Release 5
This page is part of the FHIR Specification (v5.0.0: R5 - STU). This is the current published version. For a full list of available versions, see the Directory of published versions . Page versions: R5 R4B R4 R3 R2
| Terminology Infrastructure | Maturity Level: Normative | Standards Status: Normative |
Many elements in the FHIR resources have a coded value: some fixed string (a sequence of characters) assigned elsewhere that identifies some defined "concept". The sequence of characters and its meaning may be defined in one of several places:
These methods of defining codes are collectively called "code systems". This list is far from complete; there are many ways to define code systems, and they vary widely in sophistication and size.
Throughout this specification, coded values are always treated as a pair
composed of "system" and "code", where the system is a URL that identifies the
code system that defines the codes. Note that system values are always
case sensitive. Different code systems make their own rules as to whether the codes
they define are case sensitive or not. Note that all the codes defined by FHIR
itself are case sensitive and SHALL be used in the provided case (usually, but
not always, lowercase).
The FHIR framework for using coded values is based on the fundamental framework defined in section 5 of the
HL7 v3 Core Principles document,
including the separation between code systems and value sets.
The general pattern for representing coded values is using the following four elements:
| system | A URI that identifies the system (see below) |
| version | A string value representing a version of the original code system (see Code System Versioning) |
| code | A string value that identifies a concept as defined by the code system |
| display | A description of the concept as defined by the code system |
The Coding datatype represents this pattern. This example shows a LOINC code with the LOINC system, the version of LOINC used for the definition, and the display assigned by LOINC:
{
"system" : "http://loinc.org",
"version" : "2.62",
"code" : "55423-8",
"display" : "Number of steps in unspecified time Pedometer"
}
| When codes are carried in resources, one of several different datatypes is used: | |
| code | The instance represents the code only. The system is implicit - it is defined as part of the definition of the element, and not carried in the instance. |
| Coding | A datatype that has a code and a system element that identifies where the definition of the code comes from |
| CodeableConcept | A type that represents a concept by plain text and/or one or more coding elements (See the datatype notes for a discussion of code translations and using text in CodeableConcept) |
| CodeableReference | A type that can have either a reference to another resource, or a to a concept using a CodeableConcept |
| In addition, the following datatypes also carry coded values, or content that can be treated as a code and bound to a valueset: | |
| Quantity | The instance has system and code elements for carrying a code for the type of unit, and these can be bound to a value set |
| string | The instance carries a string. In some cases, applications may wish to control the set of valid strings for a particular element, so the string value can be treated as a coded element (like code) |
| uri | Like string, URIs can be treated as a coded element |
Notes:
The set of coded values that is allowed in an element is known as a "value set". Anywhere these datatypes are used, the specification "binds"> a value set to the element, and for the types code, Coding, and CodeableConcept, always does.
The difference between a code system and a value set is an important distinction that is easily missed by implementers, since the difference is often overlooked in system design. For instance, it's not unusual to see an application table that is a mixed list of codes, containing some LOINC codes and also some additional in-house codes. Quite often, there is no explicit differentiation between them; only the fact that a code happens to look like a LOINC code betrays its origin.
The code system identifier of the concept is necessary for interoperable data exchange. Each code system is assigned a uri. The combination of the code system identifier (uri) and the concept identifier (e.g. code) is unique. Note that code system version may be necessary to achieve uniqueness when the code system does not follow concept permanence rules.
In the case of the mixed list example from the previous paragraph, there are two code systems: LOINC (http://loinc.org)
and a local one with the uri: http://example.com/codesystems/additional-test-codes.
The value set definition includes concept identifier (codes) from each of the two code systems.
ValueSet.url (e.g. "http://example.com/fhir/ValueSet/test-codes") is the canonical identifier for the value set.
The value set uri is not the same as the code system uri. Concepts are always identified by
code and system,
except for the simple datatype code where the code system is fixed in a specification
(e.g. core specification, implementation guide, profile) and not represented explicitly.
