Compositional Grammar
The syntax used to represent SNOMED CT expressions is called the Compositional Grammar. This is a formal grammar describing the symbols you can use and how the different parts of an expression can be constructed. Please refer to Compositional Grammar - Specification and Guide for the full detail on this.
While alternative syntaxes are possible, the compositional grammar syntax is considered to be the normative standard for interoperability purposes. Therefore, the recommended practice for anyone supporting postcoordination within their clinical information system, is to represent expressions using this syntax.
Structure
Figure 2.2-1 illustrates the key parts of a SNOMED CT expression through an example. The expression used in this example represents the stated view of the concept 16554361000119106 | PET CT of brain| .
Figure 2.2-1: Expression parts.
A SNOMED CT Compositional Grammar expression begins with an optional definition status, contains one or more focus concepts (represented by a concept identifier) and optionally has a refinement. Each refinement may contain grouped or ungrouped attributes (or both). An attribute consists of the attribute name (represented by a concept identifier) together with the value of the attribute. The attribute value is either an expression or a concrete value (i.e. string, integer, decimal or boolean). Please note that, in the context of an expression, the word 'Attribute' is used to refer to the name/value pair within a refinement. However, in the context of the SNOMED CT concept model, the word 'Attribute' is often used to refer specifically to the 'attribute name' (i.e. the concept used as the relationship type).
| Expression part | Description | General | Recommendation for the implementation of postcoordination |
|---|---|---|---|
| Definition status | A SNOMED CT postcoordinated expression may be used to express a clinical meaning that is either equivalent to or a subtype of the given expression. | '===' is used to state that the expression (on the right-hand side) is semantically equivalent to the left-hand side. '<<<' is used to state expression (on the right-hand side) is a subtype of the left-hand side | For the scope of this guide, the default definition status for postcoordinated expressions is equivalent, i.e. '==='. This means that, unless stated otherwise, the expression represents the full semantics of what is being represented. This principle supports an unambiguous interpretation of the meaning of the expression. |
| Focus concept | The part of a SNOMED CT expression that represents the primary clinical idea. | According to the compositional grammar syntax, SNOMED CT expressions may contain multiple focus concepts. Often, the inferred view of a precoordinated concept includes multiple focus concepts, representing the direct parents of the concept. | For the scope of this guide, all expressions may only contain a single focus concept. The reason for this is that multiple focus concepts in a single postcoordinated expression can lead to ambiguity and confusion. Restricting expressions to one focus concept supports clarity and precision of the clinical meaning being represented, making it easier for healthcare providers to understand and use the information accurately. |
| Refinement | The part of a SNOMED CT expression that modifies or adds defining properties to the focus concept. | According to the compositional grammar syntax, the refinement of a SNOMED CT expression is everything to the right of the ':' . | Postcoordinated expressions may include a refinement to specify either a refinement or a qualification of the focus concept. |
| Attribute group | An association between a set of attribute value pairs that causes them to be considered together within a concept definition or postcoordinated expression. | An expression may contain multiple attribute groups, and each attribute group is surrounded by curly braces. When all attribute-value pairs in a refinement belong to the same attribute group the braces around the attribute group are optional. | Systems enabling postcoordination may support specific transformations of expressions to ensure that appropriate groups are applied for certain attribute-value pairs or set of attribute-value pairs. Such patterns may be defined to support specific use cases while ensuring the correct inferred view of the expression. |
| Attribute-value pair | A combination of an attribute name and an attribute value used to specify a defining characteristic of a clinical idea or a concept. | Each attribute-value pair represents a characteristic that applies to the meaning being expressed. | Postcoordinated expressions may contain one or more attribute-value pairs. |
