FHIR Extensions Pack
5.3.0-ballot-tc1 - (September 2025 Ballot) International flag

FHIR Extensions Pack, published by HL7 International / FHIR Infrastructure. This guide is not an authorized publication; it is the continuous build for version 5.3.0-ballot-tc1 built by the FHIR (HL7® FHIR® Standard) CI Build. This version is based on the current content of https://github.com/HL7/fhir-extensions/ and changes regularly. See the Directory of published versions

Extension: Target Constraint

Official URL: http://hl7.org/fhir/StructureDefinition/targetConstraint Version: 5.3.0-ballot-tc1
Standards status: Trial-use Maturity Level: 2 Responsible: HL7 International / Clinical Decision Support Computable Name: TargetConstraint

Specifies a constraint that is enforced on instantiated (or target) resources. This extension can be applied to definitional resources (such as ActivityDefinition or Measure) and indicates that the constraint should be enforced on resources that are instantiated from the definition. For example, this extension can be used to define a constraint such as numerator count must be less than or equal to denominator count.

This extension allows constraints to be specified as part of definitional resources as an alternative to having to define a profile to enforce the constraint.

Context of Use

This extension may be used on the following element(s)

Usage info

Usages:

  • This Extension is not used by any profiles in this Implementation Guide

You can also check for usages in the FHIR IG Statistics

Changes since version 5.2.0:

  • No changes

    • Formal Views of Extension Content

    Description of Profiles, Differentials, Snapshots, and how the XML and JSON presentations work.

    This structure is derived from Extension

    Summary

    Complex Extension: Specifies a constraint that is enforced on instantiated (or target) resources. This extension can be applied to definitional resources (such as ActivityDefinition or Measure) and indicates that the constraint should be enforced on resources that are instantiated from the definition. For example, this extension can be used to define a constraint such as numerator count must be less than or equal to denominator count.

    • key: id: A unique identifier for the invariant (unique in the context of the resource).
    • requirements: markdown: A brief description of why the constraint is necessary or appropriate.
    • severity: code: Whether the constraint indicates an error condition or a warning.
    • expression: Expression: An expression providing a formal representation of the invariant condition.
    • human: string: A free text expression of the rule to display to the user if the rule is violated.
    • location: string: Relative paths to the elements this rule is enforced against.

    Maturity: 2

    This structure is derived from Extension

    NameFlagsCard.TypeDescription & Constraints    Filter: Filtersdoco
    .. Extension 0..* Extension A constraint on the target resource
    ... Slices for extension Content/Rules for all slices
    .... extension:key 1..1 Extension Unique identifier
    ..... extension 0..0 Extension
    ..... url 1..1 uri "key"
    ..... value[x] 1..1 id Value of extension
    .... extension:requirements 0..1 Extension Why the constraint is defined
    ..... extension 0..0 Extension
    ..... url 1..1 uri "requirements"
    ..... value[x] 1..1 markdown Value of extension
    .... extension:severity 1..1 Extension error | warning
    ..... extension 0..0 Extension
    ..... url 1..1 uri "severity"
    ..... value[x] 1..1 code Value of extension
    Binding: ConstraintSeverity (required)
    .... extension:expression 1..1 Extension The invariant expression
    ..... extension 0..0 Extension
    ..... url 1..1 uri "expression"
    ..... value[x] 1..1 Expression Value of extension
    .... extension:human 1..1 Extension Human-readable rule
    ..... extension 0..0 Extension
    ..... url 1..1 uri "human"
    ..... value[x] 1..1 string Value of extension
    .... extension:location 0..* Extension Relative path to elements
    ..... extension 0..0 Extension
    ..... url 1..1 uri "location"
    ..... value[x] 1..1 string Value of extension
    ... url 1..1 uri "http://hl7.org/fhir/StructureDefinition/targetConstraint"
    ... value[x] 0..0 Value of extension

