/A
id		0..1	string	There are no (further) constraints on this element Element id Extension.id Short description Unique id for inter-element referencing Definition Unique id for the element within a resource (for internal references). This may be any string value that does not contain spaces. Data type string Mappings rim: n/a
extension	I	0..*	Extension	There are no (further) constraints on this element Element id Extension.extension Short description Additional content defined by implementations Alternate names extensions, user content Definition May be used to represent additional information that is not part of the basic definition of the element. To make the use of extensions safe and manageable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension. Comments There can be no stigma associated with the use of extensions by any application, project, or standard - regardless of the institution or jurisdiction that uses or defines the extensions. The use of extensions is what allows the FHIR specification to retain a core level of simplicity for everyone. Data type Extension Sliced: Unordered, Open, by url(Value) Extensions are always sliced by (at least) url Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` ext-1: Must have either extensions or value[x], not both `extension.exists() != value.exists()` Mappings rim: n/a rim: N/A
url		1..1	uriFixed Value	Element id Extension.url Short description identifies the meaning of the extension Definition Source of the definition for the extension code - a logical name or a URL. Comments The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition SHALL be a URI for the Structure Definition defining the extension. Data type uri Fixed value https://fhir.hl7.org.uk/StructureDefinition/Extension-UKCore-EthnicCategory Mappings rim: N/A
value[x]		1..1	Binding	Element id Extension.value[x] Short description A code classifying the person's ethnicity Definition A code classifying the person's ethnicity. Binding A set of codes that define the ethnicity of a person, as specified by the person. UKCoreEthnicCategory (extensible) Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` Mappings rim: n/a rim: N/A
valueCodeableConcept			CodeableConcept	Data type CodeableConcept

Differential View

ExtensionUKCoreEthnicCategory (Extension)	I		Extension	Element id Extension Short description Ethnic category Definition The ethnicity of the subject. Data type Extension Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` ext-1: Must have either extensions or value[x], not both `extension.exists() != value.exists()` Mappings rim: n/a rim: N/A
id		0..1	string	There are no (further) constraints on this element Element id Extension.id Short description Unique id for inter-element referencing Definition Unique id for the element within a resource (for internal references). This may be any string value that does not contain spaces. Data type string Mappings rim: n/a
extension	I	0..*	Extension	There are no (further) constraints on this element Element id Extension.extension Short description Additional content defined by implementations Alternate names extensions, user content Definition May be used to represent additional information that is not part of the basic definition of the element. To make the use of extensions safe and manageable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension. Comments There can be no stigma associated with the use of extensions by any application, project, or standard - regardless of the institution or jurisdiction that uses or defines the extensions. The use of extensions is what allows the FHIR specification to retain a core level of simplicity for everyone. Data type Extension Sliced: Unordered, Open, by url(Value) Extensions are always sliced by (at least) url Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` ext-1: Must have either extensions or value[x], not both `extension.exists() != value.exists()` Mappings rim: n/a rim: N/A
url		1..1	uriFixed Value	Element id Extension.url Short description identifies the meaning of the extension Definition Source of the definition for the extension code - a logical name or a URL. Comments The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition SHALL be a URI for the Structure Definition defining the extension. Data type uri Fixed value https://fhir.hl7.org.uk/StructureDefinition/Extension-UKCore-EthnicCategory Mappings rim: N/A
value[x]		1..1	Binding	Element id Extension.value[x] Short description A code classifying the person's ethnicity Definition A code classifying the person's ethnicity. Binding A set of codes that define the ethnicity of a person, as specified by the person. UKCoreEthnicCategory (extensible) Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` Mappings rim: n/a rim: N/A
valueCodeableConcept			CodeableConcept	Data type CodeableConcept

Hybrid View

ExtensionUKCoreEthnicCategory (Extension)	I		Extension	Element id Extension Short description Ethnic category Definition The ethnicity of the subject. Data type Extension Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` ext-1: Must have either extensions or value[x], not both `extension.exists() != value.exists()` Mappings rim: n/a rim: N/A
id		0..1	string	There are no (further) constraints on this element Element id Extension.id Short description Unique id for inter-element referencing Definition Unique id for the element within a resource (for internal references). This may be any string value that does not contain spaces. Data type string Mappings rim: n/a
extension	I	0..*	Extension	There are no (further) constraints on this element Element id Extension.extension Short description Additional content defined by implementations Alternate names extensions, user content Definition May be used to represent additional information that is not part of the basic definition of the element. To make the use of extensions safe and manageable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension. Comments There can be no stigma associated with the use of extensions by any application, project, or standard - regardless of the institution or jurisdiction that uses or defines the extensions. The use of extensions is what allows the FHIR specification to retain a core level of simplicity for everyone. Data type Extension Sliced: Unordered, Open, by url(Value) Extensions are always sliced by (at least) url Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` ext-1: Must have either extensions or value[x], not both `extension.exists() != value.exists()` Mappings rim: n/a rim: N/A
url		1..1	uriFixed Value	Element id Extension.url Short description identifies the meaning of the extension Definition Source of the definition for the extension code - a logical name or a URL. Comments The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition SHALL be a URI for the Structure Definition defining the extension. Data type uri Fixed value https://fhir.hl7.org.uk/StructureDefinition/Extension-UKCore-EthnicCategory Mappings rim: N/A
value[x]		1..1	Binding	Element id Extension.value[x] Short description A code classifying the person's ethnicity Definition A code classifying the person's ethnicity. Binding A set of codes that define the ethnicity of a person, as specified by the person. UKCoreEthnicCategory (extensible) Constraints ele-1: All FHIR elements must have a @value or children `hasValue() or (children().count() > id.count())` Mappings rim: n/a rim: N/A
valueCodeableConcept			CodeableConcept	Data type CodeableConcept

Table View

Extension		..
Extension.url		..
Extension.value[x]	CodeableConcept	1..

