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4.9 Resource ValueSet - Content

Terminology Infrastructure icon Work GroupMaturity Level: N Normative (from v4.0.0)Security Category: Anonymous Compartments: No defined compartments

A ValueSet resource instance specifies a set of codes drawn from one or more code systems, intended for use in a particular context. Value sets link between CodeSystem definitions and their use in coded elements.

4.9.1 Scope and Usage

The FHIR terminology specification is based on the concepts of code system and value set originally defined in HL7 v3 Core Principles icon:

Value sets have 2 aspects:

  • .compose: A definition of which codes are intended to be in the value set ("intension")
  • .expansion: The list of codes that are actually in the value set under a given set of conditions ("extension") - see Value Set Expansion

The ValueSet resource can carry either the .compose or the .expansion, both of them, or neither of them (if only the metadata is being represented). There is an "$expand" operation which can be used to ask a server to generate an expansion given the composition rules, in a particular context, and a "$validate-code" operation which can be used to ask a server to check whether a given code or concept is in the value set in a particular context.

4.9.2 Boundaries and Relationships

Value Sets are used by many resources:

For a full list of uses, see below.

The Characteristics of the ValueSet resource are derived from Formal Value Set Definitions:

  • The ValueSet resource design is based on the functionality described in the OMG CTS 2 icon specification, along with metadata in the HL7 Value Set Definition specification. Value set resources can be converted to CTS2 value set or code system instances.
  • The value set resource is aligned with the Value Set Definition icon (VSD) project. Not all of the elements defined by the VSD are part of the base resource - some are defined as part of the ValueSet Extensions. In the ValueSet resource, the compose element is the VSD "Content Logical definition".

4.9.3 Background and Context

When using value sets, proper differentiation between a code system and a value set is important. This is one very common area where significant clinical safety risks occur in practice. Implementers should be familiar with the content in Using Codes in Resources.

4.9.3.1 ValueSet Identification

A value set has 3 identifiers:

  • ValueSet.id: the logical id on the system that holds the value set - this changes as it moves from server to server (this id, with the server address prepended, is called the 'literal identity' of the resource)
  • ValueSet.url: the canonical URL that never changes for this value set - it is the same in every copy. The element is named url rather than uri for legacy reasons and to strongly encourage providing a resolvable URL as the identifier whenever possible. Ideally, it should be a literal URL that is the location of the master version of the value set, though this is not always possible
  • ValueSet.identifier: A system/value pair that is used to identify the value set in other contexts (such as an OID in an HL7 v3 icon specification)

In addition, any expansion for the value set also has ValueSet.expansion.identifier which uniquely identifies each expansion. For further information regarding resource identification, see Resource Identity.

This means that each value set has 2 different URLs that can be used to reference it - its canonical URL (the url element), and its local location from which it may be retrieved (which includes the id element). Because it is common practice to copy (cache) value sets locally, most references to value sets use the canonical URL.

For example, the value sets published as part of FHIR all have a location ("literal") URI which is the URL where they may be accessed in the FHIR specification itself. Note, though, that while a new version of the FHIR Specification is being prepared, value sets that are published in the drafts will not be found in the current published FHIR specification at their canonical URL.

Alternatively, the identifier and version elements may be used to reference this value set in a design, a profile, a CDA icon template or HL7 v3 icon message (in the CD data type valueSet and valueSetVersion properties). These different contexts may make additional restrictions on the possible values of these elements. The identifier is generally not needed when using value sets in a FHIR context, where the canonical URL is always the focus.

4.9.3.2 Intensional vs Extensional

A value set may be described as intensional or extensional. The terms intensional and extensional come from the fields of mathematical logic and set theory.

An intensionally defined value set may be algorithmically defined e.g. all codes with the word diabetes in the description. A key benefit of intensionally defined value sets is that their expansion can be dynamically updated without changing the value set definition. This helps healthcare organizations keep current for example, when new drugs (and their associated codes) become available or codes for diseases and other clinical concepts are added.

Extensional value sets, meanwhile, are enumerated lists of codes where each code is listed individually. This gives the author and user of the value set more control over the which codes are in the value set, but there is a greater maintenance burden to ensure that the value set is kept up to date.

4.9.4 References to this Resource

4.9.5 Resource Content

Structure

NameFlagsCard.TypeDescription & Constraintsdoco
.. ValueSet N DomainResource A set of codes drawn from one or more code systems
+ Warning: Name should be usable as an identifier for the module by machine processing applications such as code generation

Elements defined in Ancestors: id, meta, implicitRules, language, text, contained, extension, modifierExtension
Interfaces Implemented: MetadataResource
... url ΣC 0..1 uri Canonical identifier for this value set, represented as a URI (globally unique)
+ Warning: URL should not contain | or # - these characters make processing canonical references problematic
... identifier Σ 0..* Identifier Additional identifier for the value set (business identifier)

... version Σ 0..1 string Business version of the value set
... versionAlgorithm[x] ΣTU 0..1 How to compare versions
Binding: Version Algorithm (Extensible)
.... versionAlgorithmString string
.... versionAlgorithmCoding Coding
... name ΣC 0..1 string Name for this value set (computer friendly)
... title ΣT 0..1 string Name for this value set (human friendly)
... status ?!Σ 1..1 code draft | active | retired | unknown
Binding: PublicationStatus (Required)
... experimental Σ 0..1 boolean For testing only - never for real usage
... date Σ 0..1 dateTime Date last changed
... publisher ΣT 0..1 string Name of the publisher/steward (organization or individual)
... contact Σ 0..* ContactDetail Contact details for the publisher

... description T 0..1 markdown Natural language description of the value set
... useContext ΣTU 0..* UsageContext The context that the content is intended to support