| Attribute name | The concept that represents the attribute type in a defining relationship or postcoordinated expression. | Each attribute-value pair has a name which is represented by a concept. All of the concepts that can be used to name attributes are subtypes of the concept 410662002 | Concept model attribute (attribute)| . The SNOMED CT concept model specifies the domain for all attributes. The domain for an attribute is the set of concepts which may be defined or refined, using the particular attribute | Attribute-names used in the refinement of a postcoordinated expression should be valid for the domain determined by the focus concept. If implementations extend the domain for certain attributes, transformations should be supported to convert the expressions into a concept-model compliant format. For example, adding a laterality to a procedure violates the concept model rules, but transformations may be applied to put the expression into a valid form where the laterality applies to the body structure being the site of the given procedure. |
| Attribute value | A concept that represents the target of a relationship or the value of an expression refinement in a postcoordinated expression. | An attribute name is associated with a value (that creates an attribute-value pair) when used in the definition of a concept or in a postcoordinated expression. The permitted range of values for an attribute depends on the rules specified in the SNOMED CT concept model. | Attribute values should belong to the acceptable range for the selected focus concept and attribute. Note that although an attribute value fall within the range of the attribute, the focus concept of the expression may alter/constraint the acceptable range. For example, stating a |finding site (attribute)| of |eye structure| to the clinical finding |fracture of femur| will not be valid, despite the fact that |eye structure| falls within the allowed range of the |finding site (attribute)|. The definition of the focus concept (|fracture of femur|) constraints the allowed range to the concept 71341001 |Bone structure of femur|, or any subtype hereof. |
Refinement Types
Overall, postcoordination may be performed to either refine or qualify the meaning of a SNOMED CT concept. The refinement part of a postcoordinated expression may thus represent either a 'refinement' or a 'qualification' of the stated focus concept.
Refinement
Refining the meaning of a focus concept is done by applying more specific values to particular attributes of the concept. The definition of the focus concept, therefore, determines the refinements that are possible for the concept.
Consider the following expression representing the meaning 'Spiral fracture of tibia':
The focus concept of this expression has the following stated definition:
The attribute value of used in the expression (|Fracture, spiral (morphologic abnormality)|) is a subtype of the attribute-value present in the definition of the concept (|Fracture (morphologic abnormality)|), as shown below.
Figure 2.2-2: 73737008 |Fracture, spiral (morphologic abnormality)| is a subtype of 72704001 |Fracture (morphologic abnormality)|.
Qualification
Qualifying the meaning of a concept can be done by adding a qualifying characteristic to a concept, e.g. by applying and attribute-value pair to the focus concept, where that attribute-value pair does not refine an existing attribute-value pair present in the definition of the focus concept. If a particular qualifying characteristic is applied to a concept, the resulting expression represents a more tightly defined subtype of that concept. All attribute-value pairs stated to qualify a focus concept should fall within the acceptable domain and range determined by the SNOMED CT concept model.
Examples of qualifications include:
The concept 53084003 | Bacterial pneumonia (disorder)| may be qualified according to its clinical course (<< 288524001 |Courses (qualifier value)| ) or severity ( << 272141005 |Severities (qualifier value)| )
The concept 71651007 |Mammography| may be qualified according to its priority (<< 272125009 |Priorities (qualifier value)|) or intent (<< 363675004 |Intents (nature of procedure values) (qualifier value)|)
Examples
| Expression | Type | Inferred view of focus concept | Description |
|---|---|---|---|
| Refinement | This expression represents a refinement of the focus concept 125605004 | Fracture of bone (disorder)|, because the stated finding site value (12611008 | Bone structure of tibia (body structure)| ) is a subtype of 272673000 |Bone structure (body structure)| | ||
| Refinement | === 108189003 |Abdomen excision (procedure)| + 175898006 |Kidney operation (procedure)| + 118959001 |Removal from urinary tract (procedure)| : { 260686004 |Method (attribute)| = 129304002 |Excision - action (qualifier value)| , 405813007 |Procedure site - Direct (attribute)| = 64033007 |Kidney structure (body structure)| } | This expression adds a more specific procedure site value than the one used in the definition of the focus concept | |
| Qualification | This expression adds an attribute-value pair which does not represent or refines a defining property of the focus concept. | ||
| Qualification | This expression adds an attribute-value pair which does not represent or refines a defining property of the focus concept. |
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