Regenstrief InstituteSNOMED International

LOINC Ontology Release Notes

Version 2.0, September 2025

This release is compliant with the SNOMED CT RF2 specification and contains a complete set of full and snapshot release files. The production content is released in the 11010000107 |LOINC Extension module|, aligned with the 2025-07-01 version of the International release of SNOMED CT, and Version 2.81 of LOINC, released August 2025.

This release contains over 41,000 SNOMED CT concepts representing both active and discouraged laboratory LOINC Terms. The majority of the LOINC concepts (~78%) included are sufficiently defined. In terms of coverage of the most used LOINC concepts, this release of the LOINC Ontology features:

  • 88 of the top 100 most used
  • 1533 of the top 2000 most used, and
  • 14,115 of the top 20,000 most used LOINC concepts.

New Orderable Groupers Concepts

There are approximately 2,500 concepts in a new Class named LABORDERS.ONTOLOGY. The rationale for creating the new concepts was twofold:

  1. People using the new LOINC Ontology (LOINC as a SNOMED CT Extension) have requested the creation of “organizing” or “grouper concepts” so that people browsing the Ontology are not presented with a flat list of tens of thousands of concepts.

  2. There are many situations where result Lab LOINC codes are too specific for use in ordering. The SNOMED community and other international LOINC users have asked for codes that are at a more abstract granularity consistent with order codes typically found in laboratory order catalogues.

    There are differences between the requirements for organizing concepts and orderable concepts:

    • Orders need to be fully specified. The lab needs to have enough information to know what analysis needs to be performed.
    • Organizing codes need to subsume something or they are not useful.

The approach we used to make the new LABORDERS.ONTOLOGY codes was extracting the set of unique concepts that are combinations of Component, System (specimen), and a new Property we called measurement. Measurement subsumes more specific kinds of measurements like mass concentration, substance concentration, number fraction, etc. With version 2.81, we made an arbitrary decision to only make new codes for the 2,547 items where the new concept subsumes three or more result concepts. These new concepts are not intended for and must not be used as result codes in result messages. Examples, tutorials, education, and training will be provided to emphasize the correct use of these new concepts and to try to prevent their misuse.

Areas of content specifically excluded from this release include:

  • Panels where the panel members are not also included in the release
  • Veterinary specific LOINC Terms
  • LOINC Terms that have a status of “Deprecated”
  • All other domains of LOINC including clinical, documents, surveys, etc.

The ‘SNOMED CT Implementation Guide for the LOINC Ontology’ (http://snomed.org/loinc) contains information about development of the ontology and its implementation in systems.

Annotations

An annotation of ‘LOINC comment’ with value “Discouraged” is included on the discouraged LOINC Terms, e.g., 26461-4|Nucleated erythrocytes/Erythrocytes in Blood.

These annotations can be found in the file der2_scsRefset_ComponentAnnotationStringValueSnapshot_LO1010000_20250921.txt with the column of referencedComponentId set to the SCTID of the relevant concept.

Known issues in this extension release

Content Issues

  • Phrases that are not compliant with rules for proper descriptions in SNOMED CT, e.g., “non-prob.amp.tar”.
  • LOINC Terms defined by LOINC Part (LP) “Antibiotic XXX” (‘other antibiotic’).
  • LOINC Terms modeled as process observables where the Component value is mapped to a SNOMED CT calculation, e.g., 104805-7|Creatinine renal clearance predicted.
  • Some fully specified names (FSNs) contain inconsistencies based on the terming of the SNOMED CT attribute value concepts.
  • The FSN of LOINC Terms defined by Component LP ‘Observation’ and Property LP ‘PrThr’ are not displaying correctly, e.g. 88171-4|Microscopic observation [Presence] in Cornea or Conjunctiva by Acid fast stain
  • The FSN of LOINC Terms when the concept has a divisor and an adjustment, e.g., 30905-4|Cobalamin (Vitamin B12) [Ratio] in 24 hour Urine –post dose cyanocobalamin+intrinsic factor/post dose cyanocobalamin.
  • New orderable groupers concepts do not subsume concepts that are modeled as processes

Validation Issues

The following validation issues have been identified in the version 2.0 of the LOINC Ontology. While these are valid problems according to SNOMED International validation tools, they were assessed as unlikely to present a material issue for implementers or to pose any adverse impact in clinical settings.

Duplicate FSNs between active and inactive content

FSNs are required to be unique in active content in SNOMED CT, and even with inactive concepts, SNOMED International would normally reactivate a previously retired concept rather than create a new one with the same FSN. However, in the case of the LOINC Ontology, the link between LOINC Nums and SCTIDs in the alternate identifier table means that we may see the same FSN being generated when a LOINC concept has been replaced by another LOINC concept that uses the same parts. We cannot reuse a pre-existing concept because the LOINC Num remains bound to it for all time – even when both the LOINC concept and the SNOMED concept have been inactivated.

