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4.8 Resource CodeSystem - Content

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

The CodeSystem resource is used to declare the existence of and describe a code system or code system supplement and its key properties, and optionally define a part or all of its content.

4.8.1 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:

Code systems define which codes (symbols and/or expressions) exist, and how they are understood. Value sets select a set of codes from one or more code systems to specify which codes can be used in a particular context.

The CodeSystem resource is used to declare the existence of a code system, and its key properties:

  • Identifying URL and version
  • Description, Copyright, publication date, and other metadata
  • Some key properties of the code system itself - e.g. whether it exhibits concept permanence, whether it defines a compositional grammar, and whether the codes that it defines are case sensitive
  • What filters can be used in value sets that use the code system in a ValueSet.compose element
  • What concept properties are defined by the code system

In addition, the CodeSystem resource may list some or all of the concepts in the code system, along with their basic properties (code, display, definition), designations, and additional properties. Code System resources may also be used to define supplements, which extend an existing code system with additional designations and properties.

The CodeSystem resource is not intended to support the process of maintaining code systems. Instead, the focus is on publishing the properties and optionally the content of a code system for use throughout the FHIR eco-system, such as to support value set expansion and validation. Note that the important existing (large) code systems (SNOMED CT, LOINC, RxNorm, ICD family, etc.) all have their own maintenance systems and distribution formats, and CodeSystem is generally not an efficient way to distribute their content, though it is used as one way of declaring the filters and properties associated with those code systems.

4.8.2 Boundaries and Relationships

  • Code systems are used in ValueSet resources
  • The system element in the Coding (i.e. Coding.system) refers to a CodeSystem resource instance by its canonical URL (i.e. CodeSystem.url)
  • The CodeSystem resource design is based, in part, on the code system functionality described in the HL7 CTS 2 icon specification. A CTS2 server can be used to maintain code systems which are then published using the CodeSystem resource
  • The CodeSystem resource documents the inherent structure and capabilities of code system, whereas the TerminologyCapabilities resource documents what a server hosting the code system is capable of

The CodeSystem resource declares the existence of a code system and its key properties including its preferred identifier. The NamingSystem resource identifies the existence of a code or identifier system, and its possible and preferred identifiers. The key difference between the resources is who creates and manages them - CodeSystem resources are managed by the owner or publisher of the code system, who can properly define the code system features and content. NamingSystem resources, on the other hand, are frequently defined by 3rd parties that encounter the code system in use, and need to describe the use, but do not have the authority to define the features and content. Additionally, there may be multiple authoritative NamingSystem resources for a code system, but ideally there would be only one authoritative CodeSystem resource (identified by its canonical URL) that is provided by the code system publisher, with multiple copies distributed on additional FHIR servers or elsewhere and used where needed.

Both Code System supplements and Concept Maps may be used to define relationships between concepts in different systems. ConceptMaps are assertions of the relationships between different concepts that are associated with particular contexts of use, while CodeSystem supplements are used to define inherent properties and semantics of the concepts in the code system

4.8.3 Background and Context

When using code systems and 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.

4.8.3.1 CodeSystem Identification

A code system has three identifiers. The first two can be used to reference the code system in the FHIR context:

  • CodeSystem.id: The logical id on the system that holds the CodeSystem resource instance - this typically is expected to change as the resource moves from server to server. The location URI is constructed by appending the logical id to the server base address where the instance is found and the resource type. This URI should be a resolvable URL by which the resource instance may be retrieved, usually from a FHIR server, and it may be a relative reference typically to the server base URL.
  • CodeSystem.url: The canonical URL that never changes for this code system - 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. This canonical URL is used to refer to all instances of this particular code system across all servers and systems. Ideally, this URI should be a URL which resolves to the location of the master version of the code system, though this is not always possible.

For example, for CodeSystems that are part of the FHIR specification, CodeSystem.url is the url where the CodeSystem may be accessed in the FHIR specification. However, while a new version of the FHIR specification is being prepared, new code systems that are published in CI build versions will not be found in the released FHIR specification version.

The third code system identifier is used typically for external references to the code system outside of FHIR:

  • CodeSystem.identifier: A system/value pair that is used to identify the code system in other contexts (such as an OID in an HL7 v3 icon specification)

For further information regarding resource identification, see Resource Identity.