... jurisdiction ΣXD 0..* CodeableConcept Intended jurisdiction for value set (if applicable)
Binding: Jurisdiction ValueSet (Extensible)

... immutable Σ 0..1 boolean Indicates whether or not any change to the content logical definition may occur
... purpose T 0..1 markdown Why this value set is defined
... copyright T 0..1 markdown Use and/or publishing restrictions
... copyrightLabel TTU 0..1 string Copyright holder and year(s)
... approvalDate TU 0..1 date When the ValueSet was approved by publisher
... lastReviewDate TU 0..1 date When the ValueSet was last reviewed by the publisher
... effectivePeriod ΣTU 0..1 Period When the ValueSet is expected to be used
... topic XD 0..* CodeableConcept E.g. Education, Treatment, Assessment, etc
Binding: Definition Topic (Example)

... author TU 0..* ContactDetail Who authored the ValueSet

... editor TU 0..* ContactDetail Who edited the ValueSet

... reviewer TU 0..* ContactDetail Who reviewed the ValueSet

... endorser TU 0..* ContactDetail Who endorsed the ValueSet

... relatedArtifact TU 0..* RelatedArtifact Additional documentation, citations, etc

... compose 0..1 BackboneElement Content logical definition of the value set (CLD)
.... lockedDate Σ 0..1 date Fixed date for references with no specified version (transitive)
.... inactive Σ 0..1 boolean Whether inactive codes are in the value set
.... include ΣC 1..* BackboneElement Include one or more codes from a code system or other value set(s)
+ Rule: A value set include/exclude SHALL have a value set or a system
+ Rule: A value set with concepts or filters SHALL include a system
+ Rule: Cannot have both concept and filter

..... system ΣC 0..1 uri The system the codes come from
..... version Σ 0..1 string Specific version of the code system referred to
..... concept C 0..* BackboneElement A concept defined in the system

...... code 1..1 code Code or expression from system
...... display 0..1 string Text to display for this code for this value set in this valueset
...... designation C 0..* BackboneElement Additional representations for this concept
+ Rule: Must have a value for concept.designation.use if concept.designation.additionalUse is present

....... language 0..1 code Human language of the designation
Binding: All Languages (Required)
Additional BindingsPurpose
Common Languages Starter Set

....... use C 0..1 Coding Types of uses of designations
Binding: Designation Use (Extensible)
....... additionalUse CTU 0..* Coding Additional ways how this designation would be used
Binding: Designation Use (Extensible)

....... value 1..1 string The text value for this designation
..... filter ΣC 0..* BackboneElement Select codes/concepts by their properties (including relationships)

...... property Σ 1..1 code A property/filter defined by the code system
...... op Σ 1..1 code = | is-a | descendent-of | is-not-a | regex | in | not-in | generalizes | child-of | descendent-leaf | exists
Binding: Filter Operator (Required)
...... value Σ 1..1 string Code from the system, or regex criteria, or boolean value for exists
..... valueSet ΣC 0..* canonical(ValueSet) Select the contents included in this value set

..... copyright TU 0..1 markdown A copyright statement for the specific code system included in the value set
.... exclude 0..* see include Explicitly exclude codes from a code system or other value sets

.... property TU 0..* string Property to return if client doesn't override

... expansion 0..1 BackboneElement Used when the value set is "expanded"
.... identifier 0..1 uri Identifies the value set expansion (business identifier)
.... next TU 0..1 uri Opaque urls for paging through expansion results
.... timestamp 1..1 dateTime Time ValueSet expansion happened
.... total 0..1 integer Total number of codes in the expansion
.... offset 0..1 integer Offset at which this resource starts
.... parameter 0..* BackboneElement Parameter that controlled the expansion process

..... name 1..1 string Name as assigned by the client or server
..... value[x] 0..1 Value of the named parameter
...... valueString string
...... valueBoolean boolean
...... valueInteger integer
...... valueDecimal decimal
...... valueUri uri
...... valueCode code
...... valueDateTime dateTime
.... property TU 0..* BackboneElement Additional information supplied about each concept

..... code 1..1 code Identifies the property on the concepts, and when referred to in operations
..... uri 0..1 uri Formal identifier for the property
.... contains C 0..* BackboneElement Codes in the value set
+ Rule: SHALL have a code or a display
+ Rule: SHALL have a code if not abstract
+ Rule: SHALL have a system if a code is present

..... system C 0..1 uri System value for the code
..... abstract C 0..1 boolean If user cannot select this entry
..... inactive 0..1 boolean If concept is inactive in the code system
..... version 0..1 string Version in which this code/display is defined
..... code C 0..1 code Code - if blank, this is not a selectable code
..... display C 0..1 string User display for the concept
..... designation 0..* see designation Additional representations for this item

..... property TU 0..* BackboneElement Property value for the concept

...... code 1..1 code Reference to ValueSet.expansion.property.code
...... value[x] 1..1 Value of the property for this concept
....... valueCode code
....... valueCoding Coding
....... valueString string
....... valueInteger integer
....... valueBoolean boolean
....... valueDateTime dateTime
....... valueDecimal decimal
...... subProperty 0..* BackboneElement SubProperty value for the concept

....... code 1..1 code Reference to ValueSet.expansion.property.code
....... value[x] 1..1 Value of the subproperty for this concept
........ valueCode code
........ valueCoding Coding
........ valueString string
........ valueInteger integer
........ valueBoolean boolean
........ valueDateTime dateTime
........ valueDecimal decimal
..... contains 0..* see contains Codes contained under this entry