Invalid Characters

Eight FSNs were flagged for use of a hash symbol (#) or ampersand (&), which is contrary to SNOMED International Editorial Policy. For example:

Code SCTID FSN
53273-9 590721010000104 Urinalysis type of non-squamous epithelial cells panel [#/volume] – Urine by Computer assisted method (observable entity)
50184-1 71841010000104 Mass concentration of calcitonin in serum or plasma at point in time pre 2 mg/kg calcium & 0.5 ug/kg pentagastrin IV (observable entity)

Additionally, 2672 synonyms were reported as containing one or more characters (@, $, # or ). The LOINC Long Common Name is used directly as the FSN for panels, and is therefore not subject to SNOMED Editorial policy.

Descriptions exceeding 255 characters

Fully specified names (FSNs) in the LOINC Ontology are determined programmatically by assigning attribute values to slots in a template. Because of this, LOINC concepts that use individual parts with long names can combine to form an FSN with more than the current limit of 255 characters. SNOMED International has given notice that it intends to extend this character limit, and so systems designed to work with the LOINC Ontology (and SNOMED CT in general) are encouraged not to “hard code” field size limits for descriptions.

Validation reported 5 descriptions (over a total of 3 concepts) have exceeded 255 characters:

Code SCTID FSN
48643-1 562611010000103 Volume rate per area of glomerular filtration rate in point in time in plasma or serum or whole blood by modification of diet in renal disease creatinine calculation formula relative to 1.73 square meters body surface area and adjusted for African race (observable entity)
48642-3 562601010000101 Volume rate per area of glomerular filtration rate in point in time in plasma or serum or whole blood by modification of diet in renal disease creatinine calculation formula relative to 1.73 square meters body surface area and adjusted for non-African race (observable entity)
62238-1 602541010000107 Volume rate per area of glomerular filtration rate calculation in point in time in plasma or serum or whole blood by Chronic Kidney Disease Epidemiology Collaboration creatinine calculation formula relative to 1.73 square meters body surface area (observable entity)

Non-compliance to Machine Readable Concept Model (MRCM) specifications

Since the LOINC Extension is produced using a template based transformation of officially published LOINC concepts, the LOINC Ontology team (representing both SNOMED International and Regenstrief Institute) are not concerned that modeling will be introduced that will ultimately contravene the SNOMED International Machine Readable Concept Model (MRCM). A number of modeling questions have been raised through this collaboration, and each time this has resulted in an expansion of the MRCM to allow for a wider range of expression. We have not yet experienced any situation where an MRCM restriction has been found to indicate a content problem, or require a change to the LOINC Ontology.

MRCM validation checks report a small number of model contraventions – particularly in the area of cardinality (i.e., rules about how many of each attribute can appear in a concept), which we expect to have resolved by the time the next release is published.

Case significance validation

In general, LOINC is less prescriptive about case significance than SNOMED CT. SNOMED CT has a policy of capitalizing descriptions, and providing a separate indicator to say if this capital can safely be made lower case. There are concepts in LOINC where descriptions deliberately start with a lower case character when Tall man lettering is being used, for example in 614921010000103 |fentaNYL and Norfentanyl panel – Serum or Plasma (observable entity)|.

Specific to the version 2.0 release, 13 descriptions have been flagged as failing to start with a capital letter, and all of these relate to the appropriate use of tall man lettering.

Additional validation issues arise from the use of “initial character case insensitive” in SNOMED CT, which has no representation in LOINC. This has resulted in 2048 warnings that the guidance that “Active terms sharing first word with case-sensitive term should share case sensitivity.” has not been followed.

Technical Notes

In order to align naming conventions with international standards, the naming convention for both the Association and Simple refset files will be refined from this release onwards.

Going forward from the September 2025 LOINC Ontology release, the Association refset file names will therefore change from:

  • der2_cRefset_AssociationReferenceSnapshot_LO1010000_20250921.txt
  • der2_cRefset_AssociationReferenceFull_LO1010000_20250921.txt

…to…

  • der2_cRefset_AssociationSnapshot_LO1010000_20250921.txt
  • der2_cRefset_AssociationFull_LO1010000_20250921.txt

The Simple refset file names will likewise change from:

  • der2_Refset_SimpleRefsetSnapshot_LO1010000_20250921.txt
  • der2_Refset_SimpleRefsetFull_LO1010000_20250921.txt

…to…

  • der2_Refset_SimpleSnapshot_LO1010000_20250921.txt
  • der2_Refset_SimpleFull_LO1010000_20250921.txt