This means that each code system has 2 different URIs 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). Most references to code systems use the canonical URL because it is stable and does not change from one FHIR server instance to another.

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

4.8.4 References to this Resource

4.8.5 Resource Content

Structure

NameFlagsCard.TypeDescription & Constraintsdoco
.. CodeSystem N DomainResource Declares the existence of and describes a code system or code system supplement
+ Warning: Name should be usable as an identifier for the module by machine processing applications such as code generation
+ Rule: Within a code system definition, all the codes SHALL be unique
+ Warning: If there is an explicit hierarchy, a hierarchyMeaning should be provided
+ Warning: If there is an implicit hierarchy, a hierarchyMeaning should be provided
+ Rule: If the code system content = supplement, it must nominate what it's a supplement for

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

... version Σ 0..1 string Business version of the code system (Coding.version)
... 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 code system (computer friendly)
... title ΣT 0..1 string Name for this code system (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 code system
... useContext ΣTU 0..* UsageContext The context that the content is intended to support

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

... purpose T 0..1 markdown Why this code system 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 CodeSystem was approved by publisher
... lastReviewDate TU 0..1 date When the CodeSystem was last reviewed by the publisher
... effectivePeriod ΣTU 0..1 Period When the CodeSystem 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 CodeSystem

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

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

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

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

... caseSensitive Σ 0..1 boolean If code comparison is case sensitive
... valueSet Σ 0..1 canonical(ValueSet) Canonical reference to the value set with entire code system
... hierarchyMeaning ΣC 0..1 code grouped-by | is-a | part-of | classified-with
Binding: Code System Hierarchy Meaning (Required)
... compositional Σ 0..1 boolean If code system defines a compositional grammar
... versionNeeded Σ 0..1 boolean If definitions are not stable
... content ΣC 1..1 code not-present | example | fragment | complete | supplement
Binding: Code System Content Mode (Required)
... supplements ΣC 0..1 canonical(CodeSystem) Canonical URL of Code System this adds designations and properties to
... count Σ 0..1 unsignedInt Total concepts in the code system
... filter Σ 0..* BackboneElement Filter that can be used in a value set

.... code Σ 1..1 code Code that identifies the filter
.... description Σ 0..1 string How or why the filter is used
.... operator Σ 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 What to use for the value
... property Σ 0..* BackboneElement Additional information supplied about each concept

.... code Σ 1..1 code Identifies the property on the concepts, and when referred to in operations
.... uri Σ 0..1 uri Formal identifier for the property
.... description Σ 0..1 string Why the property is defined, and/or what it conveys
.... type Σ 1..1 code code | Coding | string | integer | boolean | dateTime | decimal
Binding: Property Type (Required)
... concept C 0..* BackboneElement Concepts in the code system

.... code C 1..1 code Code that identifies concept
.... display 0..1 string Text to display to the user
.... definition 0..1 string Formal definition
.... designation C 0..* BackboneElement Additional representations for the 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 Details how this designation would be used
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
.... property 0..* BackboneElement Property value for the concept

..... code C 1..1 code Reference to CodeSystem.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
.... concept C 0..* see concept Child Concepts (is-a/contains/categorizes)


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

4.8.5.1 Terminology Bindings

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

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

The lifecycle status of an artifact.
CodeSystem.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.
CodeSystem.topic DefinitionTopic Example

High-level categorization of the definition, used for searching, sorting, and filtering.
CodeSystem.hierarchyMeaning CodeSystemHierarchyMeaning Required

The meaning of the hierarchy of concepts in a code system.
CodeSystem.content CodeSystemContentMode Required

The extent of the content of the code system (the concepts and codes it defines) are represented in a code system resource.
CodeSystem.filter.operator FilterOperator Required

The kind of operation to perform as a part of a property based filter.
CodeSystem.property.type PropertyType Required

The type of a property value.
CodeSystem.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
CodeSystem.concept.designation.use DesignationUse Extensible

Details of how a designation would be used
CodeSystem.concept.designation.additionalUse DesignationUse Extensible