... scope TU 0..1 BackboneElement Description of the semantic space the Value Set Expansion is intended to cover and should further clarify the text in ValueSet.description
.... inclusionCriteria 0..1 markdown Criteria describing which concepts or codes should be included and why
.... exclusionCriteria 0..1 markdown Criteria describing which concepts or codes should be excluded and why

doco Documentation for this format icon

See the Extensions for this resource

 

Additional definitions: Master Definition XML + JSON, XML Schema/Schematron + JSON Schema, ShEx (for Turtle) , the spreadsheet version & the dependency analysis

4.9.5.1 Terminology Bindings

Path ValueSet Type Documentation
ValueSet.versionAlgorithm[x] VersionAlgorithm Extensible

Indicates the mechanism used to compare versions to determine which is more current.
ValueSet.status PublicationStatus Required

The lifecycle status of an artifact.
ValueSet.jurisdiction JurisdictionValueSet Extensible

This value set defines a base set of codes for country, country subdivision and region for indicating where a resource is intended to be used.

Note: The codes for countries and country subdivisions are taken from ISO 3166 icon while the codes for "supra-national" regions are from UN Standard country or area codes for statistical use (M49) icon.
ValueSet.topic DefinitionTopic Example

High-level categorization of the definition, used for searching, sorting, and filtering.
ValueSet.compose.include.concept.designation.language AllLanguages (a valid code from Tags for the Identification of Languages icon) Required

This value set includes all possible codes from BCP-47 (see http://tools.ietf.org/html/bcp47)
  Common Languages starter
ValueSet.compose.include.concept.designation.use DesignationUse Extensible

Details of how a designation would be used
ValueSet.compose.include.concept.designation.additionalUse DesignationUse Extensible

Details of how a designation would be used
ValueSet.compose.include.filter.op FilterOperator Required

The kind of operation to perform as a part of a property based filter.

4.9.5.2 Constraints

UniqueKeyLevelLocationDescriptionExpression
img cnl-0Warning (base)Name should be usable as an identifier for the module by machine processing applications such as code generationname.exists() implies name.matches('^[A-Z]([A-Za-z0-9_]){1,254}$')
img cnl-1Warning ValueSet.urlURL should not contain | or # - these characters make processing canonical references problematicexists() implies matches('^[^|# ]+$')
img vsd-1Rule ValueSet.compose.includeA value set include/exclude SHALL have a value set or a systemvalueSet.exists() or system.exists()
img vsd-2Rule ValueSet.compose.includeA value set with concepts or filters SHALL include a system(concept.exists() or filter.exists()) implies system.exists()
img vsd-3Rule ValueSet.compose.includeCannot have both concept and filterconcept.empty() or filter.empty()
img vsd-6Rule ValueSet.expansion.containsSHALL have a code or a displaycode.exists() or display.exists()
img vsd-9Rule ValueSet.expansion.containsSHALL have a code if not abstractcode.exists() or abstract = true
img vsd-10Rule ValueSet.expansion.containsSHALL have a system if a code is presentcode.empty() or system.exists()
img vsd-11Rule ValueSet.compose.include.concept.designationMust have a value for concept.designation.use if concept.designation.additionalUse is presentadditionalUse.exists() implies use.exists()

4.9.6 Composition Rules

A value set can be a simple list of included codes, or it can be some kind of general selection criteria using the facilities provided by the code system. For these value sets:

  • Multiple include statements are cumulative - e.g. the value set contains the union of all the includes
  • Within an include, all the criterion apply -e.g. the value set contains the intersection of the criterion
  • Within an include, a single system with selection criteria may be listed, and/or one or more value sets may be listed
  • valueSet(s) only: Codes are 'selected' for inclusion if they are in all the referenced value sets
  • If a System only is specified, the following rules apply:
    • no concept or filter: All codes defined by the code system, independent of code status, are included
    • concept: Only the enumerated codes are selected
    • filter: Any codes meeting the filter criteria are selected
  • valueSet and System: Codes are 'selected' for inclusion if they are selected by the code system selection (after checking for concept and filter) and if they are in all the referenced value sets
  • If the system reference is not version specific and filters are present, then the contents of the value set are open and change over time as the underlying code systems are updated
  • The version reference may be the special value '*', which indicates that the value set includes codes from all versions of the code system. how to handle provision of the required versions and generation of expansions is at server discretion, including for poorly behaved code systems where a code changes in meaning).

    Implementation Note: Use of this capability is subject to future clarification and conformance requirements based on implementation experience.

  • Using the property filters is only possible where the code system in use defines the relevant properties. Note that in some cases the underlying code system defines the logical concepts but not the syntax for exercising them. In such cases, the literal definitions may be provided by a third party
  • In addition to include rules, codes may be excluded. Rules for interpretation of exclude statements match those for includes, but codes in the exclude statements are never in the value set
  • Value sets may include abstract codes - that is, codes designated by the underlying code system as not for use as a selectable concept in a particular context. These abstract codes are typically used as a grouping/searching mechanism, and can be included either by enumerating them, or by using a filter.
  • Any compose.exclude SHALL be processed such that excluded codes are not found in the expansion
  • Any compose.include SHALL NOT reference a CodeSystem supplement.
  • Any compose.exclude SHALL NOT reference a CodeSystem supplement.
  • Required CodeSystem supplements SHALL be declared using either the valueset-supplement extension, or the useSupplement $expand input parameter.

