Terminology Fundamentals
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Terminology Fundamentals, published by HL7 International / Terminology Infrastructure. This guide is not an authorized publication; it is the continuous build for version 1.0.0-ballot built by the FHIR (HL7® FHIR® Standard) CI Build. This version is based on the current content of https://github.com/HL7/terminology-fundamentals-ig/ and changes regularly. See the Directory of published versions

Identification of Terminology Artifacts

Page standards status: Informative

One of the founding principles of terminology is the importance of the correct identification of artifacts.

Global Uniqueness of Terminology Identifiers

For HL7 purposes, all identifiers for terminology artifacts must be unique within their namespaces to avoid misidentification.

Globally unique identifiers may be created as either Uniform Resource Identifiers (URIs — see RFC 8820) or Object Identifiers (OIDs — see ITU-T X.660 or ISO/IEC 9834-3). OIDs are globally unique if the OID registration procedures defined by ISO in the 9834 series of standards are followed. A set of local identifiers may be made globally unique by prefixing them with a common global identifier. OIDs are created by an ISO Registration Authority, as specified in the ISO standards. HL7 is such a Registration Authority, and creates and issues OIDs for use in identifying objects in HL7 standards and implementations.

HL7 OID Registry

An OID should be globally unique, and it must also be consistently used within a context - in this case - HL7 standards. Terminology artifacts must be consistently used for Identifier or Code Systems such as Social Security Number in the USA, or HL7-defined artifacts, ISO standards, and ICD and SNOMED CT terminologies. If identifiers are not used consistently, then interoperability is degraded because communicating systems would have no way to realize when they are referencing the same or different things.

One way to produce common consistent identification is to maintain a central system where these identification concepts are registered. HL7 maintains an OID registry for this purpose. OIDs of interest to HL7 implementers may be registered on the HL7 OID registry, which includes

  • OIDs created and issued by HL7 that refer to artifacts defined by HL7 and maintained internally,
  • OIDs created and issued by HL7 that refer to externally defined artifacts, and
  • OIDs created and issued by Registration Authorities other than HL7 that refer to externally defined artifacts.

The HL7 OID registry is not limited to terminology artifacts. 

One of the primary objectives of the HL7 OID registry is to ensure that there is only one authoritative OID to be used for a given artifact in HL7 instances. For example, there is only one OID registered for the LOINC code system in the HL7 OID registry. While any organization that is an ISO Registration Authority and manages an OID root may create their own OID for the LOINC code system, these additional OIDs cannot be added to the HL7 OID registry because they would result in multiple OIDs for the same artifact. Furthermore, if an OID has been registered with HL7 for a given Code System or Identifier System, that OID shall be used when referencing that Code System or Identifier System in conformant HL7 instances. This ensures that HL7 conformant systems can rely on Code Systems and common public identifiers being identified consistently.

For more information about HL7 assigned OIDs, please visit the HL7 OID registry.

HL7 best practice supports assigning an OID to each of its Code Systems, as well as to external standard Code Systems that are being used with HL7 International and HL7 Affiliate specifications.

There is an FAQ with more information about the use of the HL7 OID registry that may be found at OID Registry Frequently Asked Questions.

HL7 Terminology (THO)

An additional resource for terminology identifiers within HL7 is the HL7 Terminology website https://terminology.hl7.org/ (THO). THO provides web-based access to terminology artifacts referenced by HL7 published artifacts in a convenient, browsable form.

THO includes content that is both internal and external to HL7. Internal content refers to terminology artifacts that are published by HL7. These include artifacts such as Code Systems and Value Sets that have been published to support HL7 specifications. Internal THO content includes both the identifiers and the complete content for Value Sets and Code Systems.

External content refers to terminology artifacts that are published and maintained outside of HL7. THO includes all known identifiers and pertinent metadata for Code Systems and Identifier Systems maintained outside of HL7. If an agreement has been made with HL7 to republish external content, THO includes all known identifiers, pertinent metadata, and the complete content.  The Terminology Services Management Group maintains the external content information and provides guidance for Validating and Requesting Identifiers for External Code Systems and Identifier Systems.

Note that there is significant overlap between the identifiers being published via the HL7 OID registry and THO.