Details of how a designation would be used

4.8.5.2 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}$')
img csd-1Rule (base)Within a code system definition, all the codes SHALL be uniqueconcept.exists() implies concept.code.combine(%resource.concept.descendants().concept.code).isDistinct()
img cnl-1Warning CodeSystem.urlURL should not contain | or # - these characters make processing canonical references problematicexists() implies matches('^[^|# ]+$')
img csd-2Warning (base)If there is an explicit hierarchy, a hierarchyMeaning should be providedconcept.concept.exists() implies hierarchyMeaning.exists()
img csd-3Warning (base)If there is an implicit hierarchy, a hierarchyMeaning should be providedconcept.where(property.code = 'parent' or property.code = 'child').exists() implies hierarchyMeaning.exists()
img csd-4Rule (base)If the code system content = supplement, it must nominate what it's a supplement forCodeSystem.content = 'supplement' implies CodeSystem.supplements.exists()
img csd-5Rule CodeSystem.concept.designationMust have a value for concept.designation.use if concept.designation.additionalUse is presentadditionalUse.exists() implies use.exists()

4.8.6 Versioning Code Systems

Most code systems evolve over time, due to corrections, clarifications, and changes to approach or underlying knowledge or reality. If these changes lead to the meanings of existing codes changing significantly, then the interpretation of the code system becomes version dependent. This significantly complicates implementation based on the code system, to the point where it is not clear that safety can be assured, so changing the meaning of an existing code SHOULD be avoided whenever possible. It is preferable to assign a new identifier to a code system when any concepts in it have a significant change in meaning (for example, the German diagnostic classification code system ICD10GM2009 has a different system to ICD10GM2008), but this also can have substantial impact on implementation, so is often not practical - for instance, SNOMED CT icon has a complex version release framework, which may lead to variations in meaning of concepts, but there is only one identifier for SNOMED CT.

For this reason, a code system MAY provide a version which can be specified in CodeSystem.version. The version SHOULD be provided whenever there are potentially significant changes in meaning across multiple releases of a code system. There is no particular format requirement for the version, though HL7 recommends a date-based approach, or sem-ver icon.

When the CodeSystem.versionNeeded is 'true', then the CodeSystem.version value SHALL be used in Coding instances that refer to the code system.

Where the terminology does not clearly define what string should be used to identify code system versions, the recommendation is to use as the version string the date (expressed in FHIR date format) on which the version of the code system that is being used was officially published.

4.8.7 Multi-part Code Systems

The simple case for a code system is that the entire code system - all the concepts and their codes, designations and properties are distributed in a single CodeSystem resource. However, for a variety of reasons, code systems may be distributed by the code system authority in a set of fragments, and other authorities may issue additional designations and properties in supplements.

4.8.8 Code System Fragments

If the CodeSystem.content value is fragment, then the resource describes part of a code system.

Code Systems may be distributed in fragments for the following reasons:

  • Different IP distribution rules for different parts of the code system
  • Special purpose modules for specific purposes
  • Distribution of proposed content for evaluation

The following rules apply to code system fragments:

  • All fragments SHALL have the same CodeSystem.url
  • Fragments can only be published by the code system authority, or according to a process defined by the authority, if they have defined one
  • Fragments cannot contain any codes, concepts or properties that would not be found in a complete representation of the code system, if one exists. Implementers should note that a "complete representation" of the code system might not exist in a single authoritative publication (e.g., SNOMED International with all modules/extensions)
  • Publishing a code system in multiple fragments can create confusion for terminology servers and terminology service consumers. Code System authorities that publish fragments should be careful and communicate their intent clearly

4.8.9 Code System Supplements

If the CodeSystem.content value is supplement, then the resource describes a code system supplement. The following rules apply to code system supplements:

  • The CodeSystem.supplements element SHALL have a value, which is the URL of the code system being supplemented
  • The CodeSystem.url for a supplement SHALL never appear in a Coding.system
  • The CodeSystem.url for a supplement must be under the control of the authority creating or publishing the supplement (e.g. not in the same space as the code system being supplemented, unless the supplement is being issued by the same authority as the original code system
  • A codesystem supplement cannot define any new CodeSystem.concept.code. i.e.: all CodeSystem.concept.code in the supplement must be a code from the "supplemented" code system

If a supplement needs to define new concepts/codes to use as property values, it can be paired with a new (possibly contained) Code System and use the Coding type for the property values.

