The Case For Removing Description Concept Non-Current Indicators

Summary 

Concept Non Current Indicators are a legacy hangover from the RF1 specification that are logically redundant and contribute to a perception that SNOMED CT is highly complex, without adding anything in the way of useful functionality.   They are not well understood generally, and do not "fit" well into the model - Inactivation indicators being applied to active components.   They cause significant complexity for SNOMED content maintainers, especially to country extensions, as they must be calculated for all translations when concepts are inactivated in the International Edition.   They must be removed when descriptions are inactivated, and also when concepts are re-activated!   There is a host of rules around how these indicators should be processed (and packaged, and validated), and yet they're entirely unnecessary.

Description

This document details the case for removing existing Concept Non-Current (CNC) Indicators and discontinuing their use.   Should this proposal be accepted by SNOMED International internally and by the relevant Advisory Groups, then a wider consultation and lengthy notice period will be required.   It seems unlikely that implementers are making use of these indicators directly, given that the information they impart is much more readily obtained from the concept itself, but this cannot be known for sure until feedback is received from the community.

Note that other description inactivation indicators exist and are being used.  These provide useful information in giving a reason for inactivation when a description does actually become inactive.   This paper is purely concerned with the CNC variant of these, which is applied to active descriptions.

Background

Concept Non Current (CNC) indicators are a type of inactivation indicator that are applied to active descriptions, when the concept they are attached to is made inactive.   They are intended to allow implementers to show some visual cue to alert users that a description they have searched for relates to an inactive concept.

The idea of this type of indicator originated in the RF1 specification, where the description object featured a "status" field that was not a Boolean field (as we have with the RF2 "active" field), but instead contained a range of potential values indicating that the description was one of: active, not active, a reason for inactivation (ie what we now express via Inactivation Indicators), or a value indicating that while the description itself was active, the concept  that it related to was not

Arguments for Removal

Redundancy

In order to determine that any active description relates to an inactive concept, the attribute value file must be parsed to check for an active CNC inactivation indicator.   This information is already present in the concept file.   Since concepts are the "primary unit of work" in SNOMED CT,  the concept file will certainly be present in any system.    As such, the CNC indicator value in the attribute value file is not needed.   It is arguably less work to look up the concept state directly on the concept (it should know!) than it is to look up a value attached to the description.  

CNC indicators took up 108Mb of file space in the July 2023 International Edition.

Logical Confusion

Concept Non-Current indicators are (in the author's experience), one of the least well understood features of SNOMED CT.    Being able to explain the rules around their use can be considered true mastery of the subject.   The logical confusion relates to the fact that this inactivation indicator is applied to a component that is not itself inactive, and in fact relates to another component entirely.   Additionally, the fact that this flag has been squeezed into (arguably) the wrong place is highlighted when we come to inactivate the description:   while the concept may remain inactive (the CNC indicator is still be true, logically speaking), but we need to use that field to contain the reason for the description being made inactive.   And so the CNC inactivation indicator must itself then be made inactive.

Readers who are confused by the above will, I hope, agree with my point; that CNC indicators are inherently confusing.

Implementation Complexity

Concept Non-Current Indicators cause additional work for all creators of SNOMED CT content, in terms of maintenance, packaging and validation.  As extensions are upgraded to a latest release of the International Edition, CNC indicators must be added to all translated descriptions where the International Edition has made a concept inactive.   They must then be removed should the description itself become inactive, or the concept is reactivated.  And replaced again should the content change.   All these changes need a set of specific validation rules that are quite the opposite of the normal set of rules around inactivation indicator behaviour.   Normally an inactivation indicator can only be applied against an inactive component, but in the case of CNC indictors, they can only be applied to active components.

Impact

The potential impact to implementors cannot be gauged until we run a consultation exercise, but certainly as far as SNOMED International's tooling is concerned, we do not actually make use of these indicators ourselves at all.   We have a lot of code to maintain and validate them, but they're never actually used.   They do not appear at all in the browser, and while they are visible in our Authoring Platform, they cannot be modified and are maintained entirely programmatically.   So the impact of removing these indicators will entirely be a case of removing and simplifying existing code.

Existing implementations that do use CNC indicators to highlight that a description relates to an inactive concept - and I will be surprised to hear that there are any - should already have access to the active state of the concept since descriptions are inherently a part of the concept.

Conclusion

It would be an act of service to future generations of SNOMED CT students, developers and implementors to inactivate all current CNC indicators and remove them from use, in order to simplify both the understanding of SNOMED CT, and implementations supporting SNOMED CT's maintenance, validation and use.   This is in line with SNOMED International's strategic goal of "Lowering barriers to adoption".

Update: Proposal for Change Roadmap (Work In Progress)

SNOMED International have received a number of pieces of feedback on this page which has said that since the redundant nature of Concept Non Current indicators is so clear, that spending time on a Community Consultation exercise asking if we should remove these indicators would not be a good use of resources across the board.   Instead we should move directly to deprecation and allow that time to be used by implementers to work out what impact this change might have on them, if any.

As such, the following table is a proposed timeline (for discussion):

This timeline will remain provisional and dependent on the availability of resources within SNOMED International and taking into account the priorities of Member Forum.

No Concept Non Current Indicators were physically harmed during the creation of this document.