Note that for some code systems, there is a single correct mechanism by which to represent codes defined by the system as a single URL. These single URLs are used in the context of the RDF format to enable ontological reasoning. The URL is often a direct reference to a web source that can provide additional definitional material about the concept. Where the mechanism is known, and defined by the code system, it is described in this specification.
The URL in a system is always a reference to a code system,
not to a value set. The system ensures that codes can be unambiguously
traced back to their original definition, and that logical comparisons, matching and
inferences can be performed consistently by different systems. For this reason, choice
of the correct URI for the system attribute is critical.
The correct value to use in the system for a given code system can be determined by working through the following list, in order:
In the unusual situation where a code system is not resolved by this list, create a temporary identifier following this pattern: terminology.hl7.org/temporary/CodeSystem/xxxx. Contact the HL7 Vocabulary co-chairs.
For publishers of code systems, the following considerations should be kept in mind when defining the correct URI to use:
Note: if the code system is made available packaged inside a ValueSet resource, the
correct URL for the system value is ValueSet.codeSystem.system, not ValueSet.uri.
All code systems define a set of concepts, assign specific codes to them, and provide definitional material to guide implementers in the correct use and understanding of the codes. Many code systems define relationships between the different concepts - is-a, part-of, classifies-with, and many other relationships. These features are represented in the CodeSystem resource, and exchanged using one of the code datatypes described above.
Some code systems define rules for how complex expressions can be built using the basic concepts defined by the code system. This is sometimes referred to as "post-coordination". Some of the more notable code systems that define grammars for expressions are:
There are many others. Any expression defined by the code system is still regarded as a 'code' and represented as such.
This example shows a SNOMED CT expression:
{
"system" : "http://snomed.info/sct",
"code" : "128045006:{363698007=56459004}"
}
No display is provided in this example. See the discussion here: https://confluence.ihtsdotools.org/x/UwbJAQ
When an element is bound to a value set, it has a binding that has these properties:
| Name | A descriptive name used when presenting information about the binding |
| Strength | How the binding should be understood - see below |
| Reference | A URL that defines the value set. Usually, this is a direct reference to a ValueSet resource, but can be a more indirect reference, where the value set is inferred |
| Description | A text description of the use of the codes. If there is no reference, this must be populated. When there is a reference, this can be used to make additional notes about the use and implementation of the value set |
A binding is always represented using an ElementDefinition.binding.
Additional Bindings and the element ElementDefinition.binding.additional
is considered Trial Use for FHIR Release 5.
In addition, elements may have additional bindings. These additional bindings do not replace the main binding, but provide additional information about the use of codes in the element. Additional bindings have the following properties:
| purpose |
The use of this additional binding:
|
| valueSet | A canonical URL that refers to the value set for the additional binding |
| documentation | Markdown that describes the use of this value set in this element, given the purpose and usage |
| shortDoco | A short plain text string, typically just a sentence, that summarizes the documentation, for use in the tabular formal of a profile |
| usage | A set of qualifiers that restrict the scope of use for the additional binding. Typically, the binding is restricted by jurisdiction/realm, but it may also be restricted to particular clinical or workflow contexts. This specification does not detail exactly how implementations determine when usage criteria apply, so this is a subject that should be addressed in implementation guides when usages are specified |
| any |
If an element repeats, the main binding applies to all the repeats equally. By default, the same
is true for an additional binding, but setting any = true means that the additional binding
can be met by any one of the repeats. This allows multiple bindings for different repeats without the
overhead of slicing. Setting any to true only really makes sense for additional bindings with
purpose = conformance
|
There are a number of places in the specification where value sets are referenced in order to bind a coded value to a value set:
| ElementDefinition.binding.valueSet | Used to bind a defined element to a value set, and also ElementDefinition.binding.additional.valueSet |
| ConceptMap.source[x] and .target[x] | Used to indicate the scope of the mapping in the Concept Map - from one value set to another |
| Questionnaire.item.answerValueSet | Indicates that answers to a set of questions come from a value set |
| ValueSet.compose.include.valueSet | The content of a value set includes the content in the imported value set too |
| OperationDefinition.parameter.binding.valueSet | Used to bind a defined parameter to a value set |
| ValueSet Reference Extension | Indicates that a coded value was chosen from the specified value set |
When referencing value sets, the reference is usually made to the definition of a value set - that is, a value set that defines what codes are in the value set. A terminology server is required to convert this definition to the actual expansion that specifies what codes are in the value set in the context of operation.
