This page describes how to use LOINC with HL7 Standards.

Using LOINC with FHIR R5

Summary

Source	LOINC is made available by the Regenstrief Institute at http://loinc.org
System	The URI http://loinc.org identifies LOINC codes
Version	Where a version is used, it should be the standard LOINC version e.g. 2.48
Code	The LOINC Code Identifier e.g. 21176-3. LOINC codes are not case sensitive. In addition, LOINC part codes and LOINC answer string ids can be used where appropriate
Display	Use either the SHORTNAME or LONG_COMMON_NAME field for the display
Inactive	Codes with Property STATUS = DEPRECATED are considered inactive for use in ValueSet.compose.inactive
Subsumption	LOINC defines the Multi-Axial Hierarchy, which is the basis for subsumption logic in LOINC
Filter Properties	Several properties are defined as described below

Copyright Issues

The terms of use for LOINC require that a notice be included with any use of LOINC codes. This notice must appear in the copyright element of any value set that includes LOINC codes (either in the code or filter elements, or in an expansion):

<copyright value="This material contains content from LOINC (http://loinc.org). 
Refer to terminology.hl7.org for copyright information. 
LOINC® is available at no cost under the license at http://loinc.org/license. LOINC® is a registered United States trademark of Regenstrief Institute, Inc."/>

Additional copyright statements may also be found in the copyright element. Some LOINC codes have 3rd party copyright statements. When these codes are included in a value set, they must carry their own copyright statement as well.

LOINC FHIR Terminology Server

Regenstrief Institute provides access to LOINC's terminology content through the HL7® FHIR® API. See the LOINC FHIR Terminology Server page for details.

Case Sensitivity

For comparison purposes, LOINC codes, displays, and property values are not case sensitive, though implementers SHOULD maintain the correct case when using LOINC codes and property values.

Use of LOINC PARTS

As described in the LOINC Manual section 10.2, LOINC Parts are a coded representation of a value for a dimension used to specify a LOINC Term which are assigned a non-semantic identifier with a "LP" prefix and a mod-10 check digit. Following the LOINC license, these part codes may be used in the following ways:

• In filter properties, as described below
• In Structure Definitions, where the structure describes the use of a set of LOINC codes
• In a ConceptMap resource, where mappings between LOINC codes and other codes are being defined

Part codes are the same LOINC system (http://loinc.org) and SHALL be represented in uppercase (e.g. LP31755-9).

Use of LOINC Answer Lists

LOINC also allocates Answer List and Answer String Ids for use in various forms and questionnaires. LOINC Answer String IDs are also valid LOINC codes:

  <coding>
    <system value="http://loinc.org"/>
    <code value="LA11165-0"/>
    <display value="Platelet anisocytosis"/>
  </coding>

LOINC Answer List Ids are actually value set identifiers. See below for how to use these.

RDF

LOINC uses the namespace http://loinc.org/rdf# as the root for LOINC concepts in the RDF space. This means that when a LOINC code is converted from the system:code pair, where the system is http://loinc.org, to the RDF ontological form, the representation is http://loinc.org/rdf#[code].

LOINC Properties

In addition to the standard properties, the following LOINC table fields are defined as code system properties when using LOINC in FHIR:

STATUS	string	Status of the term
COMPONENT	code	First major axis-component or analyte: Analyte Name, Analyte sub-class, Challenge
PROPERTY	code	Second major axis-property observed: Kind of Property (also called kind of quantity)
TIME_ASPCT	code	Third major axis-timing of the measurement: Time Aspect (Point or moment in time vs. time interval)
SYSTEM	code	Fourth major axis-type of specimen or system: System (Sample) Type
SCALE_TYP	code	Fifth major axis-scale of measurement: Type of Scale
METHOD_TYP	code	Sixth major axis-method of measurement: Type of Method
CLASS	string	An arbitrary classification of the terms for grouping related observations together
CONSUMER_NAME	string	An experimental (beta) consumer friendly name for this item. The intent is to provide a test name that health care consumers will recognize; it will be similar to the names that might appear on a lab report
CLASSTYPE	string	1=Laboratory class; 2=Clinical class; 3=Claims attachments; 4=Surveys
ORDER_OBS	string	Provides users with an idea of the intended use of the term by categorizing it as an order only, observation only, or both
DOCUMENT_SECTION	string	Classification of whether this LOINC code can be used a full document, a section of a document, or both

Note that when a $lookup operation is performed on a LOINC code, servers SHALL return the version being used (see above) in the version property. Other properties are at the discretion of the server and the client.