    doco Documentation for this format
    NameFlagsCard.TypeDescription & Constraints    Filter: Filtersdoco
    .. Extension 0..* Extension A constraint on the target resource
    ... id 0..1 id Unique id for inter-element referencing
    ... Slices for extension 4..* Extension Additional content defined by implementations
    Slice: Unordered, Open by value:url
    .... extension:key 1..1 Extension Unique identifier
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "key"
    ..... value[x] 1..1 id Value of extension
    .... extension:requirements 0..1 Extension Why the constraint is defined
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "requirements"
    ..... value[x] 1..1 markdown Value of extension
    .... extension:severity 1..1 Extension error | warning
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "severity"
    ..... value[x] 1..1 code Value of extension
    Binding: ConstraintSeverity (required)
    .... extension:expression 1..1 Extension The invariant expression
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "expression"
    ..... value[x] 1..1 Expression Value of extension
    .... extension:human 1..1 Extension Human-readable rule
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "human"
    ..... value[x] 1..1 string Value of extension
    .... extension:location 0..* Extension Relative path to elements
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "location"
    ..... value[x] 1..1 string Value of extension
    ... url 1..1 uri "http://hl7.org/fhir/StructureDefinition/targetConstraint"

    doco Documentation for this format

    This structure is derived from Extension

    Summary

    Complex Extension: Specifies a constraint that is enforced on instantiated (or target) resources. This extension can be applied to definitional resources (such as ActivityDefinition or Measure) and indicates that the constraint should be enforced on resources that are instantiated from the definition. For example, this extension can be used to define a constraint such as numerator count must be less than or equal to denominator count.

    • key: id: A unique identifier for the invariant (unique in the context of the resource).
    • requirements: markdown: A brief description of why the constraint is necessary or appropriate.
    • severity: code: Whether the constraint indicates an error condition or a warning.
    • expression: Expression: An expression providing a formal representation of the invariant condition.
    • human: string: A free text expression of the rule to display to the user if the rule is violated.
    • location: string: Relative paths to the elements this rule is enforced against.

    Maturity: 2

    Differential View

    This structure is derived from Extension

    NameFlagsCard.TypeDescription & Constraints    Filter: Filtersdoco
    .. Extension 0..* Extension A constraint on the target resource
    ... Slices for extension Content/Rules for all slices
    .... extension:key 1..1 Extension Unique identifier
    ..... extension 0..0 Extension
    ..... url 1..1 uri "key"
    ..... value[x] 1..1 id Value of extension
    .... extension:requirements 0..1 Extension Why the constraint is defined
    ..... extension 0..0 Extension
    ..... url 1..1 uri "requirements"
    ..... value[x] 1..1 markdown Value of extension
    .... extension:severity 1..1 Extension error | warning
    ..... extension 0..0 Extension
    ..... url 1..1 uri "severity"
    ..... value[x] 1..1 code Value of extension
    Binding: ConstraintSeverity (required)
    .... extension:expression 1..1 Extension The invariant expression
    ..... extension 0..0 Extension
    ..... url 1..1 uri "expression"
    ..... value[x] 1..1 Expression Value of extension
    .... extension:human 1..1 Extension Human-readable rule
    ..... extension 0..0 Extension
    ..... url 1..1 uri "human"
    ..... value[x] 1..1 string Value of extension
    .... extension:location 0..* Extension Relative path to elements
    ..... extension 0..0 Extension
    ..... url 1..1 uri "location"
    ..... value[x] 1..1 string Value of extension
    ... url 1..1 uri "http://hl7.org/fhir/StructureDefinition/targetConstraint"
    ... value[x] 0..0 Value of extension