4.9.6.1 Compose

VSD Content Logical Definition (CLD) is analogous to FHIR ValueSet.compose. The ValueSet author creates the expression (intensional definition), the extensional list, or a text description of ValueSet.scope and ValueSet.description. There are three ways to accomplish ValueSet definitions in FHIR:

  1. Computable Expression Option 1
    1. ValueSet.compose
  2. Computable Expression Option 2
    1. Extension ValueSet.valueset-expression
      1. If valued with ValueSet.compose, must be an expression that aligns with the ValueSet.compose
      2. If valued without ValueSet.compose, must be a computable expression (e.g., ECL)
  3. Non-computable Expression
    1. Extension ValueSet.valueset-rules-text
      1. An expression that is not computable and is meant to be interpreted by a human
      2. May be valued with or without ValueSet.compose

4.9.6.2 Filters

VSD describes filters for the Content Logical Definition. FHIR uses filters in the ValueSet.compose to implement the VSD features.

Scenario FHIR Implementation
When the ValueSet.compose is an expression (ValueSet.expression), declare the type of expression here

compose.include.filter.op = "property"

compose.include.filter.op = regex

compose.include.filter.value = "property value"
When the ValueSet.compose describes Concepts to include based on CodeSystem properties

compose.include.filter.property = "property name"

Either the Value or Expression must be provided

Value:

compose.include.filter.op = "="

compose.include.filter.value =the value

Expression:

  • compose.include.filter.op = "regex"
  • compose.include.filter.value =the Expression
When the ValueSet.compose defines the types of relationships between concepts to include in the ValueSet Expansion

compose.filter.property = "RelationshipType"

(note: filter property is a string in VSD, code in FHIR)

compose.filter.property.op = "="

compose.filter.property.value = a code defined by the CodeSystem

Example:

to find all concepts in SNOMED CT with the SNOMED CT laterality qualifier relationship to the SNOMED CT concept "left":

filter.property.name = '272741003'

filter.property.op = '='

filter.property.value = '7771000'

4.9.6.2.1 Code Systems Note

Each Code System defines which filters can be used in ValueSet.compose.include.filter. All code systems have base filters and any additional filters defined in (CodeSystem.filter).

HL7 Terminology defines filters for various published code systems:

4.9.6.3 Union and Intersection

ValueSet.compose may reference other ValueSets.

Intersection:

  • If the ValueSet definition contains references to multiple ValueSets within a single ValueSet.compose.include
    • The resulting expansion includes only those concepts that are in each expansion of the included ValueSets

Union:

  • If the ValueSet definition contains references to ValueSets using multiple ValueSet.compose.include elements
    • The resulting expansion includes all concepts from the ValueSet expansions

For expansion guidance, and how to manage exclude, see Value Set Expansion

When a ValueSet definition includes other ValueSets, use the ValueSet.valueset-compose-include-ValueSetTitle extension to provide the human readable name of the included Value Set.

4.9.6.4 Referencing CodeSystem Fragments

The VSD Partition aligns with FHIR CodeSystem Fragments.

Code System fragments are a unique and identifiable distinct segment of an overall Code System namespace. While most Code Systems are in essence one fragment and, therefore, do not have identifiable fragments, some Code Systems can be made up of a collection of distinct segments. SNOMED CT is an example of such a Code System. The SNOMED CT International Edition is the base fragment and can be used alone, but there are many distinct additional fragments that can be added to the international core to create what is known as a SNOMED CT Edition.

The CodeSystem referenced in a ValueSet.compose may reference a CodeSystem fragment.

4.9.7 ValueSet Alignment with VSD

4.9.7.1 Author and Publisher

Author, Steward and Publisher are roles associated with a Value Set Definition.

ValueSet Author is the person or organization that creates and may modify the ValueSet. The Steward is the person or organization responsible for the content, maintenance and life cycle of the ValueSet. The Publisher is the person or organization that makes the ValueSet available. The publisher or a terminology server might provide the ValueSet expansions.

In FHIR, ValueSet.publisher satisfies the requirements for both publisher and steward.

4.9.7.2 Scope and Description

Description and Purpose are standard elements that appear in many FHIR resources. Neither fully supports the intent of the VSD Scope element, however ValueSet.description does support Focus as defined in VSD.

In combination with ValueSet.description, ValueSet.scope is used by a ValueSet.author to communicate the semantic space the ValueSet expansion is intended to cover to other authors and users and to create specific descriptions of what is included and what is excluded. Inclusion criteria should not be used to repeat the Content Logical Definition (ValueSet.compose) (see the first example below).

When a ValueSet definition includes other ValueSets, use ValueSet.description to provide information about the included value sets in context of the parent value set.

Purpose may be used for the ValueSet.author to describe why the ValueSet was created. ValueSet.description may be used to provide the ValueSet Focus as described in VSD.

Example:

Value Set Name Description Scope Inclusion Criteria Scope Exclusion Criteria
Body Site Value Set All SNOMED CT anatomic structures, locations, abnormal structures that can be considered to describe an anatomical site. Bad Example: SNOMED CT concepts descending from the Anatomical Structure (91723000) or Acquired body structure (body structure) (289115004) or Anatomical site notations for tumor staging (body structure) (258331007) or Body structure, altered from its original anatomical structure (morphologic abnormality) (118956008) or Physical anatomical entity (body structure) (91722005). None
Familial Hypercholesterolemia This value set contains terms defining familial hypercholesterolemia, regardless of genetic origin. Good Example: Terms identifying familial hypercholesterolemia diagnoses. Familial combined hyperlipidemia, polygenic hypercholesterolemia, pure hypercholesterolemia not specifically familial. There are no ICD-9-CM codes specific to familial hypercholesterolemia, hence no ICD-9-CM codes are included in this value set.

4.9.7.3 Name and Title

The VSD element Name aligns with ValueSet.title which does not support preference or language. Use the ValueSet.valueset-otherTitle extension when a language or preference is necessary. ValueSet.title or ValueSet.valueset-otherTitle must be valued.

4.9.7.4 Include Active Concepts Only

VSD and FHIR support the constraining the $expand operation to return only active concepts as defined by the CodeSystem.