4.8.10 Display, Definition and Designations

Concepts have both a display and a definition. The display is a short text that represents the meaning of the concept to human users, while the definition is a more formal statement of the meaning of the concept, which is often longer. All concepts SHOULD have a display and a definition, though it is not mandatory because there are many cases where no such properties are provided, in spite of their utility and importance for clear and safe communication.

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

4.8.11 Concept Properties

Each code system can define one or more concept properties. Each concept defined by the code system may have one or more values for each concept property defined by the code system. Typical uses for properties include:

  • Tracking administrative status (inactive, deprecation date)
  • Providing additional statements about the meaning of the concept
  • Defining structured relationships with other concepts in the code system
  • Assigning scoring values to the concepts

Properties are identified by their master URI (CodeSystem.property.uri), and then, by their code (CodeSystem.property.code), which is used both internally within the code system resource (CodeSystem.concept.property.code) and also externally, in the following places:

  • ConceptMap: ConceptMap.element.target.dependsOn.property and ConceptMap.element.target.product.property
  • ValueSet: ValueSet.compose.include.filter.property can refer to any defined code system property
  • $lookup operation: In Parameters.parameter.name when returning information about a code
  • $translate operation: In Parameters.parameter.part.name for dependencies and products
  • $find-matches operation: In Parameters.parameter.name when providing codes, and in Parameters.parameter.name when asking for codes

Properties are defined using the following elements:

Name Details Description
code code Used to identify the property, in the places shown in the list immediately above this table
uri optional uri Reference to the formal meaning of the property. One possible source of meaning is the Concept Properties code system. This part of the definition is optional, but is recommended to provide an additional level of definitional consistency
description optional string A description of the property- why it is defined, and how its value might be used
type code | Coding | string | integer | boolean | dateTime | decimal The type of the property value. The values for properties of type "code" (CodeSystem.concept.property.valueCode) are drawn from the codes defined in the CodeSystem instance.

Note that properties provide a common view of concept relationships that is common across all code systems. Some code systems define properties with more sophistication, such as groups of properties, or subsumption relationships between properties (e.g. SNOMED CT). Servers providing support for these code systems will need to know full details about the underlying relationships in order to provide the correct information about concepts and their properties, but this information does not surface in the resources or operations defined by the FHIR specification.

4.8.12 Defined Concept Properties

In order to assist with consistency between code systems, the following basic property URIs are defined in the Concept Properties code system:

http://hl7.org/fhir/concept-properties#status : code A property that indicates the status of the concept. If the property is identified by this URL, then it SHALL use at least these status values (where appropriate):
  • active - the concept is for normal use
  • experimental - provided for trial, but may be removed in the future
  • deprecated - the concept is outdated and planned to be withdrawn from use
  • retired - still present for historical reasons, but no longer allowed to be used
http://hl7.org/fhir/concept-properties#inactive : boolean True if the concept is not considered active - e.g. not a valid concept any more. Property type is boolean, default value is false
http://hl7.org/fhir/concept-properties#effectiveDate : date The date at which the concept status was last changed
http://hl7.org/fhir/concept-properties#deprecationDate : date Date Concept was deprecated
http://hl7.org/fhir/concept-properties#retirementDate : date Date Concept was retired
http://hl7.org/fhir/concept-properties#notSelectable : boolean This concept is a grouping concept and not intended to be used in the normal use of the code system (though may be used for filters etc.). This is also known as 'Abstract'
http://hl7.org/fhir/concept-properties#parent : code An immediate parent of the concept in the hierarchy
http://hl7.org/fhir/concept-properties#child : code An immediate child of the concept in the hierarchy
http://hl7.org/fhir/concept-properties#partOf : code The concept identified in this property (by its code) contains this concept as a component
http://hl7.org/fhir/concept-properties#synonym : code This property contains an alternative code that may be used to identify this concept instead of the primary code
http://hl7.org/fhir/concept-properties#comment : string A string that provides additional detail pertinent to the use or understanding of the concept
http://hl7.org/fhir/concept-properties#itemWeight : decimal A numeric value that allows the comparison (less than, greater than) or other numerical manipulation of a concept (e.g. Adding up components of a score). Scores are usually a whole number, but occasionally decimals are encountered in scores

Code System representations SHOULD use these properties where possible.

