2.7 Maintaining Postcoordinated Expressions

As SNOMED CT evolves with each new release, maintaining the relevance and accuracy of postcoordinated expressions becomes imperative. Postcoordinated expressions allow for detailed and nuanced clinical concepts by combining multiple SNOMED CT codes. However, without a robust maintenance process, these expressions can become outdated, leading to failures in subsumption by any concept in SNOMED CT and exclusion from Expression Constraint Language (ECL) results.

Importance of Maintenance After Each Update

Alignment with Latest SNOMED CT Versions

• Concept Evolution: SNOMED CT regularly updates its content, which may include the addition of new concepts, modification of existing ones, or retirement of outdated concepts.
• Accuracy in Clinical Representation: Expressions that reference inactive concepts will no longer effectively represent valid clinical meanings, leading to misinterpretation of patient data.
• Interoperability: Ensuring that the substrate and the expressions are up-to-date maintains interoperability between systems using the latest versions of SNOMED CT.

Impact on Data Retrieval and Analytics

• ECL Queries: Outdated expressions won't appear in ECL query results, hindering data analytics and reporting.
• Clinical Decision Support: Inaccurate expressions can affect decision-making processes that rely on current clinical terminologies.

Implementing the Maintenance Process

1. Support Expression Versions

• Non-Semantic Identifiers: Assign a unique, non-semantic identifier to each expression, such as:
  • A "16" partition SNOMED Identifier
  • A sequential number
  • A Universally Unique Identifier (UUID)
• Version Control: Maintain a history of all changes to the expression's value over time while keeping the identifier constant.
• Clinical Record Integrity: By preserving identifiers, clinical records remain consistent even as the underlying expressions are updated.

2. Generate Reports of Outdated Expressions

• Post-Upgrade Audit: After upgrading to a new SNOMED CT edition, generate a comprehensive report listing expressions that require updates.
• Identification Criteria: Expressions may need updating if they:
  • Contain references to retired concepts
  • No longer comply with updated SNOMED CT modeling principles
• Actionable Insights: Use the report to prioritize and plan maintenance activities.

3. Attempt Automatic Updates Using Historical Associations

• Leverage SNOMED CT Historical Data: Utilize historical associations provided by SNOMED CT to map outdated concepts to current ones.
• Configuration of Confidence Levels:
  • High Confidence: Automatic replacements using "Same As" associations.
  • Moderate Confidence: Consider "Replaced By" or "Partially Equivalent To" associations but flag for review.
  • Low Confidence: Manual verification is required for associations with less certainty.
• Automation Tools: Implement software capable of processing these associations to update expressions where appropriate.

4. Manual Review and Updates

• Complex Cases: Not all expressions can be updated automatically due to:
  • Ambiguities in historical associations
  • Changes in clinical practice or terminology
  • Complex postcoordination that lacks direct replacements
• Quality Assurance: Manual revisions of random samples can ensure that the process is working as expected.

Challenges in the Maintenance Process

Resource Intensiveness

• Time and Effort: Manual reviews are time-consuming and require significant human resources.
• Specialized Knowledge: Requires expertise in both clinical domains and SNOMED CT's structure and updates.

Complexity of Changes

• Evolving Concepts: Understanding the nuances of how concepts have changed or been restructured.
• Interdependencies: Changes in one area may affect multiple expressions across different clinical domains.

Tool Limitations

• Automation Boundaries: Current tools may not fully automate the update process, especially for complex expressions.
• Reporting Accuracy: Generating accurate reports of outdated expressions can be challenging without sophisticated tooling or querying capabilities.

Benefits of using Close to User Form

Close to user form is a simpler form of an expression that is later transformed into a "classifiable form". Close-to-user form expressions include only the essential concepts required to convey the desired meaning, and fewer concepts in the expression means that it is less susceptible to be impacted by changes in the dependencies. The steps of automatic updates and manual reviews are much simpler to implement and execute when using close-to-user form expressions.

Best Practices for Effective Maintenance

• Regular Updates: Schedule maintenance activities aligned with SNOMED CT release cycles.
• Store expressions using close-to-user form: the simpler expressions facilitate maintenance.
• Comprehensive Documentation: Keep detailed records of changes made to expressions for future reference and auditing.
• Stakeholder Engagement: In complex cases, a multidisciplinary team, including clinicians, terminologists, and IT professionals, is involved.
• Continuous Training: Ensure staff are up-to-date with the latest SNOMED CT changes and maintenance methodologies.
• Robust Testing: Validate updated expressions in a test environment before deploying to production systems.