SNOMED International proposes to add an additional relationship file to the International Edition of SNOMED CT which will - initially - contain numeric values for medicinal products.  At the same time, existing drugs strengths and counts expressed using concepts (which represent those same numeric values) will be inactivated.  This enhancement will increase the analytical power of SNOMED CT through the use of numeric queries and assist with interoperability by removing the need for extension maintainers to all - separately - add concepts representing numbers in order to publish their own drug dictionaries.  SNOMED International ask that users read this briefing paper and then respond with questions and feedback via this Google form.


SNOMED International proposes to further enhance SNOMED CT by adding the capability to express Concrete Domain values.  This enhancement will initially target improvements in the Pharmaceutical / biologic product hierarchy.  Specifically supplying clinical drug strengths as true numbers, rather than using the existing work-around of concepts which represent those numbers.

Background and Rationale

SNOMED International seek to make SNOMED CT richer, more precise and more useful.   This drive is hampered when particular types of information cannot be fully represented using the existing RF2 release distribution format.   In the case of numerics - for example the weight of a particular medicinal substance used in a drug product - it is desirable to be able to represent that amount, and then be able to ask questions like "List all drugs that contain >= 100mg of Paracetamol".

A work around for this particular attribute was put in place in 2017, namely to create a concept to represent every number needed in the International Drug Model.   This solution results in fully correct classification and has allowed the Drug Model to progress in its implementation.  However it has several drawbacks:

  1. A new number concept must be created every time a new one is required.  National Release Centres (NRCs) may wish to use numbers that are not contained in the International Edition, and indeed many NRCs maintain substantially sized drug dictionaries so their burden is increased.  Each NRC would then be creating their own number concepts, some of which might then appear in the International Edition, causing further work to resolve.
  2. SI and Implementers need to take care to normalize drug strengths to avoid unwanted equivalencies.   For example, the SI drug model would always represent 0.5g as 500mg since the classifier would not recognise those two strengths as being the same thing - they're represented by different concepts.   Using concrete domains allows equivalences between different units to be detected.
  3. Users are not currently able to directly ask questions that involve numeric operations ( > , < , = ).   Number concepts currently need to be transformed into true numbers before this could be done.

There are other types of concrete value that are desirable, such as strings and booleans which are used already, for example, in the Singapore Drug Dictionary.  This proposal would also add the capability to express these additional data types.

Proposed Changes to RF2

The Modeling Advisory Group proposes to add a new file to the international release called, for example : SnomedCT_InternationalRF2_Production_20210731T120000Z/Full/Terminology/sct2_RelationshipConcreteValues_Full_INT_20210731.txt   with counterparts in the Snapshot and Delta folders.

This file would contain the following fields (with example rows & FSNs added for clarity):

70755260202021073119000000000002070083483150096511234567809 |Has presentation strength numerator value (attribute)|900000000000011006 |Inferred relationship (core metadata concept)|900000000000451002 |Existential restriction modifier (core metadata concept)|
707552902920210731190000000000020700834831500950021234567809 |Has presentation strength numerator value (attribute)|900000000000011006 |Inferred relationship (core metadata concept)|900000000000451002 |Existential restriction modifier (core metadata concept)|

The new concrete values file will use the same sort of relationship SCTIDs that the current file uses1.  

Existing attribute types that take concepts as numbers for values will be retired and replaced with new attributes using the same FSN, but taking a concrete value as the attribute target.   This will mean that all relationships using the original attribute types will be inactivated in the inferred relationship file, and replaced with new rows in the new relationship concrete values file.    There will be an equivalent change in the stated view, but since OWL is already capable of supporting concrete values, this will only result in changes within the OWL expressions.   There will be no structural changes in the stated view.

The range of types of the values that can be held are as specified in SNOMED Compositional Grammar.  That is:  boolean, numbers and strings.  The particular type of value used for each relationship type will be specified by the MRCM.  See MRCM for Concrete Domains.  Boolean will be restricted to true | false to avoid confusion with numeric types - always lower case.