    doco Documentation for this format

    Snapshot View

    NameFlagsCard.TypeDescription & Constraints    Filter: Filtersdoco
    .. Extension 0..* Extension A constraint on the target resource
    ... id 0..1 id Unique id for inter-element referencing
    ... Slices for extension 4..* Extension Additional content defined by implementations
    Slice: Unordered, Open by value:url
    .... extension:key 1..1 Extension Unique identifier
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "key"
    ..... value[x] 1..1 id Value of extension
    .... extension:requirements 0..1 Extension Why the constraint is defined
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "requirements"
    ..... value[x] 1..1 markdown Value of extension
    .... extension:severity 1..1 Extension error | warning
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "severity"
    ..... value[x] 1..1 code Value of extension
    Binding: ConstraintSeverity (required)
    .... extension:expression 1..1 Extension The invariant expression
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "expression"
    ..... value[x] 1..1 Expression Value of extension
    .... extension:human 1..1 Extension Human-readable rule
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "human"
    ..... value[x] 1..1 string Value of extension
    .... extension:location 0..* Extension Relative path to elements
    ..... id 0..1 id Unique id for inter-element referencing
    ..... extension 0..0 Extension
    Slice: Unordered, Open by value:url
    ..... url 1..1 uri "location"
    ..... value[x] 1..1 string Value of extension
    ... url 1..1 uri "http://hl7.org/fhir/StructureDefinition/targetConstraint"

    doco Documentation for this format

     

    Other representations of profile: CSV, Excel, Schematron

    Terminology Bindings

    Path Status Usage ValueSet Version Source
    Extension.extension:severity.​value[x] Base required ConstraintSeverity 📦5.0.0 FHIR Std.

    Constraints

    Id Grade Path(s) Description Expression
    ele-1 error **ALL** elements All FHIR elements must have a @value or children hasValue() or (children().count() > id.count())
    ext-1 error **ALL** extensions Must have either extensions or value[x], not both extension.exists() != value.exists()

    R4B

    The extension is unchanged in R4B

    R4

    The extension is unchanged in R4

    R3

    The extension is represented a little differently in R3:

    • Replace the type Expression with a set of extensions for the content of the type along with the _datatype extension
    • Note: Extensions that have been modified for R3 are still draft while real-world experience is collected
    Name Flags Card. Type Description & Constraints      Filter: Filters doco
    . . Extension 0..* Extension A constraint on the target resource
    . . . Slices for extension Content/Rules for all slices
    . . . . extension:key 1..1 Extension Unique identifier
    . . . . . extension 0 .. 0 Extension
    . . . . . url 1 .. 1 uri "key"
    . . . . . value[x] 1.. 1 id Value of extension
    . . . . extension:requirements 0..1 Extension Why the constraint is defined
    . . . . . extension 0 .. 0 Extension
    . . . . . url 1 .. 1 uri "requirements"
    . . . . . value[x] 1.. 1 markdown Value of extension
    . . . . extension:severity 1..1 Extension error | warning
    . . . . . extension 0 .. 0 Extension
    . . . . . url 1 .. 1 uri "severity"
    . . . . . value[x] 1.. 1 code Value of extension
    Binding: ConstraintSeverity ( required )
    . . . . extension:expression 1..1 Extension The invariant expression
    . . . . . Slices for extension 0 ..* Extension Extension
    Slice: Unordered, Open by value:url
    . . . . . . extension:_datatype 1..1 (Complex) DataType name 'Expression' from R5
    URL: http://hl7.org/fhir/StructureDefinition/_datatype
    . . . . . . . value[x] 1.. 1 string Value of extension
    Fixed Value: Expression
    . . . . . . extension:description 0..1 Extension Natural language description of the condition
    . . . . . . . extension 0 .. 0 Extension
    . . . . . . . url 1 .. 1 uri "description"
    . . . . . . . value[x] 1.. 1 string Value of extension
    . . . . . . extension:name 0..1 Extension Short name assigned to expression for reuse
    . . . . . . . extension 0 .. 0 Extension
    . . . . . . . url 1 .. 1 uri "name"
    . . . . . . . value[x] 1.. 1 code Value of extension
    . . . . . . extension:language 0..1 Extension text/cql | text/fhirpath | application/x-fhir-query | etc.
    . . . . . . . extension 0 .. 0 Extension
    . . . . . . . url 1 .. 1 uri "language"
    . . . . . . . value[x] 1.. 1 code Value of extension
    Binding: ExpressionLanguage ( extensible ) : The media type of the expression language.
    Additional Bindings Purpose
    MimeTypes Max Binding
    . . . . . . extension:expression 0..1 Extension Expression in specified language
    . . . . . . . extension 0 .. 0 Extension
    . . . . . . . url 1 .. 1 uri "expression"
    . . . . . . . value[x] 1.. 1 string Value of extension
    . . . . . . extension:reference 0..1 Extension Where the expression is found
    . . . . . . . extension 0 .. 0 Extension
    . . . . . . . url 1 .. 1 uri "reference"
    . . . . . . . value[x] 1.. 1 uri Value of extension
    . . . . . url 1 .. 1 uri "expression"
    . . . . . value[x] 0 .. 0 Value of extension
    . . . . extension:human 1..1 Extension Human-readable rule
    . . . . . extension 0 .. 0 Extension
    . . . . . url 1 .. 1 uri "human"
    . . . . . value[x] 1.. 1 string Value of extension
    . . . . extension:location 0..* Extension Relative path to elements
    . . . . . extension 0 .. 0 Extension
    . . . . . url 1 .. 1 uri "location"
    . . . . . value[x] 1.. 1 string Value of extension
    . . . url 1 .. 1 uri "http://hl7.org/fhir/StructureDefinition/targetConstraint"
    . . . value[x] 0 .. 0 Value of extension