There are two types of value set references in this list, direct and logical.
A direct value set reference has the type Reference, and refers directly to a ValueSet based on a URL, usually to a terminology server running a FHIR RESTful API. When accessing a value set based on this kind of reference, a system should access the URL directly (after converting a relative reference to an absolute reference according to the local context). If this process fails, the system is unable to resolve the value set and must handle the error appropriately.
Example:
GET fhir/Questionnaire/234
<Questionnaire>
...
<question>
<options>
<reference value="ValueSet/234234"/>
</options>
</question>
....
</Questionnaire>
This specifies that the values for a questionnaire come from the ValueSet with id 234234 on the same FHIR end-point. To resolve this, the system would GET fhir/ValueSet/234234
Typically, a direct reference like this is good for in-process references, in closed or carefully managed eco-systems. In a more general context, these references tend to be fragile over time because web URLs - including RESTful API URLS - are easily reassigned. For this reason, systems are encouraged to use logical value set references.
A logical value set reference has the type uri, where an absolute URI is provided that matches the one in ValueSet.url. The value set URL can - and is preferred to be - a web address that resolves directly to a fixed web address that serves as the authoritative source for that value set. Alternatively, the system can query its terminology server(s) to resolve a value set with that URL as its identity.
Example:
<StructureDefinition>
...
<element>
...
<binding>
...
<valueSet value="http://hl7.org/fhir/ValueSet/clinical-findings"/>
</binding>
...
</element>
....
</StructureDefinition>
This specifies that the element is bound to the value set with a ValueSet.url of
http://hl7.org/fhir/ValueSet/clinical-findings . One way to access this value set is
to try GET http://hl7.org/fhir/ValueSet/clinical-findings - which works, for this value set - http://hl7.org/fhir/ValueSet/clinical-findings
returns the authoritative value set for this URL.
Alternatively, the value set could be resolved using a local terminology server. If that's running a FHIR Terminology Server, then this would work like this:
GET fhir/ValueSet?url=http://hl7.org/fhir/ValueSet/clinical-findings
if the terminology server knows the value set, then it will return the value set. If the URL doesn't resolve to an authoritative value set, and the terminology server(s) don't know the value set, the system is unable to resolve the value set and must handle the error appropriately.
The value set URL is allowed to be a URI such as a UUID (e.g. urn:uuid:c0e0d027-1250-4278-8f44-33a49dc67916).
These value sets can never be accessed directly, and must come from a terminology server. Note
that HL7 Terminology defines many value sets that have a logical URL that is not resolvable
(examples for SNOMED CT , RxNorm
, and LOINC
)
Using a logical reference which is a direct reference to the authoritative value set is the easiest and most reliable approach. However, this requires suitable hosting arrangements, and cannot always be guaranteed, so it is not required.
Version specific Logical References
A value set has a two-part identifier: a url, and a version. Some value sets only ever have a single 'version'; a revision of the value set contents will cause a new url to be assigned. Others, however, maintain the same URL, and change the version. A terminology server may have multiple value sets for the same ValueSet.url with different versions.
To be precise about which version of a value set is being referred to in a value set reference, append the version to the canonical URL with a '|' like this:
<valueSet value="http://hl7.org/fhir/ValueSet/clinical-findings|0.8"/>
This is a version specific reference to a value set. Searching for this on a terminology server would look like this:
GET fhir/ValueSet?url=http://hl7.org/fhir/ValueSet/clinical-findings&version=0.8
Note that if a value set reference does not have a version, and the server finds multiple versions for the value set, the system using the value set should pick the latest version of the value set and use that. Note that this applies to all conformance resources.
Note that as a matter of ongoing development, a few elements that have coded datatypes are not bound to any value set at all. Bindings are to be provided for these elements.
