This guide does not mandate a particular way to meet the requirements and the design of an expression repository as outlined in 4.1 Requirements and 4.2 Design. However, to ensure that terminology servers supporting postcoordination are implemented in a way that doesn't compromise terminological integrity and clinical safety, this section presents considerations important for the implementation of an expression repository. Also, different approaches to implementations are be presented.
Ensuring Safety
When implementing a terminology server supporting postcoordination with SNOMED CT, careful considerations must be taken to ensure that the approach taken doesn't compromise safety. As a combined effort between the terminology server and the client, the following criteria for expressions must, therefore, be ensured:
- All expressions should conform to the concept model when represented in their classifiable form
- No ambiguity is allowed, i.e. the transformation of the expression into its classifiable form must not result in a modification of the intended meaning of the expression
Meeting these criteria can be done in several ways and depends on the responsibility taken by the terminology server and client respectively.
Transformation Levels
Each terminology server designed to enable postcoordination with SNOMED CT should clearly specify the level of transformation supported, to enable requirements to be specified for the client side, and to support decisions about the use of postcoordination for the specific implementation. Figure 5.3-1 illustrates four levels of support for postcoordination. At each level, the sum of restrictions provided by the terminology server and the client must ensure that the criteria for 'safe' postcoordinations are met.
Figure 5.3-1: Overview of the four levels for support of postcoordination.
Terminology server transformations supported at the different levels
| Transformation Level | Description | CTU | CF | Details |
|---|---|---|---|---|
| Level 0 | The terminology server will classify the expression without transformation. The client is responsible for ensuring that expressions are fully MRCM-compliant. | CTU should be fully MRCM compliant | Same as CTU | |
| Level 1 | The terminology server behaves as in level 0 if the expression is fully MRCM compliant. The terminology server will apply transformations based on known close-to-user form patterns to generate the classifiable form. The client is responsible for ensuring that expressions are fully MRCM-compliant or conform to allowed close-to-user form patterns. | CTU should be fully MRCM compliant or conform to close-to-user form patterns | Generated by transformation | |
| Level 2 | The transformation logic for these levels has not yet been agreed. At this stage, postcoordinations and transformation logic beyond the patterns described at level 1 are, therefore, not recommended. | N/A | ||
| Level 3 | N/A | |||
Table 5.3-1: Overview of the four levels
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