The VSD element ActiveOnly equal to TRUE is equivalent to ValueSet.compose.inactive equal to FALSE.

4.9.7.5 Status

The ValueSet resource defines a Value Set Definition and a Value Set Expansion. VSD describes only the characteristics of the Value Set Definition.

The VSD element ActivityStatus aligns with ValueSet.status, which is the status of the entire ValueSet resource with the exception of the ValueSet.expansion. This aligns with VSD using the word definition to mean the Content Logical Definition plus all the other non-expansion metadata. A Value Set Expansion is stateless.

Note: ValueSet.status is bound with required binding strength to the PublicationStatus value set. The following table shows how the ActivityStatus values in VSD align with the values in the PublicationStatus Value Set Expansion.

VSD Activity Status Value FHIR Publication Status Value
Preliminary Draft
Active Active
Inactive Retired
* Unknown
Deleted *

*There is no equivalent value in either VSD or the PublicationStatus Value Set.

4.9.8 Display and Designations

Concepts used in ValueSets can have a display, which is a short text that represents the meaning of the concept to human users in the context of the value set (which often has narrower meaning and therefore is amenable to shorter displays. If a display is not provided, the value set uses the display from the code system (which is the preferred approach, because overriding the display can lead to very unsafe outcomes).

When a value set enumerates codes, it is sometimes useful to define an alternative display for the code that is to be used wherever the value set is expanded and used in a UI. This facility is provided to cover the following circumstances:

  • The system that defines the code or expression doesn't provide a display for this code (or any codes).
  • The system that defines the code or expression defines multiple choices for display.
  • The system provides a very long display name that is unnecessary or inappropriate in the context of this value set (e.g. a display name of "Glucose [Mass/volume] in Serum or Plasma --10 PM specimen" for LOINC code 48991-4, when the value set only includes Glucose mass/vol in serum/plasma codes). As the display names get longer, this becomes more important.

Note that care must be taken in order to avoid "changing the meaning" of the concept by implying that it means something other than the explicit definition of the concept in the underlying code system (e.g., in the case above, using a display of "Glucose Concentration at 10pm"). For this reason, some contexts of use do not allow a display to be associated with a specific code in a value set.

Any display name for a concept provided in the value set is for display to a human user. The display in the Coding that results from a user selecting a concept from the expansion must be taken from the underlying code system definition, even if a value set is referenced explicitly in the Coding (e.g. by an extension). The correct display for a code can be determined by a $lookup operation. Any alternative display specified in the value set would go in CodeableConcept.text, perhaps appended to the UI label for the matching data element.

As an example, the LOINC icon code 55423-8 icon has a display value of "Number of steps in unspecified time Pedometer". A value set for a pick list in a patient generated data form might choose a simpler name: 

{
  "resourceType" : "ValueSet",
  "compose" : {
    "include" : [{
      "system" : "http://loinc.org",
      "concept" : [{
        "code" : "55423-8",
        "display" : "Step Count"
      }]
    }]
  }
}

The expansion generated by a terminology server will have this: 

{
  "resourceType" : "ValueSet",
  "expansion" : {
    "contains" : [{
      "system" : "http://loinc.org",
      "code" : "55423-8",
      "display" : "Step Count"
    }]
  }
}

The expansion display is taken from the value set, and this is what is displayed in the pick list. Once the user picks the code, it will appear in the Observation.code like this: 

{
  "resourceType" : "Observation",
  "code" : {
    "coding" : [{
      "system" : "http://loinc.org",
      "code" : "55423-8",
      "display" : "Number of steps in unspecified time Pedometer"
    }],
    "text" : "Step Count"
  }
}

Note that the correct value for the display is not in the expansion above. The client can either omit the display, look it up using $lookup, or the server might pre-populate it in the expansion: 

{
  "resourceType" : "ValueSet",
  "expansion" : {
    "contains" : [{
      "system" : "http://loinc.org",
      "code" : "55423-8",
      "display" : "Step Count",
      "designation" : [{
        "use" : {
          "system" : "http://snomed.info/sct",
          "code" : "900000000000003001"
        },
        "value" : "Number of steps in unspecified time Pedometer"
      }]
    }]
  }
}

Irrespective of this, the display in the expansion always goes in CodeableConcept.text.

In addition to the display, a concept can have one or more designation elements. The display is equivalent to a special designation with an implied designation.use of "primary code" and a language equal to the Resource Language. The designations can provide additional displays for other languages, as well as designations for other purposes. When using concepts, applications use the display unless the language or usage in context provides a reason to use one of the designations.

4.9.9 Value Sets with multiple code systems

Value sets may select codes from multiple code systems - either by including codes from different systems, or importing other value sets that include them. A typical use for crossing code systems is when including a set of codes, and adding a few additional codes to cover cases not catered to by the included codes (e.g. Data missing or workflow error codes).

Best Practice Note: Mixing definitional systems offers the potential for confusing, overlapping, and inconsistent definitions. Creating value sets that cross code systems should be done with care to avoid creating definitional confusion.

Value sets should only include well differentiated concepts, but many value sets and code systems do not have well differentiated concepts because of various real-world constraints.

4.9.10 Value Set Expansion

A value set can be "expanded", where the definition of the value set is used to create a simple collection of codes suitable for use for data entry or validation. There is a defined operation $expand to ask a server to perform this expansion. Expansions are most useful when a value set includes all the codes in a code system, or a set of codes by filter.

A resource that represents a value set expansion includes the same identification details as the definition of the value set, and MAY include the definition of the value set (.compose). In addition, it has an .expansion element which contains the list of codes that constitute the value set expansion. Each contained code can include nested contained codes - see below for further discussion.