XML View

<StructureDefinition xmlns="http://hl7.org/fhir">
    <id value="Extension-UKCore-EthnicCategory" />
    <url value="https://fhir.hl7.org.uk/StructureDefinition/Extension-UKCore-EthnicCategory" />
    <version value="2.3.0" />
    <name value="ExtensionUKCoreEthnicCategory" />
    <title value="Extension UK Core Ethnic Category" />
    <status value="active" />
    <date value="2023-12-12" />
    <publisher value="HL7 UK" />
    <contact>
        <name value="HL7 UK" />
        <telecom>
            <system value="email" />
            <value value="ukcore@hl7.org.uk" />
            <use value="work" />
            <rank value="1" />
        </telecom>
    </contact>
    <description value="Information that describes the patient&#39;s ethnic category." />
    <purpose value="This extension extends the Patient resource to support the exchange of ethnic category information, as a Codeable Concept." />
    <copyright value="Copyright &#169; 2021+ HL7 UK Licensed under the Apache License, Version 2.0 (the &quot;License&quot;); you may not use this file except in compliance with the License. You may obtain a copy of the License at  http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an &quot;AS IS&quot; BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. HL7&#174; FHIR&#174; standard Copyright &#169; 2011+ HL7 The HL7&#174; FHIR&#174; standard is used under the FHIR license. You may obtain a copy of the FHIR license at  https://www.hl7.org/fhir/license.html." />
    <fhirVersion value="4.0.1" />
    <mapping>
        <identity value="rim" />
        <uri value="http://hl7.org/v3" />
        <name value="RIM Mapping" />
    </mapping>
    <kind value="complex-type" />
    <abstract value="false" />
    <context>
        <type value="element" />
        <expression value="Patient" />
    </context>
    <type value="Extension" />
    <baseDefinition value="http://hl7.org/fhir/StructureDefinition/Extension" />
    <derivation value="constraint" />
    <differential>
        <element id="Extension">
            <path value="Extension" />
            <short value="Ethnic category" />
            <definition value="The ethnicity of the subject." />
        </element>
        <element id="Extension.url">
            <path value="Extension.url" />
            <fixedUri value="https://fhir.hl7.org.uk/StructureDefinition/Extension-UKCore-EthnicCategory" />
        </element>
        <element id="Extension.value[x]">
            <path value="Extension.value[x]" />
            <short value="A code classifying the person&#39;s ethnicity" />
            <definition value="A code classifying the person&#39;s ethnicity." />
            <min value="1" />
            <type>
                <code value="CodeableConcept" />
            </type>
            <binding>
                <strength value="extensible" />
                <description value="A set of codes that define the ethnicity of a person, as specified by the person." />
                <valueSet value="https://fhir.hl7.org.uk/ValueSet/UKCore-EthnicCategory" />
            </binding>
        </element>
    </differential>
</StructureDefinition>

JSON View

{
    "resourceType": "StructureDefinition",
    "id": "Extension-UKCore-EthnicCategory",
    "url": "https://fhir.hl7.org.uk/StructureDefinition/Extension-UKCore-EthnicCategory",
    "version": "2.3.0",
    "name": "ExtensionUKCoreEthnicCategory",
    "title": "Extension UK Core Ethnic Category",
    "status": "active",
    "date": "2023-12-12",
    "publisher": "HL7 UK",
    "contact":  [
        {
            "name": "HL7 UK",
            "telecom":  [
                {
                    "system": "email",
                    "value": "ukcore@hl7.org.uk",
                    "use": "work",
                    "rank": 1
                }
            ]
        }
    ],
    "description": "Information that describes the patient's ethnic category.",
    "purpose": "This extension extends the Patient resource to support the exchange of ethnic category information, as a Codeable Concept.",
    "copyright": "Copyright © 2021+ HL7 UK Licensed under the Apache License, Version 2.0 (the \"License\"); you may not use this file except in compliance with the License. You may obtain a copy of the License at  http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an \"AS IS\" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. HL7® FHIR® standard Copyright © 2011+ HL7 The HL7® FHIR® standard is used under the FHIR license. You may obtain a copy of the FHIR license at  https://www.hl7.org/fhir/license.html.",
    "fhirVersion": "4.0.1",
    "mapping":  [
        {
            "identity": "rim",
            "uri": "http://hl7.org/v3",
            "name": "RIM Mapping"
        }
    ],
    "kind": "complex-type",
    "abstract": false,
    "context":  [
        {
            "type": "element",
            "expression": "Patient"
        }
    ],
    "type": "Extension",
    "baseDefinition": "http://hl7.org/fhir/StructureDefinition/Extension",
    "derivation": "constraint",
    "differential": {
        "element":  [
            {
                "id": "Extension",
                "path": "Extension",
                "short": "Ethnic category",
                "definition": "The ethnicity of the subject."
            },
            {
                "id": "Extension.url",
                "path": "Extension.url",
                "fixedUri": "https://fhir.hl7.org.uk/StructureDefinition/Extension-UKCore-EthnicCategory"
            },
            {
                "id": "Extension.value[x]",
                "path": "Extension.value[x]",
                "short": "A code classifying the person's ethnicity",
                "definition": "A code classifying the person's ethnicity.",
                "min": 1,
                "type":  [
                    {
                        "code": "CodeableConcept"
                    }
                ],
                "binding": {
                    "strength": "extensible",
                    "description": "A set of codes that define the ethnicity of a person, as specified by the person.",
                    "valueSet": "https://fhir.hl7.org.uk/ValueSet/UKCore-EthnicCategory"
                }
            }
        ]
    }
}

Complex Extension Illustration