LOINC Filters

This section documents the property filters that can be used with the LOINC code system in value set composition statements.

LOINC Property filter

Description	Allows the selection of a set of LOINC codes with a common property value (see list above)
Property Name	One of the names listed in the "Field Name" column in LOINC Database Structure (Appendix A of the LOINC manual)
Operations Allowed	= / regex
Values Allowed	[string value]
Comments	The 6 properties COMPONENT, PROPERTY, TIME_ASPCT, SYSTEM, SCALE_TYP, and METHOD_TYP are most likely to be useful.

3rd Party Copyright

Description	Allows for the inclusion or exclusion of LOINC codes that include 3rd party copyright notices
Property Name	copyright
Operations Allowed	=
Values Allowed	LOINC | 3rdParty
Comments	LOINC = only codes with a sole copyright by Regenstrief. 3rdParty = only codes with a 3rd party copyright in addition to the one from Regenstrief.

Multi-Axial Hierarchy

Description	Allows for the selection of a set of codes base on their appearance in the LOINC multi-axial hierarchy
Property Name	parent | ancestor
Operations Allowed	= / in
Values Allowed	Part Code (or, for "in", multiple part codes separated by commas)
Comments	"parent" selects immediate parents only. For example, the code "44022-2" has the parent "LP52960-9". Ancestor includes parents transitively, e.g. "LP52960-9" eventually has a parent "LP31755-9", so the code "44022-2" is in the set of codes that have ancestor=LP31755-9

TODO: Document Ontology

LOINC Implicit Value Sets

Implicit value sets are those whose specification can be predicted based on the grammar of the underlying code system, and the known structure of the URL that refers to them. LOINC defines one set of implicit value sets: By Multi-Axial Hierarchy Entry.

If any value set resources exist with an identifier that conforms to the URL patterns specified below, the content of the resource must conform to the template provided. Profiles and other value set references are allowed to reference these value sets directly.

The value set identifier http://loinc.org/vs is a value set that contains all LOINC codes.

LOINC Answer Lists

LOINC defines a set of Answer lists, each of which contains a set of LOINC codes. LOINC answer lists are value sets. The value set canonical uri "http://loinc.org/vs/[id]" identifies a value set that contains a set of LOINC codes. For instance, the value set uri http://loinc.org/vs/LL715-4 has the following definition for LOINC 2.52:

<ValueSet xmlns="http://hl7.org/fhir">
  <text>
    <status value="generated"/>
    <div xmlns="http://www.w3.org/1999/xhtml">
      [some html that identifies that this value set
      includes all LOINC codes in this answer list]
    </div>
  </text>
  <url value="http://loinc.org/vs/LL715-4"/>
  <version value="2.52"/>
  <name value="Platelet morph"/>
  <description value="LOINC Answer List for Platelet morph"/>
  <status value="active"/>
  <date value="[optional date of LOINC release]"/>
  <compose>
    <include>
      <system value="http://loinc.org"/>
      <concept>
        <code value="LA11165-0"/>
        <display value="Platelet anisocytosis"/>
      </concept>
      <concept>
        <code value="LA11168-4"/>
        <display value="Platelet clump"/>
      </concept>
      <concept>
        <code value="LA11167-6"/>
        <display value="Platelet large fragments"/>
      </concept>
      <concept>
        <code value="LA11166-8"/>
        <display value="Platelet satellitism"/>
      </concept>
      <concept>
        <code value="LA11169-2"/>
        <display value="Platelets.agranular"/>
      </concept>
      <concept>
        <code value="LA11170-0"/>
        <display value="Platelets.giant"/>
      </concept>
      <concept>
        <code value="LA11172-6"/>
        <display value="Platelets.large"/>
      </concept>
      <concept>
        <code value="LA11171-8"/>
        <display value="Platelets.small"/>
      </concept>
    </include>
  </compose>
</ValueSet>