When a request for an expansion is received (e.g., for the $expand operation), the following process should be followed:

  • If the value set already has an expansion (e.g., a stored expansion):
    • Check that the parameters associated with the $expand operation are consistent with the parameters recorded in the expansion
    • If the parameters are inconsistent, then either generate a new expansion, or, if there is no definition (.compose), return an error
    • Additional parameters may be applied to an existing expansion if the server has validated that the result will be the same as if the expansion was generated directly from the value set definition (e.g. text filter, designations included).
  • Otherwise: For each compose.include:
    1. If there is a system, identify the correct version of the code system, and then:
      • If there are no codes or filters, add every code in the code system to the result set.
      • If codes are listed, check that they are valid, and check their active status, and if ok, add them to the result set (the parameters to the $expand operation may be used to control whether active codes are included).
      • If any filters are present, process them in order (as explained above), and add the intersection of their results to the result set.
    2. For each valueSet, find the referenced value set by ValueSet.url, expand that (e.g., using the $expand operation: GET [base]/ValueSet/$expand$url=[compose.include.valueSet]), to produce a collection of result sets. This means that expansion across imports is a recursive process.
    3. Add the intersection of the result set from the system (step 1) and all of the result sets from the value sets (step 2) to the expansion
  • For each compose.exclude, follow the same process as for compose.include, but remove codes from the expansion in step 3 instead of adding them.
  • When available, ValueSet.status and ValueSet.version from the ValueSet resource instance which contains the definition SHALL be returned in the ValueSet resource instance which contains the expansion.

The final "result set" is then represented in expansion. Note that the expansion structure is inherently ordered; this specification does not fix the meaning of use of the order. The conceptual approach described should not be understood to prohibit any implementation approach in these regards. In addition, note that the method described above is a conceptual approach; individual servers may choose to follow alternative approaches that are more efficient, as long as the outcome is the same.

The impact of Code System supplements on value set expansion - and therefore value set validation - is subject to ongoing experimentation and implementation testing, and further clarification and additional rules might be proposed in future versions of this specification.

 

4.9.10.1 Hierarchical Expansions

The expansion MAY be hierarchical - that is, it may have contains element that contain their own sub-elements, to any level of depth. This specification does not fix the meaning of the hierarchy: there is no implication about the logical relationship between the nested contain elements, and the structure cannot be used for logical inferencing. The structure exists to provide navigational assistance for helping human users to locate codes in the expansion.

Note that the CodeSystem resource and ValueSet.compose offer no direct support for defining hierarchies and groups. An expansion may include entries in the expansion that only serve an arbitrary grouping purpose, to make it easier for a human to use the list. These entries have no system or code, and must be marked as abstract.

4.9.10.2 Uniqueness of Concepts in Expansions

Value set definitions may lead to more than one instruction to include a given concept in the value set across the includes and imports. No matter how many times the definitions include a concept, it is only present in the value set once, and will only appear once in a flat expansion of the value set. Note, however, that a concept may appear more than once in a nested hierarchy when the expansion is prepared for UI use (irrespective of how many times it is included in the definitions). Note that uniqueness is based on system/version/code; it is possible to include the same concept from different versions of a code system in the same expansion, though this is generally confusing for users and should be avoided.

The codes in the expansion should be treated as case sensitive - implementers should use the correct case. Implementers can consult the definition of the underlying code systems to determine whether the code system that defines the code is case sensitive or not.

4.9.10.3 Expansion Identifiers

It is important that expansions be identified properly. Any value set definition may produce an infinite number of expansions, depending on the operation parameters. Any expansions produced must be clearly identified so that there is no confusion. The following rules apply:

  • The canonical URL for the expansion is the same as the value set it was expanded from
  • Each expansion SHALL have a unique identifier in ValueSet.expansion.identifier
  • The result of an $expand operation may use the same identifier in ValueSet.expansion.identifier as a previous expansion, but if it does, the canonical representation of the value set expansion SHALL be identical (e.g. a cached response)

4.9.10.4 Expansion Parameters

The expansion contains a set of parameters in ValueSet.expansion.parameter that record what controlled the expansion process. These parameters may be used by users of expanded value sets to check whether the expansion is suitable for a particular purpose, or to pick the correct expansion. The server decides which parameters to return in ValueSet.expansion.parameter, but at a minimum, the list SHOULD include:

  • All input parameters defined by the $expand operation and provided by the client and used by the server to generate the expansion
  • All input parameters defined by the $expand operation and their values for which the server provided default values
  • Canonical or URI references (including version) for all CodeSystems, CodeSystem Supplements and ValueSets used in the expansion

The following parameters are predefined by the $expand operation, and are suitable for use in the expansion parameters:

filter

A text filter that is applied to restrict the codes that are returned (this is useful in a UI context). The interpretation of this is delegated to the server in order to allow to determine the most optimal search approach for the context. The server can document the way this parameter works in TerminologyCapabilities..expansion.textFilter. Typical usage of this parameter includes functionality like:

  • using left matching e.g. "acut ast"
  • allowing for wild cards such as %, &, ?
  • searching on definition, designations and display(s)
  • allowing for search conditions (and / or / exclusions)

Text Search engines such as Lucene or Solr, long with their considerable functionality, might also be used. The optional text search might also be code system specific, and servers might have different implementations for different code systems

date

The date for which the expansion should be generated. if a date is provided, it means that the server should use the value set / code system definitions as they were on the given date, or return an error if this is not possible. Normally, the date is the current conditions (which is the default value) but under some circumstances, systems need to generate an expansion as it would have been in the past. A typical example of this would be where code selection is constrained to the set of codes that were available when the patient was treated, not when the record is being edited. Note that which date is appropriate is a matter for implementation policy.