Almost all the elements that have a coded datatype are bound to a value set. The bindings are associated with various degrees of flexibility as to how closely the value set should be followed:
| required | To be conformant, the concept in this element SHALL be from the specified value set. |
| extensible | To be conformant, the concept in this element SHALL be from the specified value set if any of the codes within the value set can apply to the concept being communicated. If the value set does not cover the concept (based on human review), alternate codings (or, data type allowing, text) may be included instead. |
| preferred | Instances are encouraged to draw from the specified codes for interoperability purposes but are not required to do so to be considered conformant. |
| example | Instances are not expected or even encouraged to draw from the specified value set. The value set merely provides examples of the types of concepts intended to be included. |
The precise conformance criteria for 'required' and 'extensible' binding strengths vary by the datatype to which they are applied, as described in the paragraphs below.
Irrespective of the binding strength, when a StructureDefinition is used to describe local usage, it can bind the element to a different value set in order to be much more precise about exactly which coded values can be used for these elements, and/or increase the strength of the binding. There are different rules for this, depending on the binding strength, as discussed below. Generally it is expected that jurisdictions, projects and vendors will work together to choose actual working value sets.
To be conformant, the data element SHALL contain one of the values in the expansion of the bound value set.
If the binding strength is required, the data element SHALL contain one of the values in the bound value set.
Required binding strength is used for elements where the value needs to be strictly controlled so the element can be interpreted with confidence. Required binding strength SHALL be used for elements with type code unless the element has a context-free (i.e. no binding.additional.usage present) additional binding with purpose = maximum. Exceptions to this rule need to be approved by the FHIR-I work group which will document the patterns that qualify as exceptions.
In the base FHIR specification, required binding strength is typically only used with the 'code' datatype. In profiles, it may be used more broadly when there is agreement within a context of use that a specified set of codes are the only ones that can be used.
The following rules apply when required bindings are used with the CodeableConcept datatype:
text can be provided as well, and is always recommended, but is not an acceptable substitute for the required codeIf a required binding is applied to an element with maximum cardinality > 1, the binding applies to all the elements.
Note: when a binding is applied to a CodeableReference, these rules also apply to its concept property.
The following rules apply when required bindings are used with the code datatype outside of a Coding:
When an element is bound to a required value set, derived profiles may state rules on which codes can be used, including removing codes from allowed use, but cannot specify additional codes for these elements.
For examples of Required bindings, see Terminology Binding Examples.
To be conformant, the data element SHALL contain one of the values in the expansion of the bound value set if any of the codes within the value set can apply to the concept being communicated.
Extensible bindings are used when there is consensus at the specification or profiling level about the coded values that should be used, but it is impossible to create a bounded list of codes that are known to cover all use cases, including ones that are yet to arise.
Note that it is the value set binding that is extensible, not the value set itself. In the interests of brevity and clarity, please consider all references to value set in the text below to refer to an evaluation of the content logical definition (CLD) of the extensibly-bound value set unless the reference is explicitly noted to refer to a value set definition. If the value set can be expanded, then a proper expansion is equivalent to the evaluation of the CLD.
If there is no applicable concept in the value set (based on human review), an alternate concept (either system
/code pair, system/version/code triplet, or text) may be
used instead. The alternate concept can have any level of specificity in an is-a hierarchy
(see Condition instance #2).
For clarity, this table summarizes the rules around extensibility for each bindable data type:
| CodeableConcept |
|
| Coding |
|
| Quantity |
|
| uri, string |
|
| code |
|
The same rules apply for the concept property of a CodeableReference.
If an extensible binding is applied to an element with maximum cardinality > 1, the binding applies to all the element repetitions.
For an extensibly-bound element, derived profiles may state rules on which codes can be used, but cannot select new or additional codes for these elements unless no codes with appropriate meanings are found in the extensibly-bound value set in the parent profile. You can only constrain, not relax.
Note that if the valueset-reference extension is being used and the code in the element instance is from outside the extensibly-bound value set, the extension must reference a different value set definition that the code was chosen from (or if no other value set reference is available the extension cannot be used in that instance).
See examples to help explain the difficult but important subject of Extensible bindings.
Instances are encouraged to draw from the specified codes for interoperability purposes but are not required to do so to be considered conformant.