The parent and child properties are used when performing concept lookup (using the $lookup operation), and when using properties to indicate parent/child relationships (see below).

4.8.12.1 Concept Status

Many Code Systems have a 'status' associated with the concept. This may categorize the concept as:

  • Experimental - provided for trial, but may be removed in the future
  • Active - in normal use
  • Deprecated - planned to be removed from use
  • Retired - still present for historical reasons, but no longer allowed to be used

There is wide variation in the life cycles supported by the different code systems and the words they use to describe the various status values they use, and some code systems have additional status values. HL7 uses Active, Deprecated and Retired. These ideas overlap with setting the concept to 'inactive'. In addition, concepts have dates associated with their retirement or deprecation and some code systems imply the status by providing dates.

In addition to these status codes, concepts may also be labeled 'notSelectable' (aka 'Abstract'), which means "not to be used in some circumstances", though the exact circumstances need to be clarified somewhere. This is sometimes also considered a status, though it's usually dependent on the context of use (e.g. set in a ValueSet).

4.8.13 Concept Hierarchy

Code systems may be presented hierarchically using either nested concept elements or properties. When exclusively using nested concept elements the code system hierarchy is a single tree, and concepts have only one parent. When using properties, polyhierarchies are allowed (e.g. concepts may have more than one parent).

If a code system hierarchy is represented using properties (typically, concepts that are subsumed by more than one other concept), nested concepts SHOULD NOT be used to represent any parts of the hierarchy structure in the Code System resource; the relationships between concepts should be defined using properties.

Operations based on the codeSystem resource SHALL have the same result whether or not the relationships are represented explicitly as properties or implicitly using the CodeSystem resource hierarchy.

4.8.14 Subsumption Testing

The words 'subsume', 'subsumes', 'subsumed' and 'subsumption' are defined in relation to the type of hierarchy (i.e. the value of CodeSystem.hierarchyMeaning) identified for the code system that is being represented by the CodeSystem resource. Concept A is considered to be subsumed by Concept B if it comes under Concept B in the hierarchy, or if a parent/child relationship is declared in the properties, and the hierarchyMeaning is "is-a").

Where a CodeSystem does not declare its hierarchy meaning directly, then the code system documentation must be consulted manually to determine how subsumption is determined. If there is no definition, none of the subsumption based features can be used with the code system.

Subsumption based logic arises explicitly or implicitly in the following places in the FHIR specification:

4.8.15 Filters

The following filters are defined for all code systems:

Property Name Operation Value Definition Notes
code regex [string] Matches any concept with the code matching the supplied string The regex flavor is determined by the server
[property] = [string] Includes all codes that have a property value equal to the specified string, where [property] is the code for any defined property. Where the type of the property is on of integer, dateTime or decimal, the FHIR search prefixes and semantics apply (see below)
[property] in [string,string...] Includes all codes that have a property value equal to one of the specified strings, where [property] is the code for any defined property The values cannot include ",", since it is being used as a delimiter
[property] regex [string] Includes all codes that have a property value matching the specified regex
[property] exists true or false When value is "true", includes all codes that have the specified property. When value is "false", includes all codes that lack the specified property
designation = [string] Matches any concept with a designation equal to the supplied string Designations considered include the concept.display, which counts as a designation
designation regex [string] Matches any concept with a designation matching the supplied string Designations considered include the concept.display, which counts as a designation

The following filters are defined for all code systems that support "is-a" relationships between concepts. (The filters can be used regardless of whether the "is-a" relationships are conveyed as concept properties or as concept nesting):

Property Name Operation Value Definition Notes
concept is-a [string] concept code Includes all concept ids that have a transitive is-a relationship with the concept Id provided as the value, including the provided concept itself (include descendant codes and self).
concept descendent-of [string] concept code Includes all concept ids that have a transitive is-a relationship with the concept Id provided as the value, excluding the provided concept itself (i.e. include descendant codes only).
concept is-not-a [string] concept code The specified property of the code does not have an is-a relationship with the provided value.
concept generalizes [string] concept code Includes all concept ids that have a transitive is-a relationship from the concept Id provided as the value, including the provided concept itself (i.e. include ancestor codes and self).
concept child-of [string] concept code Only concepts with a direct hierarchical relationship to the index code and no other concepts. This does not include the index code in the output.
concept descendent-leaf [string] concept code Includes concept ids that have a transitive is-a relationship with the concept Id provided as the value, but which do not have any concept ids with transitive is-a relationships with themselves.

