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The simplest form in which any concept can be stored is as a single Identifier. This is referred to as a precoordinated expression, because all aspects of a potentially multifaceted concept are precoordinated into a single discreet form. SNOMED CT contains more than a quarter of a million concepts, and thus allows a wide range of clinical statements to be expressed in precoordinated form.
The procedure "Laparoscopic emergency appendectomy" has at least three distinct facets: "removal of appendix","using a laparoscope" and "as an emergency procedure". SNOMED CT includes a concept that precoordinates these facets.
The concept 174041007 |laparoscopic emergency appendectomy| has the following defining characteristics: 260870009 |priority|= 25876001 |emergency|, 116680003 |is a|= 80146002 |appendectomy|, [ 2 425391005 |using access device|= 86174004 |laparoscope|.
A multi-faceted concept can be stored using a combination of Identifiers for its individual facets. This is referred to as postcoordination, because the various aspects of the concept are coordinated during data entry rather than in the preparation of the terminology. Three types of postcoordination are described in the following sections.
postcoordination by refinement
Example: Total replacement of hip using a Sheehan total hip prosthesis - postcoordinated A postcoordinated expression based on the concept 52734007 |total hip replacement| can be used to record an instance of this procedure. The definition of this concept includes 363699004 |direct device|= 304120007 |total hip replacement prosthesis| and the value of this attribute can be refined to 314580008 |Sheehan total hip prosthesis|(which is a subtype of 304120007 |total hip replacement prosthesis|). Therefore, the following postcoordinated expression can be created and used to represent this procedure: 52734007 |total hip replacement|: 363699004 |direct device|= 314580008 |Sheehan total hip prosthesis|.
Another common use of refinement is to represent a situation such as a family history, or a planned procedure. In this case, a concept representing the general type of situation can be refined by applying a clinical finding or procedure.
Example: Family history of temporal arteritis - postcoordinated A postcoordinated expression based on the concept 281666001 |family history of disorder| can be used to record a family history of any disorder. The definition of this concept includes 246090004 |associated finding|= 64572001 |disease| and the value of this attribute can be refined to 400130008 |temporal arteritis|(which is a subtype of 64572001 |disease|). Therefore, the following postcoordinated expression can be created and used to represent this family history: 281666001 |family history of disorder|: 246090004 |associated finding|= 400130008 |temporal arteritis|.
Postcoordination by qualification
Qualification is a type of postcoordination in which a concept is made more specific by applying value to attributes that are permitted by the Concept Model. Unlike refinement, the attributes applied need not be present in the definition of the concept that is being qualified.
Example: Laparoscopic emergency appendectomy - postcoordinated A postcoordinated expression based on the concept 80146002 |appendectomy| can be used to record an instance of this procedure by separately specifying the access instrument and priority. The concept 80146002 |appendectomy| does not have defined values for the attributes 260870009 |priority| and 425391005 |using access device| but the Concept Model permits these to be added to subtypes of 71388002 |procedure|. Therefore, the following postcoordinated expression can be created: 80146002 |appendectomy|: 260870009 |priority|= 25876001 |emergency|, 425391005 |using access device|= 86174004 |laparoscope| This postcoordinated expression is equivalent to the definition of the concept 174041007 |laparoscopic emergency appendectomy|. However, the postcoordinated approach can also be applied to procedures for which there is no precoordinated concept.
postcoordination by combination
"Gallstones with cholecystitis" could be represented by combining the concepts for the disorders "gallstones" and 76581006 |cholecystitis| as a single postcoordinated statement. Neither of these concepts is really a qualifier of the other since it could equally well be regarded as 25924004 |Calculus of gallbladder with cholecystitis|. SNOMED CT allows Concepts to be combined in postcoordinated statement.
Combinations like this should only be used to represent concepts that can be regarded as discreet reusable clinical statements. They should not be used to construct arbitrarily complex representations of multiple statements to a particular record.
Some concepts, such as the first and last examples above, can be represented in either a postcoordinated or precoordinated form. However, there are other concepts, like the second example above, for which no precoordinated Concept exists in SNOMED CT. Although future releases of SNOMED CT will include new precoordinated Concepts, there will always be some clinical Concepts that require postcoordination.
This guide does not specify a single right way to represent postcoordinated expressions. Alternative representations have different profiles of advantages and disadvantages. The choice of representation depends on functional requirements including performance, information model of the software application and the communication standards to be supported.
Some alternative representations are summarized below. These summaries illustrate some of the main options and do not go into extensive technical detail. Detailed design may lead to further alternatives that are not documented here.
Each of the following summaries assumes that SNOMED CT expressions are stored in (or associated with) one or more fields within particular types of record entry. The expression is only one part of the data in that record entry.
Parsable text representation
- Each clinical statement is recorded as a row in a relational database table (or as an element in an XML document);
The schema for representation of clinical statements contains a field (or element) for representation of the SNOMED CT expression;
Unrestricted relational representation
An unrestricted relational database representation of a postcoordinated expression requires that a data item that may be expressed using SNOMED CT is modeled in a way that permits an indeterminate number of attribute-value pairs to be appended to a focus concept. In addition, the value within each attribute-value pair must be able to be refined by addition of nested attribute-value pairs.
This offers a flexible and extensible approach but adds significantly to database design complexity. Disadvantages arising from this complexity include storage capacity requirements and the impact on writing queries and retrieval performance.
Restricted relational representation
- Each clinical statement is recorded as a row in a relational table.
The clinical statements table contains a field for a Concept Identifier.
The clinical statements table also contains fields for a specified number of qualifiers. These fields may be provided in different ways:
Each qualifier is represented by two Concept Identifier fields (one for the attribute and one for the value) and an optional field for relationshipGroup field. With this option the only restriction is the total number of qualifiers or modifiers that can be stored for each Concept.
Each qualifier is represented as a single Concept Identifier and carries the value of a qualifier attribute specific to that field. This restricts the usable qualifiers to those specified in the database schema.
Similar to above, but with different sets of qualifying attributes available according to the semantic type of the primary Concept in the statement. There are various ways of implementing this approach to ensure that the appropriate interpretation is applied to each row of the table.
Combined Concepts may be represented by explicitly combining two rows of the clinical statements table.
Unlike the representations discussed in previous subsections, this approach limits the expressivity of postcoordinated statements. The advantage of this restricted approach is that it reduces the number of joins involved in retrieval queries. In some software environments this may significantly improve performance.
The balance between demands for flexibility and performance depends on user requirements. Therefore, limitations in expressivity may be acceptable for some users or user communities but not for others. However, it should be noted that these limitations might cause difficulties when communications are received from systems that support richer forms of expression.
- Each clinical statement is recorded as a row in a relational table or as an element in an XML representation.
The clinical statements table (or element) contains a field (or element) for representation of the concept.
- Various alternative XML representations could fulfill this role.
Representation as precoordinated content
User input includes also a text label for the expression, and the new concept is created, usually a team of expert SNOMED CT modelers review the new concept for quality assurance. Other implementations requires that user enter only the text label, and then the modelers team can associate the label to an existing concept, or create a new concept in a local extension using the label as a Description and adding the new Relationships for the concept definition.
This approach is called Managed Content Additions (MCA). Has some advantages like having all new content available for text searches by users, and allowing the use of a description logics classifier for inferring Relationships and super-types, avoiding the need of complex real-time expressions computations. On the other having a centralized team of experts represents an expensive approach and a possible bottleneck for terminology development, as the experts need to review all content additions in the system.
Storing and retaining original expressions
Transforming an expression to a normal form may be necessary to support effective data retrieval. However, even quite small minor corrections to the definition of a concept in future releases may significantly alter the resulting normal form of the same expression.
Therefore, it is recommended that:
- The primary or original record should be stored using the representation that is as close as possible to the form in which it was recorded.
If transformations to alternative representations are used to enhance the efficiency of retrieval, these should be stored as secondary supporting tables or indices:
This has the advantage that these alternative forms can be regenerated based on the most up to date set of definitions when a new release of SNOMED CT is installed, without affecting the integrity of the original records.