    doco Documentation for this format

    Notes:

    When reporting the results of this custom validation rule via an OperationOutcome (issue), the following property mappings should be considered:

    Issue property Constraint property Comments
    severity severity  
    code 'Invariant' Fixed code value from issue-type CodeSystem
    details.coding.system Questionnaire.url Canonical URL of Questionnaire that contains this constraint
    details.coding.code key  
    details.coding.display human This is the human readable error message
    details.text human This value can also be displayed to a user (instead of the coding.display above) and is often useful to also prefix the {}human{} value with the display text from the item(s)
    diagnostics expression The validation fhirpath expression (helps diagnose issues with validation rules)
    expression evaluate location The location expression should be evaluated on the QuestionnaireResponse (or item) to provide the location(s) of the error to permit a User Interface to show the error associated with the specific field(s) that caused the error/warning

    The location component is a FHIRPath expression that should be executed on the target resource (or item that the constraint is associated with) to locate the path of the property(s) that should be flagged to the user when the constraint is violated.

    When the location property is missing, it is assumed that the path of the answer being validated should be reported in the issue, and the path to the item in the target resource being validated should be used in the issue.expression field

    e.g. A Questionnaire that has a group (g1) with 2 items (acc-date and review-date).

    A validation rule will be added on the g1 item:

    "url":"http://example.org/Questionnaire/abc",
...
"linkId":"q1",
"text": "Question 1",
{ 
    "key": "seq-dt", 
    "severity": "error", 
    "expression" : "item.where(linkId='acc-date').answer.first().trace('acc-date') < %resource.authored.trace('authored')", 
    "human": "The accident date must be before the completion date of the form", 
    "location": "item.where(linkId='acc-date').answer.first()" 
}

    Example issue created from the above invariant example

    {
  "severity": "error",
  "code": "Invariant",
  "details": {
    "coding": [{
      "system": "http://example.org/Questionnaire/abc", 
      "code" : "seq-dt", 
      "display": "The accident date must be before the review date"
    }],
    "text":"Question 1: The accident date must be before the completion date"
  },
  "diagnostics": "item.where(linkId='acc-date').answer.first().trace('acc-date') <  %resource.authored.trace('authored') acc-date: 2024-03-17; authored: 2024-02-01",
  "expression": ["QuestionnaireResponse.item[0].item[1].answer[0].value[0]"]
}

    Search Parameters for this Extension

    (none found)