4.8.15.1 FHIR Search Semantics

If the = filter is used with a property that is defined on the CodeSystem, and if the property has a type of integer, dateTime or decimal, then the FHIR search rules apply. This means that the following prefixes may be used in the value of the filter:

  • eq: the code has a value for the property on the code and it is equal to the provided value
  • ne: the code has a value for the property on the code and it is not equal to the provided value
  • gt: the code has a value for the property on the code and it is greater than the provided value
  • lt: the code has a value for the property on the code and it is less than the provided value
  • ge: the code has a value for the property on the code and it is greater or equal to the provided value
  • le: the code has a value for the property on the code and it is less or equal to the provided value
  • sa: the code has a value for the property on the code and it is starts after the provided value
  • eb: the code has a value for the property on the code and it is ends before the provided value
  • ap: the code has a value for the property on the code and it is approximately the same to the provided value

The semantics of prefixes are as explained on the search page with regard to precision and range considerations, particularly on dates and decimals. Note that eq is not redundant - in it's absence, the semantics are for a direct string match, not a smart match as defined by the search semantics.

Some Examples: 

{ "property": "retiredDate", "operator": "=", "value": "lt2021-01-01" }

Any code that was retired before 2021. Regarding precision: 

{ "property": "retiredDate", "operator": "=", "value": "eq2022" }

would match a concept with a retiredDate of 2022-09-12. However: 

{ "property": "retiredDate", "operator": "=", "value": "2022" }

would not match a concept with a retiredDate of 2022-09-12.

4.8.16 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 A code defined in the code system CodeSystem.concept.code
content-mode token not-present | example | fragment | complete | supplement CodeSystem.content
context token A use context assigned to the code system (CodeSystem.useContext.value.ofType(CodeableConcept)) 30 Resources
context-quantity quantity A quantity- or range-valued use context assigned to the code system (CodeSystem.useContext.value.ofType(Quantity)) | (CodeSystem.useContext.value.ofType(Range)) 30 Resources
context-type TU token A type of use context assigned to the code system CodeSystem.useContext.code 30 Resources
context-type-quantity TU composite A use context type and quantity- or range-based value assigned to the code system On CodeSystem.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 code system On CodeSystem.useContext:
  context-type: code
  context: value.ofType(CodeableConcept)		 30 Resources
date date The code system publication date CodeSystem.date 31 Resources
derived-from TU reference A resource that the CodeSystem is derived from CodeSystem.relatedArtifact.where(type='derived-from').resource
(Any)		 10 Resources
description string The description of the code system CodeSystem.description 29 Resources
effective TU date The time during which the CodeSystem is intended to be in use CodeSystem.effectivePeriod 12 Resources
identifier token External identifier for the code system CodeSystem.identifier 35 Resources
jurisdiction token Intended jurisdiction for the code system CodeSystem.jurisdiction 27 Resources
language token A language in which a designation is provided CodeSystem.concept.designation.language
name string Computationally friendly name of the code system CodeSystem.name 28 Resources
predecessor TU reference The predecessor of the CodeSystem CodeSystem.relatedArtifact.where(type='predecessor').resource
(Any)		 10 Resources
publisher string Name of the publisher of the code system CodeSystem.publisher 31 Resources
status token The current status of the code system CodeSystem.status 35 Resources
supplements TU reference Find code system supplements for the referenced code system CodeSystem.supplements
(CodeSystem)
system uri The system for any codes defined by this code system (same as 'url') CodeSystem.url
title string The human-friendly name of the code system CodeSystem.title 28 Resources
topic TU token Topics associated with the CodeSystem CodeSystem.topic 10 Resources
url uri The uri that identifies the code system CodeSystem.url 34 Resources
version token The business version of the code system CodeSystem.version 32 Resources