To maintain structural compatibility with the existing relationship file, the various columns will have the same layout and usage.  The value column in the concrete values file replaces the target column in the relationship file.  Otherwise, the two files will be identical such that they could be loaded into the same data structures if an implementer chose to do so.

Proposed changes to the SI Drug Model 

Following established principles of retiring concepts when there is a change in meaning,  SNOMED International will inactivate the existing strength / concentration  attributes which use concepts-as-numbers and replace them with new ones (using the same FSNs) and switch the target/value to the corresponding concrete numeric.   This change will be done programatically as a technical solution - which will be made generally available for NRCs and other generators of SNOMED content to use.

The attribute type concepts which will be replaced are as follows (italics indicate attributes not currently used by International Drug Model):

766953001 |Count of active ingredient (attribute)|732944001 |Has presentation strength numerator value (attribute)|733724008 |Has concentration strength numerator value (attribute)|
766952006 |Count of base of active ingredient (attribute)|732946004 |Has presentation strength denominator value (attribute)|733723002 |Has concentration strength denominator value (attribute)|
784276002 |Count of clinical drug type (attribute)| 766954007 |Count of base and modification pair (attribute)| 


With the changes introduced by SNOMED International in 2018, classification of SNOMED CT is performed directly from OWL.   OWL supports concrete domains, and so the existing "stated" OWL reference set is capable of representing concrete domains without further modification.   Class (ifiers also, in general, support concrete domains.  The two areas where changes will be required are:

  1. In Tooling, to allow authors to enter a concrete value rather than a concept as the target of a relationship.
  2. In the distribution archive, adding a new relationship file to allow the concrete values to be used 2.  


Your Current or Planned Usage

Impact of Change

Detail of Impact

No use of International Drug Model, or usage limited to using descriptions and hierarchical structure.

There is no need to load the new concrete value relationship file for this use case.

SNOMED International Managed Service users and users of SI open source software.

SNOMED International tooling will be enhanced in good time to support the proposed changes.

Maintenance of drug concepts which form part of the 373873005 |Pharmaceutical / biologic product (product) hierarchy.

Extension concepts in this hierarchy will need to have their strengths and ingredient counts expressed using concrete values in the Stated OWL refset, in order to properly classify and subsume appropriately. The concrete value relationships will need to be split into their own file in order to be compatible with the International Edition.

SNOMED International software such as Snowstorm and the SNOMED OWL Toolkit will be updated to perform these steps automatically.

Proposed Schedule for Change

The table below shows the features that will be introduced over the next few releases of the SNOMED CT International Edition. This is an optimistic timeline that has been driven by the desire to fulfil obligations for successful delivery of work being done in the Pharmaceutical / biologic product and Substance hierarchies.




Stated OWL Axiom file changes

Inferred Relationship file changes

Additional Features in International Release

Technical preview





A technical preview will be produced, with drug concept strengths and counts expressed as concrete values in the new file

Expected publication prior to February 2021.


Existing drug concept axioms will be updated to use concrete values.

Existing strength and count attribute types will be inactivated, replaced with new ones using the same FSNs.

Existing drug concept strength and count relationships inactivated.

Existing strength and count attribute types will be inactivated, replaced with new ones using the same FSNs.

New separate concrete value relationship file will express these same attributes using numeric values.

New attributes will be used, although they will take have the same FSNs as the current attributes.

MRCM will include new rows to indicate that the new attribute types are expected to take a concrete domain - specifically numbers - as target values.


Next Steps

SNOMED International request that users of SNOMED CT provide feedback on this proposal via this form. SNOMED International will respond to feedback received on this page until the end of this consultation exercise on 31 December 2019. 


1 Technically, this means using Partition 02 for these concrete value relationship SCTIDs in the International Edition, as we do for all relationship types.   Extensions will continue to use partition 12. 

2 SNOMED International seek to minimise disruption to consumers of SNOMED CT and here propose adding new, optional artefacts, rather than modifying existing structures.


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