FHIR Terminology Service

This page is a draft that is intended to be incorporated into the FHIR specification for DSTU 2

Introduction

The FHIR specification includes support for the provision of a consumer level terminology service. The following facilities are provided:

• Editing Value Sets
• Getting a list of codes for UI
• validating that a code is correct
• translating from one code system to another
• maintaining a closure table

This page describes how these services are provided to support both servers and clients. A server that supports all the functionality described here can be described as a "FHIR Terminology Service", and SHALL conform to this conformance statement - [link to be provided].

Security

For the operational services, OAuth is generally not an appropriate form of security for an infrastructural service like a terminology server. A terminology server may choose not to authenticate the clients in any fashion, but can do so in order to limit or account for usage. Since terminology servers do not directly handle patient information, it is not necessary to protect the communications with SSL, but hackers may be able to infer information about patients to observing the kind of codes that are retrieved, so encryption, possibly with client verification of credentials, may still be a good idea.

For the value set maintenance server, OAuth may be appropriate, or some other login method might be used.

Code Systems and Value Sets

The FHIR specification is based on the Core Principles (link). A brief glossary of terms used:

• code system: defines a set of codes with meanings (also known as enumeration, terminology, classification, and/or ontology)
• value set: selects a set of codes from those defined by one or more code systems

Note: Proper differentiation between a code system and a value set is important. For instance, it's not unusual to see a mixed list of codes containing a set of LOINC codes with some additional in-house codes, with no explicit differentiation between them. In FHIR, each code system that defines codes has a URL, and the codes they define are a pair ("Code Pair") - a name with a namespace. So in this example, there is two code systems: LOINC (http://loinc.org) and a local one (e.g. http://example.com/codesystems/additional-test-codes). Then there's a value set, which says that it includes some codes from each of those two namespaces - a set of Code Pairs. The value set itself gets a URL as an identifier (e.g. http://example.com/fhir/ValueSet/test-codes) - this identifies the set of Code Pairs, but is never used the namespace in a code pair. In FHIR, Code Pairs are always represented as "code" and "system", except for the simple type "code" data type where the namespace (e.g. system) is fixed in the schema and not represented explicitly. The URL in a system element is always a reference to a code system, not to the value set.

ValueSet Resource

FHIR defines a single resource named "ValueSet" which defines a set of Code Pairs. A value set resource can include code pairs defined elsewhere by:

• importing set of code pairs from other value sets
• including codes from code systems by listing (enumerating) the codes (this is known as an extensional definition)
• including sets of codes from code systems based on their properties (this is known as an intensional definition)

In addition, a Value Set resource can define it's own code system. Codes defined in this inline code system are also included in the value set using the URL explicitly assigned to the code system in the valueset.

The Value Set resource is the fundamental building block for a terminology server - it manages a repository of value sets, allows selected users to edit them, and provides services based on the definitions in the value sets.

External Code Systems

In order to be used with a value set, code must be defined somewhere. They can be defined as part of an inline code system definition, or they can be defined elsewhere, and then used in a value set by quoting the correct namespace. The FHIR specification defines a set of namespaces (link) for commonly encountered code systems, and defines how some work with FHIR (links, SNomed, LOINC, RxNorm). These code systems are often large, and have many internally defined properties that are part of their formal definitions. Inline code systems are not appropriate for these code systems; FHIR does not provide an formal representation at all. Instead, it is assumed that these are externally known.

Most useful terminology servers will make one or more of these external code systems available for use within the value sets that they manage. The list of additional terminologies that a terminology supports beyond those defined in it's value sets is published to clients by (still to figure out how this works).

Implementation Note

When a terminology server exposes an external code system, it makes a set of services available internally that serve the operational interfaces below. The internal server depends on the following logical statements for a terminology:

• it's URL (namespace, and how versioning works)
• what codes are valid
• what properties can be used to select codes
• what implicit value sets exist

The FHIR specification itself defines these things for (links, SNomed, LOINC, RxNorm).

Note: A terminology service may choose to expose additional external code system specific related functionality such as exploration, or structured search, but these services are outside the scope of the FHIR terminology service.