Here is an example of a LOINC Answer list used in a Questionnaire question:

<item>
  <code>
    <system value="http://loinc.org"/>
    <code value="11125-2"/>
    <display value="Plat morph Bld"/>
  </code>
  <type value="Coding"/>
  <answerValueset>
    <reference value="http://loinc.org/vs/LL715-4"/>
  </answerValueset>
</item>

Multi-Axial Hierarchy Entry

A value set with an identifier of "http://loinc.org/vs/[partcode]" must conform to this template, where [partcode] is a part code from the multi-axial hierarchy:

<ValueSet xmlns="http://hl7.org/fhir">
  <text>
    <status value="generated"/>
    <div xmlns="http://www.w3.org/1999/xhtml">
      [some html that identifies that this value set
      includes all LOINC codes subsumed by the identified
      Multi-Axial Hierarchy Part Code]
    </div>
  </text>
  <url value="http://loinc.org/vs/[partcode]"/>
  <version value="[optional - but strongly recommended - LOINC version]"/>
  <name value="LOINC Value Set from Multi-Axial Hierarchy code [partcode]"/>
  <description value="All LOINC codes for [partcode or name]"/>
  <status value="active"/>
  <date value="[optional date of LOINC release]"/>
  <compose>
    <include>
      <system value="http://loinc.org"/>
      <filter>
        <property value="ancestor"/>
        <op value="="/>
        <value value="[partcode]"/>
      </filter>
    </include>
  </compose>
</ValueSet>

LOINC and Data Elements

LOINC codes and their properties are describing a data element that can have a value. Functionally, this overlaps with FHIR's notion of a Logical Data Model. A logical data model can be created for each LOINC code based on it's properties. The representation of each logical model is aligned with LOINC's RDF representation.

<StructureDefinition xmlns="http://hl7.org/fhir"> 
  <id value="[LOINC code]"/>
  <url value="http://loinc.org/owl#[LOINC code]"/>
  <version value="{LOINC version number]"/>  <!-- though this is not required -->
  <name value="[display - adjusted]"/> <!-- converted to meet regex by removing spaces, non-supported characters -->
  <title value="[display]"/>
  <status value="[derived from STATUS]"/>
  <experimental value="false"/>
  <date value="[last changed date]"/>
  <publisher value="http://loinc.org"/>
  <copyright value="This content LOINC® is copyright © 1995 Regenstrief Institute, Inc.
      and the LOINC Committee, and available at no cost under the license at 
      http://loinc.org/terms-of-use"/>
  <differential>
    <element>
      <path value="[LOINC code]"/>
      <short value="[display]"/>
      <definition value="[definition]"/>
      <min value="0"/>
      <max value="1"/>
      <type>
        <code value="[mapped type - see below]"/>
      </type>
      <!-- if this is a LOINC question with linked answers -->
      <binding>
        <strength value="required"/>
        <valueSet value="http://loinc.org/vs/[linked answer list code]"/>
      </binding>
    </element>
  </differential>
</StructureDefinition>

The canonical URL http://loinc.org/rdf#[LOINC code] can be used to refer to logical model elements defined by LOINC such as e.g. in Questionnaire.item.definition.

The appropriate type is determined by mapping from the LOINC specified SCALE_TYP for v3 to FHIR using this table:

SCALE_TYP	FHIR data type
Qn	Quantity
Ord	CodeableConcept
OrdQn	Quantity and CodeableConcept (e.g. both in the data element; either is allowed)
Nom	CodeableConcept
Nar	markdown
Multi	Attachment
Doc	Attachment