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4.8 Resource CodeSystem - Content

Terminology Infrastructure Work Group

Maturity 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.

4.8.4 References to this Resource

• Resource References: itself, Coding, ConceptMap, TerminologyCapabilities and ValueSet
• Extension References: ValueSet Supplement
• Profile References: CQL Library, ELM Library, FHIRPath Library, Logic Library and Module Definition Library

4.8.5 Resource Content

 

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 while the codes for "supra-national" regions are from UN Standard country or area codes for statistical use (M49) .
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 )	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

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 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 .

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. See further comments below
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.

Properties do not have to have a uri the formally defines them, but they SHOULD have a uri, and this is particularly important when using properties across multiple code systems. The following guidelines apply:

• By convention, property URIs SHOULD have the form {canonical}#{code} where {canonical} is the CodeSystem URI, and {code} is the code for the property
• The convention {canonical}#{code} is also used in some contexts to refer directly to a concept defined by the CodeSystem, so if there is a concept defined with the same code as the property, it is treated as a 'backing' concept, to allow reasoning and use in value sets
• Properties are explicitly defined, not implied by the existence of a concept with the same code, and the definitions and properties of the concept to not contribute to the definition of the property
• CodeSystems MAY (and should!) use properties defined in other code system. The Concept Properties Code System exists for this purpose, and the properties defined there SHOULD always be used where applicable
• When using a property defined in another code system, a code system SHOULD provide a full definition - code, uri, description, type, and valueSet where applicable
• In such cases, the definition of the property must be consistent with the source CodeSystem. CodeSystems MAY contain additional descriptive text around the use of the property, and MAY narrow the applicable value set if there is one
• If any of these features are not redeclared, applications SHOULD assume that the source CodeSystem definitions apply

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 (deprecated: use alternateCode, and see the discussion below)
http://hl7.org/fhir/concept-properties#alternateCode : code	This property contains the code for another representation of this concept, with the same real-world meaning, in this code system
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
http://hl7.org/fhir/concept-properties#order : decimal	A property that indicates the order of the concept amongst its siblings in the code system e.g. severity. The code system specific purpose of the ordering is defined elsewhere
http://hl7.org/fhir/concept-properties#definition : string	The definition of the concept. This corresponds to CodeSystem.concept.definition and is not used in the definition of CodeSystems but is used when clients request the definition property be included in an expansion

Code System representations SHOULD use these properties where possible and these codes for these properties. Code System representations SHOULD regard the codes for these properties (e.g. status, inactive, notSelectable, etc.) as reserved values in CodeSystem.property.code values and that when used they SHOULD refer to the properties defined in the concept properties code system.

In the absence of CodeSystem.property data elements being provided, when properties with these code values are added to a concept (in CodeSystem.concept.property.code) the intended property SHOULD be assumed to refer to the defined concept properties in the http://hl7.org/fhir/concept-properties code system.

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).

4.8.12.2 Synonyms and Alternate Codes

Many code systems define multiple codes for the same real world concept. Some code systems do this as a matter of policy (e.g. case-sensitive vs case insensitive) while others arise over time as the editorial process unfolds. Some established code systems treat these as synonym codes for the same concept, while others treat them as synonym concepts - duplicated concepts with the same real world meaning, though the definition sometimes differ. Another reason for duplicated concepts is to deal with implied poly-heirarchical concepts in systems that do not support formal poly-heirarchies.

Whatever the reason, in FHIR code systems, all kinds of synonyms are represented the same way: as duplicate concepts with a declared relationship between them. The relationship is declared using the 'alternateCode' property, which identities another concept with the same real-world meaning that the code system defines.

In most cases (but not necessarily all), the concepts related with the alternateCode property will have the same:

• property values (with the exception of alternateCode, and administrative properties)
• designations

The alternateCode property allows CodeSystem resource instances to represent multiple codes for a concept defined in a code system. The correct way to use the alternateCode property for an instance of CodeSystem.concept (A):

• the alternateCode value must be the code associated with a different CodeSystem.concept (B) in the same CodeSystem; and
• B SHALL contain an instance of the alternateCode property with the value of the code for A. I.e. reflexivity is required (see csd-6 above)
• All relationships shall be explicit - relationships (including transitivity and parent/child) must not be assumed.

Note to Balloters: This section is newly introduced in R6 after extensive consideration in committee, and comments are sought.

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:

• CodeSystem $subsumes operation
• CodeSystem $lookup operation
• ConceptMap $closure operation
• Search by subsumption
• ValueSet $expand operation
• ValueSet $validate-code operation

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)	9 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)	9 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