R4 Ballot #1 (Mixed Normative/Trial use) Release 5 Preview #2
This page is part of the FHIR Specification (v3.3.0: R4 Ballot 2). The current version which supercedes this version is 4.0.1 . For a full list of available versions, see the Directory of published versions
Vocabulary Work Group   Maturity Level : 5 N Normative (from v4.0.0) Compartments : N/A
Normative Candidate Note:
This page is candidate normative content for R4 in has been approved as part of an ANSI standard. See the Conformance Package . Once normative, it will lose it's Maturity Level, and breaking changes will no longer be made. for further details.

Given a code/system, or a Coding, get additional details about the concept, including definition, status, designations, and properties. One of the products of this operation is a full decomposition of a code from a structure terminology structured terminology.

When invoking this operation, a client SHALL provide both a system and a code, either using the system+code parameters, or in the coding parameter. Other parameters are optional.

The official URL for this operation definition is


Formal Definition (as a OperationDefinition ).

URL: [base]/CodeSystem/$lookup

This is an idempotent operation

In Parameters:
Name Cardinality Type Binding Profile Documentation
code 0..1 code

The code that is to be located. If a code is provided, a system must be provided

system 0..1 uri

The system for the code that is to be located

version 0..1 string

The version of the system, if one was provided in the source data

coding 0..1 Coding

A coding to look up

date 0..1 dateTime

The date for which the information should be returned. Normally, this is the current conditions (which is the default value) but under some circumstances, systems need to acccess this information as it would have been in the past. A typical example of this would be where code selection is constrained to the set of codes that were available when the patient was treated, not when the record is being edited. Note that which date is appropriate is a matter for implementation policy.

displayLanguage 0..1 code

The requested language for display (see ExpansionProfile.displayLanguage) $expand.displayLanguage)

property 0..* code

A property that the client wishes to be returned in the output. If no properties are specified, the server chooses what to return. The following properties are defined for all code systems: url, name, version (code system info) and code information: display, definition, designation, parent and child, and for designations, lang.X where X is a designation language code. Some of the properties are returned explicit in named parameters (when the names match), and the rest (except for lang.X) in the property parameter group

Out Parameters:
Name Cardinality Type Binding Profile Documentation
name 1..1 string

A display name for the code system

version 0..1 string

The version that these details are based on

display 1..1 string

The preferred display for this concept

designation 0..*

Additional representations for this concept

designation.language 0..1 code

The language this designation is defined for

designation.use 0..1 Coding

A code that details how this designation would be used

designation.value 1..1 string

The text value for this designation

property 0..*

One or more properties that contain additional information about the code, including status. For complex terminologies (e.g. SNOMED CT, LOINC, medications), these properties serve to decompose the code

property.code 1..1 code

Identifies the property returned

property.value 0..1 code | Coding | string | integer | boolean | dateTime | decimal

The value of the property returned

property.description 0..1 string

Human Readable representation of the property value (e.g. display for a code)

property.subproperty 0..*

Nested Properties (mainly used for SNOMED CT decomposition, for relationship Groups)

property.subproperty.code 1..1 code

Identifies the sub-property returned

property.subproperty.value 1..1 code | Coding | string | integer | boolean | dateTime | decimal

The value of the sub-property returned

property.subproperty.description 0..1 string

Human Readable representation of the property value (e.g. display for a code)

Note that the $lookup operation is more than just a code system search - the server finds the concept, and gathers the return information from the underlying code system definitions.

Request: Looking up a code/system (Request):

GET [base]/CodeSystem/$lookup?system=http://loinc.org&code=1963-8

Request: Lookup using a Coding (Request):

POST [base]/CodeSystem/$lookup
[other headers]
<Parameters xmlns="http://hl7.org/fhir">
    <name value="coding"/>
    <system value="http://loinc.org"/>
    <code value="1963-8"/>

Response: When the request can be processed ok

HTTP/1.1 200 OK

HTTP/1.1 200 OK

[other headers]
  "resourceType" : "Parameters",
  "parameter" : [
    "name" : "name",
    "valueString" : "LOINC"
    "name" : "version",
    "valueString" : "2.48"
    "name" : "designation",

    "name" : "display",

    "valueString" : "Bicarbonate [Moles/volume] in Serum"
    "name" : "abstract",
    "valueString" : "false"
      "name" : "designation",
    "part" : [
      "name" : "value",
      "valueString" : "Bicarbonate [Moles/volume] in Serum "

      "valueString" : "Bicarbonate [Moles/volume] in Serum"


Response: An error - e.g. the requested code is not known

HTTP/1.1 400 Bad Request
[other headers]
  "resourceType": "OperationOutcome",
  "id": "exception",
  "text": {
    "status": "additional",
    "div": "<div xmlns=\"http://www.w3.org/1999/xhtml\">Code "ABC-23" not found</div>"
  "issue": [
      "severity": "error",
      "code": "not-found",
      "details": {
        "text": "Code "ABC-23" not found"


For more information about operations, including how they are invoked, see Operations .