The LOINC Multiaxial Hierarchy is a computer-generated organization of all LOINC codes

The LOINC Multiaxial Hierarchy (MAH) organizes LOINC codes based on multiple axes of each term. LOINC Parts comprise all of the branches in the MAH, and LOINC terms comprise all of the leaf nodes. The MAH is generated using an automated process based on the separate Component hierarchy, which is maintained by hand, as well as the System and Class for each term. These hierarchies are not meant to be pure ontologies, and rather are tools for organizing sets of codes within different domains.

In the RELMA desktop mapping program, the MAH can be used to browse, search, and visualize LOINC terms.

Multiaxial Hierarchy view in RELMA

The MAH was originally created for the Laboratory domain, and over time, specific clinical domains (e.g., Radiology, Document Ontology) were added. The MAH released with LOINC Version 2.61 was a major update that incorporated all LOINC terms (across all domains), and going forward we plan to continue including all LOINC terms in the hierarchy.

The MAH contains four top-level branches to match the four broad types of LOINC codes: Laboratory, Clinical, Attachments, and Survey. All of the domains other than Radiology (which is contained within the Clinical branch) are organized by Class, Component, and finally the System. Radiology terms are organized by System (Region imaged and/or Imaging focus) followed by the Method (Modality).

Each top-level branch (except for Attachments) contains a "NotYetCategorized" node, which includes all terms in a given Class that could not be placed elsewhere in the hierarchy based on an issue with Component placement in the Component tree. Note that none of the Survey Components are included in the Component hierarchy, so all of the Survey terms are in the "SurveyNotYetCategorized" node. Over time, we intend to more fully develop the hierarchies in these areas and reduce the number of LOINC terms in "NotYetCategorized" branches.

File Contents

A description of this artifact's primary file, MultiAxialHierarchy.csv, follows below.


LOINC Multiaxial Hierarchy
Version: 3.5
Released: 2021-08-23
Size: 3 MB (ZIP)

  • Loinc_2.71_MultiAxialHierarchy_3.5
  • LoincReleaseNotes.pdf
  • Loinc_2.71_License_5.2.txt
  • MultiAxialHierarchy.csv
  • MultiAxialHierarchyReadMe.txt

API Access

The hierarchy of all LOINC terms cannot be retrieved from the LOINC API using HL7 FHIR at this time. We are working to make the parent and child(ren) relationships for each Part and the parent Part for each LOINC term available through the FHIR API.

See our documentation for more information on how to use the Terminology Server including example requests.

File Description


This file contains all LOINC terms and uses the following columns to represent them in a hierarchy.

Column Name Description
PATH_TO_ROOT The series of Part numbers delimited by dots (.) that represent the path from the root of the hierarchy to an individual LOINC Part or term.
SEQUENCE The order in which the LOINC Part or term specified in the CODE column appears under its immediate parent (i.e., the Part in the IMMEDIATE_PARENT column).
IMMEDIATE_PARENT The parent LOINC Part that immediately precedes a given code in the hierarchy. Note that the value contained in this field is always a LOINC Part number and can never be a LOINC term number.
CODE The LOINC Part or LOINC term whose hierarchy path is being described in a given row.
CODE_TEXT The name of the LOINC Part or the Short Name of the LOINC term specified in the CODE column.

Version History

Version 3.0 Alpha 1

Significant updates to the hierarchy creation process were implemented for the LOINC 2.66 release, including:

  1. Using the Primary Component Parts that include Suffixes rather than the core Component (e.g., HIV 1 Ab instead of HIV 1);
  2. Including the Class in addition to the Component+System in the Multiaxial Parts that organize terms;
  3. Using Class Parts for the higher level nodes; and
  4. Taking the Super System into account

These changes resulted in correction of several issues:

  1. Placement of terms with a Challenge and/or a Method under the Component+System+Class rather than Component alone;
  2. Limiting inclusion of CHEM and CHAL terms to their respective branches rather than duplicating across branches;
  3. Separation of concepts with different meanings based on the suffix, e.g., Hepatitis B virus surface Ab and Hepatitis B virus surface Ag; and
  4. Separation of terms with Super Systems from those without, such as Ser/Plas and Ser/Plas^Donor

For this release, we also began adding more Components for Type 2 (Clinical) terms to the Component hierarchy, specifically for Cardiology, Dental, Ophthalmology/Optometry, and Respiratory/Ventilator, so that terms in these Classes are no longer placed under the "ClinicalNotYetCategorized" node. In addition, we added Panel Class nodes under their corresponding Classes (e.g., PANEL.MICRO under MICRO), so that panel terms are better organized in the hierarchy.

All of these changes, especially the use of the Primary Component and addition of the Class to the Multiaxial Parts, resulted in the creation of thousands of new Multiaxial Parts and led to thousands of existing Multiaxial Parts to become obsolete. These obsolete Parts have been deprecated and are included in the new ChangeSnapshot artifact so that users will have a record of the Multiaxial Parts that are no longer used.

Because of the multiple, significant changes to the approach to constructing the hierarchy, this release constitutes a major version update. We seek user feedback about each of the major changes, but most specifically about the approach to including the Class in the construction of the hierarchy nodes.