offset

Paging support - where to start if a subset is desired (default = 0). Offset is number of records (not number of pages)

count

Paging support - how many codes should be provided in a partial page view. Paging only applies to flat expansions - servers ignore paging if the expansion is not flat. If count = 0, the client is asking how large the expansion is. Servers SHOULD honor this request for hierarchical expansions as well, and simply return the overall count

includeDesignations

Controls whether concept designations are to be included or excluded in value set expansions

designation

A token that specifies a system+code that is either a use or a language. Designations that match by language or use are included in the expansion. If no designation is specified, it is at the server discretion which designations to return

includeDefinition

Controls whether the value set definition is included or excluded in value set expansions. This includes all elements of the ValueSet, including extensions, with the exception that the id and meta elements would be specific to the expansion, not to the definition value set, and the expansion itself is filled out.

activeOnly

Controls whether inactive concepts are included or excluded in value set expansions. Note that if the value set explicitly specifies that inactive codes are included, this parameter can still remove them from a specific expansion, but this parameter cannot include them if the value set excludes them

excludeNested

Controls whether or not the value set expansion may nest codes or not (i.e. ValueSet.expansion.contains.contains). If excludeNested is set to true, the expansion MUST be flat (no nesting). If excludeNested is set to false (default), however, nesting is possible but not required

excludeNotForUI

Controls whether or not the value set expansion might include

One purpose of such concepts is helping a user navigate through the list efficiently. If excludeNotForUI is set to true, the concepts as described above will be excluded from the expansion. If excludeNotForUI is set to false (default), all concepts as described above may be part of the expansion. In the FHIR Specification itself, the value set expansions are generated with excludeNotForUI = false, and the expansions used when generating schema / code etc., or performing validation, are all excludeNotForUI = true.

excludePostCoordinated

Controls whether or not the value set expansion includes post coordinated codes

displayLanguage

Specifies the language to be used for description in the expansions i.e. the language to be used for ValueSet.expansion.contains.display

exclude-system

Code system, or a particular version of a code system to be excluded from the value set expansion. The format is the same as a canonical URL: [system]|[version] - e.g. http://loinc.org|2.56

system-version

Specifies a version to use for a system, if the value set does not specify which one to use. The format is the same as a canonical URL: [system]|[version] - e.g. http://loinc.org|2.56

check-system-version

Edge Case: Specifies a version to use for a system. If a value set specifies a different version, an error is returned instead of the expansion. The format is the same as a canonical URL: [system]|[version] - e.g. http://loinc.org|2.56

force-system-version

Edge Case: Specifies a version to use for a system. This parameter overrides any specified version in the value set (and any it depends on). The format is the same as a canonical URL: [system]|[version] - e.g. http://loinc.org|2.56. Note that this has obvious safety issues, in that it may result in a value set expansion giving a different list of codes that is both wrong and unsafe, and implementers should only use this capability reluctantly. It primarily exists to deal with situations where specifications have fallen into decay as time passes. If the value is overridden, the version used SHALL explicitly be represented in the expansion parameters

The count and offset parameters are important. If the expansion is a page out of the whole expansion, the offset and count parameters SHALL be populated. Clients can reliably use the count/offset parameters to determine whether the whole expansion is returned.

Other parameters that servers may be required to use:

[canonical]#CodeSystem.content [content] : The content value for the code system for the canonical URL. Applications generating expansions SHALL use this parameter if the CodeSystem.content value is "fragment"
[canonical1]#supplement [canonical2] : Indicates that the specified supplement (canonical2) contributed to the content of the expansion for the code system canonical1 (by influencing selection, or providing designations). Applications generating expansions SHALL use this parameter to record that a supplement was used during the expansion process

There's also some parameters that only go in the response to an expand, so that the server can inform the consumer of the expansion which value sets, code systems and supplements were used as the basis for the expansion:

used-system uri The canonical URL (system | {version} if available) of a code system that was used during the expansion of the value set
used-supplement uri The canonical URL (system | {version} if available) of a code system supplement that was used during the expansion of the value set
used-valueset uri The canonical URL (system | {version} if available) of a different value set that was used during the expansion of the value set

Beyond all these parameters, servers MAY define their own parameters, though Terminology server authors are requested to bring additional parameters to HL7 (via 'Propose a change' link below) in the interests of interoperability.

Servers can also create and store Provenance statements about the expansion, or AuditEvent records of the expansion process if further transparency is required. These resources can contain considerable detail about the various inputs to the process, and any significant decisions by the expansion engine. Further details around this (and profiles on the relevant resources) may be provided in future versions of this specification or related implementation guides.

Request for Feedback: The existing set of parameters are intended to cover the vast majority of use cases, but there are some cases where the parameters do not provide enough control to a client, particularly with regard to combinations of parameters, and the interplay between code system versions and other parameters. Implementers may need to define their own value sets to meet these requirements.

Ongoing feedback is welcome at [link to be provided]

4.9.10.5 Storing Expansions

Whether to store expanded value sets, or simply to store their definitions and expand on the fly is a matter for system deployment. Some servers, including public value sets servers, only store expansions. However, any system that stores an expansion must be concerned with how to determine whether the expansion is still current, and this requires deep knowledge of how the expansion was created. A system with a dedicated terminology server that returns expansions on demand avoids this problem, but leaves open the question of how to audit the specific expansion that was used for a particular case. One solution to this is to use a dedicated terminology server, and have the clients ask for expansions on demand based on the value set definitions, and for the server to store (and reuse as appropriate) the returned expansion (when it reuses the expansion, ValueSet.expansion.identifier will be the same). If expansions are shared, users need to be aware of how expansion identifiers (which may be server specific) work.

