Introduction

A discussion paper from the Content Managers Advisory Group, entitled Discussion Paper - Allowance of Extensions to Modify Core Content has been submitted to SNOMED International management to resolve issues surrounding the interpretation of what national extensions are allowed to modify in the International release of SNOMED CT.  The discussion paper outlines a number of proposed clarifications to the interpretation of what is allowed and details the requirements for those changes.

Background

The primary concern expressed is the interpretation of Clause 4.1 of the SNOMED CT® AFFILIATE LICENSE AGREEMENT, which states: 

“Subject to clause 2.1.4, the Licensee may not modify any part of the SNOMED CT Core distributed as part of the International Release or as part of a Member’s National Release.”

Clause 2.1.4 states:

 “(The Licensor grants the Licensee, license to) modify the manner of formatting of the copy of the SNOMED CT Core distributed to the Licensee as part of the International Release or as part of a Member’s National Release”

 According to the discussion paper, there are two primary issues associated with this clause:

 "This restriction on modifications has been interpreted generally in two ways. 

1. The RF2 distribution must not be modified, beyond appending additional rows. Overwriting data in the distribution files - such that an extension violates the append only model is not allowed (editor's addition). Additionally, the full history provided by the international release must provided, i.e. extensions may not omit anything;

2. Core components must not be modified.
   Overriding the international content through either:

• the addition of new versions for core components within extensions; or

• the addition of relationships to core concepts and changing their DL definition.

Restrictions covered by B have been shown to be impractical and prohibit proper quality terminology authoring."

Proposed actions allowed for National Extensions

The discussion paper proposes seven actions that national extensions may want to perform in the course of their content development and maintenance:

1. Create new concepts.

2. Fully define concepts they create.

3. Classify terminology extensions.

4. State additional IS A relationships against core (international) concepts.

5. Retire (redundant) IS A relationships (not necessarily stated).

6. Add additional defining (non-IS A) relationships to primitive core concepts.

7. Retire content considered "inappropriate" - concepts, descriptions or relationships

The paper then goes into substantial detail justifying the need to perform each of these actions.  A request was forwarded to SNOMED International management for review and clarification of the interpretation of clause 4.1 and acceptance of the allowable proposed actions available to national extension managers.

This document attempts to clarify the feasibility of each of the proposed actions and provides justification for the acceptance or rejection of each, in order to make the interpretation of Clause 4.1 more clear.

Evaluation of proposed allowable actions for national extensions

Create new concepts

The current interpretation of Clause 4.1 with regards to creation of new concepts is that the new concepts would necessarily be leaf nodes to the existing International release.  This type of content, by its very nature would have no impact on the international content, as it would be distal in the hierarchy to all international content.  National extensions have found that, due to gaps in the current International release, it is often necessary to insert intermediate (i.e. non-leaf) concepts into the terminology to meet user requests as well as to provide additional navigational support.  These intermediate concepts may be either primitive or fully-defined.

The creation of intermediate concepts necessarily results in changes to the relationships in the core terminology.  These changes can be of almost any type including: assignment of new parent(s), retirement of inferred or stated parents, inheritance of new/additional defining relationships, removal of inferred relationships by the classifier, etc. 

Proposed policy

SNOMED International recognizes that SNOMED CT is not always comprehensive in its terminological coverage in particular domains and that there exist "gaps" in the hierarchies, where concept coverage is not adequate for extensions.  It is proposed that intermediate concepts may be added to extensions under the following guidelines:

1. The addition of the concept does change the transitive closure for the International Release, except to add additional rows that include the new intermediate concept). The transitive closure of the national edition must be a superset of the transitive closure of the international edition. This necessitates the retention of all transitive IS A relationships from the core.
2. No stated defining attribute of core content are inactivated in the extension.
3. Inferred |is a| relationships should only be inactivated due to redundancy, but should still represent a valid relationship in the transitive closure. See "Retire (redundant) IS A relationships (not necessarily inferred)". 
4. Additional inferred defining relationships to not create crossovers (i.e. subtypes with attribute values that are supertypes of the values represented by the same attribute of the parent)
5. Additional inferred IS A relationships of subtypes do not represent a change in meaning as represented by the Fully Specified Name. 
6. The intermediate concepts are submitted for promotion to the international release as soon as possible.

Fully define concepts they create

Full definition of intermediate concepts will in many cases result in the reassignment of core relationship due to inferences assigned from the classifier (see next section).

Proposed policy

Classify terminology extensions

The ability to classify extensions is an essential part of leveraging the description logics represented in the International Release for the proper maintenance of extension content.  However, 

Proposed policy

State additional IS A relationships against core (international) concepts

Missing stated IS A relationships for core content is usually the result of the existence of intermediate primitive concepts that editors are unaware of.  These intermediate primitive concepts are under review, but the volume makes it unlikely that all will be identified and resolved in the very near future.  Identification of these missing stated relationships by national extensions provides an opportunity for incremental improvement of the core terminology.   

Proposed policy

1. Addition of new stated IS A relationships to core content is not allowed.

Retire (redundant) IS A relationships (not necessarily stated)

Redundant IS A relationship exist in SNOMED CT where variations in modeling style have been used and hierarchically related terms have been assigned as stated relationships. These are considered internal quality errors in the and are being incrementally addressed through the SNOMED CT Internal quality improvement program.  Many of these are already identified (See: https://dailybuild.ihtsdotools.org/qa/  Pattern 1) and are being addressed by SNOMED editors as resources allow.  Redundant relationships that impact extension structure should be reported to SNOMED International for prioritized disposition.  

Proposed policy

1. Relationships identified as redundant should be reported to SNOMED Int'l as soon as possible to increase its priority.   
2. International stated relationships must not be retired. 
3. International inferred concept may be retired in a national edition is if it is redundant, but it must still be part of the transitive closure. 
4. Where retirement of stated relationships are needed, a request should be made to SNOMED International to promote the intermediate concept.
   

Add additional defining (non-IS A) relationships to primitive core concepts

The incomplete modeling of core content resulting in a plethora of primitive content is well-recognized.  However, correction of these deficiencies within a national extension can lead to a number of unintended consequences including: 1) the impact on the classification process and the potential change in meaning of descendent concepts (i.e. loss of subtypes) 2) eventual remodeling of the primitive concept in the International release could lead to conflicts between the national edition and the International release.

It is recognized that there may be cases where proper structuring of a national extension requires the full definition of currently primitive core conte

, and (b) perhaps we should require NRCs who add defining attribute relationships to international concepts to submit these for promotion to / inclusion in the international edition. 

Of course, allowing the definition of international concepts to be changed in an extension makes cross-border, cross-organisation exchange/management/querying of health records an even harder problem than it already is ... so the more than can be done and shared in the international edition the better.

Proposed policy

Retire content considered "inappropriate" - concepts, descriptions or relationships

It is recognized that there are components within the International release that are erroneous. These is a current issue tracker IHTSDO-724 - Retire inappropriate synonyms, that will address the presence of inappropriate synonyms. The existence of other components deemed inappropriate by a national extension are often context or realm dependent.  The inactivation of international release components within a national extension can result in substantial deviation from the International release and thus an alternative representation of SNOMED core content.  The integrity of SNOMED as an internationally consistent terminology would thus be compromised. 

Proposed policy

1. 1. Inactivation of components (concepts, descriptions, stated relationships) originating in the International release, by a national extension, is not allowed.
   2. Identified inappropriate components shall be communicated to SNOMED International via the content request system as internal quality errors to be addressed by the SNOMED International editorial team
   3. The identification of these errors as impacting a national extension will be added to the request in order to escalate its priority in resolution by the editorial team.