Preferred bindings are used when there is consensus at the specification level about the coded values that are the best to be used, but there is recognition that some implementation contexts are unable to use the recommended codes for a variety of reasons. Applications should consider adopting the preferred value set wherever possible, as these preferred value sets are the most likely to serve interoperability purposes in the future. For these bindings:
For examples of Preferred bindings, see Terminology Binding Examples.
When an element is bound to a preferred value set, derived profiles MAY bind the element to any value set they choose.
Instances are not expected or even encouraged to draw from the specified value set. The value set merely provides examples of the types of concepts intended to be included.
Example bindings are used when an element has a very broad meaning (such as List.code), or there is no consensus over the correct codes to be used. For these bindings:
Some other coded value MAY be used, or (for a CodeableConcept), a text alternative MAY be provided. Example value sets are provided to assist implementers to understand the correct use of an element. Value sets based on code systems such as SNOMED CT that have restrictive license terms will only be used as example bindings in the base FHIR specification, though implementation guides for particular jurisdictions may adopt value sets that require licenses. In addition, well-specified realm-specific ValueSets may also be used as example bindings in the base specification.
For examples of Example bindings, see Terminology Binding Examples.
When an element is bound to an example value set, derived profiles may bind the element to any value set they choose.
A binding strength of 'required' or 'extensible' does not indicate that ALL of the codes in the bound value set will be supported. It constrains the set of codes that are allowed to be shared. If systems flag an element as mustSupport, the minimumValueSet extension SHOULD be used to identify a specific subset (possibly the full set) of the bound codes that must be supported by implementers.
If no MIN value set is specified, then there is no guidance for implementers regarding which code(s) must be supported. However, at least one of the codes MUST be supported.
When binding a ValueSet to an element, values may be limited by declaring a maximum value set in the additional bindings.
A binding may be further constrained to declare a value set for use when the binding strength is 'extensible' or 'preferred'. This value set is the value set from which additional codes can be taken from. This defines a 'required' binding over the top of the 'extensible' binding.
Systems with legacy or external data that was constructed without an awareness of the terminology requirements asserted in a binding may have trouble complying with the expectations of the binding. Both 'required' and 'extensible' bindings impose an expectation of systems to map their existing data to the value set. For 'required' bindings, if no mapping is possible, the element can't be sent (not even with an extension). If the element and its ancestor elements all have a 'min' cardinality of 1 or more, then the system cannot produce a conformant instance. For extensible bindings, if a mapping has been tried and there is no corresponding concept, then the legacy data may be freely sent. However, this still imposes the expectation of performing a mapping, and this might not be possible if new externally sourced data with arbitrary codes is being regularly received. If no mapping has been performed, then the element would need to be omitted, or the instance would not be able to claim conformance to the specification that imposed the binding.
R5 introduces some new binding capabilities through the Additional Binding extension. This extension allows specifications to specify more refined types of bindings. In particular, the 'current' binding allows a specification to differentiate between terminology requirements that apply to data captured by a system from the time it complies with a specification as opposed to historical or externally sourced data. Implementers that find the bindings asserted by specifications are too onerous for external or historical data, but are reasonable for 'current' data, might encourage specification authors to consider making use of this Additional Binding extension to better reflect the expectations of systems.
Servers may support a variety of implementation guides. There is no guarantee that all data they have available on their FHIR interfaces will necessarily comply with all implementation guides. On the other hand, some clients might not be able to safely consume data that does not comply with a given IG. In environments where there will be a need to expose a mixture of IG-conformant and non-IG conformant data, but some consumers will depend on data being conformant to safely consume, then Resource.meta.profile can be used to explicitly flag the instances that conform. Clients can then filter using the _profile search parameter to ensure they only receive data they can safely handle - with the known cost that they won't necessarily have access to all data that exists.
FHIR has defined a Terminology Service specification which sets requirements for systems that support the use of codes, value sets and code systems.
FHIR ®© HL7.org 2011+. FHIR R5 hl7.fhir.core#5.0.0 generated on Sun, Mar 26, 2023 15:26+1100.
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