4.9.10.6 Searching for codes in value sets

The search parameters defined on ValueSet include the code parameter. This is intended to allow a consumer to find all the value sets that include a particular code. However, fully evaluating this search parameter is extremely onerous for a server. Further, whether a code is in a value set depends on the context in which expansions are performed (see the $expand operation). For this reason, the degree to which the search parameter is supported can be declared in the Terminology Capability statement.

4.9.11 Combinations of .compose and .expansion

Use cases for the different combinations:

  • .compose only: Provides the value set definition (CLD). This is the expected format from the value set publisher.
  • .expansion only: Provides the value set expansion, which might or might not be persisted (see 4.8.7 Value Set Expansion above). The "expansion only" format may be returned in the $expand operation output (at the terminology server's discretion).
  • Both .compose and .expansion: The terminology server often may include the definition as well as the expansion in the $expand operation output. The resource instance including the expansion might or might not be persisted.
  • Neither .compose nor .expansion: This is a valid format for ValueSet resource instances. A primary use case is the return from a summary search (i.e. _summary=true). This may be especially useful to reduce the performance overhead when returning a summary of extensionally defined value sets (which may include a large number of concepts in the value set definition).

4.9.12 Implicit Value Sets

Instead of a ValueSet resource as the definition, implicit value sets are defined in a specification which references the underlying code system structures and includes a prescribed URI pattern to identify the value set. HL7 has defined implicit value set URI patterns for some key code systems.

Implicit value sets allow the URI to serve as the basis for ValueSet operations such as $expand and $validate-code without the need to create a defining ValueSet resource instance.

Some advantages of using implicit value sets are that they may be used:

  • With read-only terminology servers because a ValueSet resource cannot be posted
  • With HTTP GET requests because they are cacheable
  • To enable text searches (google like) for concepts across the entirety of large code systems (e.g., SNOMED CT)
  • To provide a convenient way to reference all or parts of code systems in the definition of a ValueSet or ConceptMap

In some cases it is not possible to express a value set definition using ValueSet.compose - but it may be possible to express the value set definition with an implicit value set URI. An example of this scenario is all SNOMED CT concept IDs that identify reference sets.

If there is an explicit value set resource with the same URI as a known implicit value set, it SHALL conform the pattern described in the definition of the implicit value set. It is up to the discretion of the server how to handle explicit instances when it is also able to process requests for the implicit value set.

Implementation Note: If the relevant server(s) support implicit value sets, implementers are discouraged from creating their own explicit value sets with the same URI, as their existence may create confusion.

Implementers SHOULD NOT create ValueSet resources where the ValueSet.url value matches the pattern of a known implicit value set.

Implicit value set URIs can be used anywhere a value set URI can be used. Support for implicit value set URI patterns varies across terminology servers.

Some (but not all) code systems have implicit value set URI patterns defined by HL7 and documented for use with FHIR terminology services. Code system publishers may also define implicit value set URI patterns. FHIR terminology servers might or might not support any or all of these URI patterns. Caution should be exercised when using value set URI patterns that have not been defined by HL7 or the code system publisher.

The following links describe the currently defined FHIR implicit value set URL patterns for these listed code systems:

4.9.13 Search Parameters

Search parameters for this resource. See also the full list of search parameters for this resource, and check the Extensions registry for search parameters on extensions related to this resource. The common parameters also apply. See Searching for more information about searching in REST, messaging, and services.

Name Type Description Expression In Common
code token This special parameter searches for codes in the value set. See additional notes on the ValueSet resource ValueSet.expansion.contains.code | ValueSet.compose.include.concept.code
context token A use context assigned to the value set (ValueSet.useContext.value.ofType(CodeableConcept)) 30 Resources
context-quantity quantity A quantity- or range-valued use context assigned to the value set (ValueSet.useContext.value.ofType(Quantity)) | (ValueSet.useContext.value.ofType(Range)) 30 Resources
context-type TU token A type of use context assigned to the value set ValueSet.useContext.code 30 Resources
context-type-quantity TU composite A use context type and quantity- or range-based value assigned to the value set On ValueSet.useContext:
  context-type: code
  context-quantity: value.ofType(Quantity) | value.ofType(Range)		 30 Resources
context-type-value TU composite A use context type and value assigned to the value set On ValueSet.useContext:
  context-type: code
  context: value.ofType(CodeableConcept)		 30 Resources
date date The value set publication date ValueSet.date 31 Resources
derived-from TU reference A resource that the ValueSet is derived from ValueSet.relatedArtifact.where(type='derived-from').resource
(Any)		 9 Resources
description string The description of the value set ValueSet.description 29 Resources
effective TU date The time during which the ValueSet is intended to be in use ValueSet.effectivePeriod 12 Resources
expansion uri Identifies the value set expansion (business identifier) ValueSet.expansion.identifier
identifier token External identifier for the value set ValueSet.identifier 35 Resources
jurisdiction token Intended jurisdiction for the value set ValueSet.jurisdiction 27 Resources
name string Computationally friendly name of the value set ValueSet.name 28 Resources
predecessor TU reference The predecessor of the ValueSet ValueSet.relatedArtifact.where(type='predecessor').resource
(Any)		 9 Resources
publisher string Name of the publisher of the value set ValueSet.publisher 31 Resources
reference uri A code system included or excluded in the value set or an imported value set ValueSet.compose.include.system
status token The current status of the value set ValueSet.status 35 Resources
title string The human-friendly name of the value set ValueSet.title 28 Resources
topic TU token Topics associated with the ValueSet ValueSet.topic 10 Resources
url uri The uri that identifies the value set ValueSet.url 34 Resources
version token The business version of the value set ValueSet.version 32 Resources