This guide does not mandate a particular way to meet the requirements and the design of an expression repository as outlined in Requirements and 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.

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. The diagram below 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.

At this level, the terminology server requires all submitted expressions to already be in valid classifiable form. This form is a syntactically valid and concept model compliant representation expression that can be directly used as input to the classifier, enabling expressions to be classified together with other SNOMED CT content.

Implementation Advice

To enable postcoordination at Level 0, it is important that the system used to create the expressions supports any transformation needed to represent the expressions in their classifiable form. This may be enabled using one of the following techniques:

• Form-based Creation

• Expression Templates

These techniques will support the creation of expressions that fully comply with the concept model rules and avoid some of the common mistakes created. For example using attributes that are incorrect for the domain or incorrectly grouped.

Consider the situation where the following expression is created to represent the meaning of a "Disorder of left lung":

At level 0, this expression would not be accepted by the terminology server, because the MRCM only permits those concepts that are both within the << 91723000 |Anatomical structure (body structure)| domain and also listed in the to be qualified using . The focus concept of the expression is neither.

As a result of this rule, the action required to make the expression classifiable, is to add a nested expression where the laterality is applied to the value of the attribute that is part of the NNF for the same focus concept, , as follows:

Consider the situation where the following expression is created to express the meaning of a "Procedure on left lung":

At level 0, this expression would not be accepted by the terminology server, because is specified as being 'grouped', which requires a role group to be explicitly stated. As a result of this rule, the action required to make the expression classifiable, is to place the attribute in a group, as follows:

Terminology servers at level 0 will reject any expressions not conforming to the classifiable form.

The table below provides examples of a set of expressions with an indication of their acceptability and potential issues at level 0.

Terminology servers at Level 1 will accept any expressions represented in valid classifiable form, as in Level 0. Additionally, some expressions that are not already in classifiable form will also be accepted. These will mainly be syntactically compliant expressions that are not MRCM compliant, but which the server can unambiguously transform to derived expressions that are MRCM compliant, using predefined automated transforms.

Loose Attributes

A loose attribute is any attribute within a CTU expression that is not in an attribute group AND the MRCM specifies:

• that it should be grouped, or

• that it is in the wrong domain

Level 1 transformations of CTU expressions may apply to expressions meeting the following criteria.

• Transformations can only apply where there is no definition status

• There must be only one focus concept

• Level 1 transformations can only apply to loose attributes (as defined above)

Expression syntax, concepts being active and MRCM attribute-range validation should be applied before the transformation step. Implementations should fail fast if these are not met and not attempt transformation. This simplifies the preconditions for specific transformations.

The process of applying the transformations is iterative. This includes: collecting the set of remaining loose attributes, then applying the next transformation, and repeating until each transformation has been applied once. When each specific transformation is applied it should be greedy and transform all the loose attributes that meet the conditions of that transformation.

The list of transformations must be applied in a specific order; to ensure that the same classifiable expression is reached every time by all implementations.

The allowed patterns can be summarized as follows:

1. Refining Existing Attribute

2. Adding a Self-grouped Attribute

3. Adding Severity to Clinical Findings

4. Lateralizing Clinical Findings

The following sections elaborate on the preconditions and behaviour of each pattern.

In this case, the terminology server will accept attributes-value pairs with the following characteristics:

• The attribute is already present in the NNF of the focus concept

• The attribute value stated in the expression is a subtype of the attribute value of the focus concept

Important Notes

• If no groups are stated in the CTU expression (i.e. attributes are declared as loose attributes), the terminology server will accept attribute-value pairs correctly belonging to role groups of the focus concept as follows:

  • The terminology server will accept attribute-value pairs that refine one or more of the attribute-value pairs of the focus concept, i.e.

    • The attribute type should be the same as, or a subtype of, an attribute that already exists in the NNF of the focus concept, for which

This transformation will be applied to all loose attributes that meet the following conditions:

• The attribute applies to the MRCM domain of the focus concept (see table below)

• Only one instance of the attribute is stated in the CTU expression

  • Although the MRCM allows multiple instances, this transformation condition aims to avoid misinterpretation of the intended meaning

Please refer to the Editorial Guide for instructions on the use and the implied meaning of these attributes:

• Clinical Finding Defining Attributes

• Procedure Defining Attributes

As part of the expression transformation process, the terminology server will place the attributes in groups on their own, as shown in the examples below.

At level 1, the terminology server will accept CTU expressions matching the following pattern:

This means that CTU expressions that includes the severity attribute will be accepted if the following criteria are met:

• The focus concept is a subtype of |Clinical finding| (but not a subtype of )

• The attribute value must be a subtype of

• When applied as a loose attribute, only one instance of the attribute may be stated in the CTU expression

As part of the transformation process, the terminology server will place the attribute in a group on its own, similar to the self-grouped attributes.

The terminology server will accept CTU expressions matching the following pattern:

The transformation of expressions following this pattern will include the following steps:

1. Copy the defining properties and role groups of the focus concept (as present in the NNF) to the classifiable form expression

2. Replace the value of the |Finding site| attribute with a nested expression:

From:

To:

Important Notes

When applying the concept as the value for the attribute, the transformation should be done in accordance with the modeling applied to the content in the International Edition of SNOMED CT.

This means that the transformation steps described above should be applied twice; one for each lateral half, as illustrated in the example below.

The terminology server will accept CTU expressions matching the following pattern:

The transformation of expressions following this pattern will include the following steps:

1. Copy the defining properties and role groups of the focus concept (as present in the NNF) to the classifiable form expression

2. Replace the value of the |Procedure site| attribute (or any of its subtypes / @siteAttribute) with a nested expression:

From:

To:

When applying the concept as the value for the attribute, the transformation should be done in accordance with the modeling applied to the content in the International Edition of SNOMED CT.

This means that the transformation steps described above should be applied twice; one for each lateral half, as illustrated in the example below.

At level 1, the terminology server will accept CTU expressions matching the following pattern:

This pattern allows attributes applicable to the domain to be used to add contextual information to clinical finding concepts.

The generated classifiable form must be based on the following expression with as the focus concept and the following default attributes and values unless otherwise specified by the CTU. The value of the Associated finding attribute will be replaced by the focus concept of the CF expression.

At level 1, the terminology server will accept CTU expressions matching the following pattern:

This pattern allows attributes applicable to the domain to be used to add contextual information to procedures.

The generated classifiable form must be based on the following expression with as the focus concept and the following default attributes and values, unless otherwise specified by the CTU. The value of the Associated procedure attribute will be replaced by the focus concept of the CF expression.