With the form-based approach, the combination of values recorded in different data entry controls results in specific expressions to be formed.
With this approach, using an expression template can help to ensure that the selected values are added in the appropriate places within the expression.
The example shown in the image below illustrates how an expression template is used to apply a laterality to a selected diagnosis. In this example, if a user enters the values Open fracture of radius as the diagnosis, and the value left in the side field, the values entered are then used to populate the slots in the expression template and form an expression representing the meaning of an 'open fracture of radius with a laterality of left'.
Expression templates associated with a data entry form should be created during the design phase of the implementation, and then populated with values at run time, when the user selects values in each field. Please see section /expression template https://github.com/SNOMED-Documents/snomed-postcoordination-guide/blob/main/4-expressions-in-the-ehr/4.1-preparing-expression-repository/4.1.3-creating-expressions.md#id-4.1.3.3-expression-templatesfor details on expression templates.
When using the form-based approach for entering postcoordinated expressions in an EHR, it is recommended to store the data in an information structure that matches the fields on the form and to record the concept identifier and display term for each of the fields, within that information structure. This enables the system to efficiently display health records that have been recorded previously, and also provides full traceability of what the user selected at the point of data entry. It also enables the possibility of re-generating a postcoordinated expression at a later date that is potentially different from that which was originally generated, e.g. if an error were found in the original template or if structural changes in SNOMED force a redesign of the expression template.
In addition to the storage of the individual concept identifiers, the derived expression (in terse form) may also be stored to meet specific EHR-requirements. Generating the derived expression may be done using an expression template.
Terminology services
Enabling form-based entry of expressions in the EHR requires the EHR to dynamically access at runtime SNOMED CT concepts as determined by the terminology binding specified for each data element. At the point of data entry, no specific services for postcoordination are required, however, if the resulting expression is stored as a single expression, or subsequently communicated, terminology services are required to add the expression to the expression repository. For this purpose, the following services are required:
(including required sub-services)
When enabling postcoordination in an EHR, it is important to be aware of how the approach selected for entry of the expression impacts the storage and the display of the expression.
There are three main approaches to entering postcoordinated clinical data into an electronic health record.
The following pages will elaborate on each of these approaches and clarify key requirements for each. Furthermore, best practice approaches to the storage and display of expressions will also be presented.
Except where agreed business processes require structured and standardised recording, using free text fields are often a necessity to support the clinical documentation and established work processes. Free text in Electronic Health Records enables the user to enter information using the terms and phrases of his/her own choice.
With this approach, the clinical user enters the information in a dedicated text box, and the text is subsequently analyzed by a natural language processing (NLP) tool. The NLP tool is designed to codify the key terms as SNOMED CT concepts, and the rules specified in the Machine Readable Concept Model (MRCM) may be applied to help identify the relationships between these key terms, to form SNOMED CT expressions.
The image below illustrates how the phrase "The patient has an open fracture of the left radius" is processed by an NLP tool to form the expression:
With this approach, the expressions are usually created at run-time, as the user types in the free text. This requires the system to incorporate a SNOMED CT- enabled NLP service that works in the local dialect.
Using the free-text approach with an NLP tool to generate the expressions, it is recommended, and often required for medicolegal reasons, to store both the free text as it was entered by the user, and also the coded expression that was generated by the NLP tool and confirmed by the user. You should also store the terms associated with the expression that was displayed to the user when they confirmed that it accurately reflects the meaning of the free text.
Terminology services
Enabling NLP with expressions in the EHR requires the EHR to add newly created expressions derived from the NLP algorithm. For this purpose, the following services may be required:
Lookup Expression
Add Expression
Get Display Term. This may be required to display a human readable representation of the generated expression, to enable the end-user to confirm the result of the NLP analysis.
So, in this approach the expressions are usually created at run-time, as the user types in the free text. This requires the system to incorporate a SNOMED CT enabled NLP service that works in the local dialect. So, in this approach the expressions are usually created at run-time, as the user types in the free text. This requires the system to incorporate a SNOMED CT enabled NLP service that works in the local dialect.
42945005 |Open fracture of radius| : 272741003 |Laterality| = 7771000 |Left| . 
When using pre-populated expressions, the user is presented with a list of clinical phrases (āinterface termsā), using a pick list, checkboxes, or another type of data entry control.
The essence of this approach is that the user is presented with a clinical phrase representing a specific clinical meaning, and in the backend of the system, this clinical phrase is associated with a postcoordinated expression (instead of a precoordinated concept).
In the example shown in the image below, the user interface includes a list of phrases such as āOpen fracture of left radiusā, and āOpen fracture of right radiusā. Some of these phrases may be associated with precoordinated content (either via mapping or the released SNOMED CT descriptions), while other phrases may be associated with postcoordinated expressions. In this example, the phrase āOpen fracture of left radiusā and āOpen fracture of left ulnaā is associated with postcoordinated expressions.
This approach requires the expressions to be created and mapped to an interface term by a SNOMED CT implementer. This occurs as part of the terminology binding process in the design stage of the implementation. Different approaches to the creation of the expressions exist and depend on the tools available. These approaches are described in the following pages.
Using postcoordinated expressions for items in a prepopulated list, or predefined values in a UI, each expression is associated with one field on the user interface. For precoordinated concepts, the best practice is to record the concept identifier as the code, and the term that was displayed or selected by the user as the original text.
The recommended approach to the storage of predefined postcoodinated expressions is similar to the storage of precoordinated concepts. This means that the display term used for the expression should be recorded in the health record, but the ācodeā should either be the terse form of the full expression (i.e. the expression with the terms and spaces removed), or the unique identifier associated with the expression when this was added to the expression repository.
Terminology services
Enabling prepopulated expressions in the EHR requires the EHR to access expressions already created in the expression repository. For this purpose, the following services may be required:
