SNOMED CT Editorial Guide

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SNOMED CT contains concepts that include context information, and concepts that are regarded as context-free. A concept includes context information if the name of the concept explicitly represents information that might otherwise be represented by another less context-rich concept in a particular place within an electronic health record or EHR. 

In SNOMED CT, context describes the effects of embedding a concept in a clinical situation, i.e. when it is used in an EHR.

For example, 

    • When the concept  22298006 | Myocardial infarction (disorder)|  is used in an EHR, it takes on a specific contextualized meaning. The meaning might be an assertion by the person entering the information, that on a given date, the patient was diagnosed with a myocardial infarction. Or, it may be used to document a complication of smoking, a protocol for chest pain, a medication contraindication, a part of a patient's medical history, a possible diagnosis justifying a diagnostic test, a diagnosis excluded by a diagnostic test, a patient's family history, etc.
    • The concept for breast cancer, 254837009 | Malignant neoplasm of breast (disorder)| , might be used to indicate either a current diagnosis of breast cancer, a family history of breast cancer, or a past history of breast cancer. Each of these three meanings differs in regard to the context in which breast cancer is described. 
      • Current diagnosis of breast cancer indicates that the breast cancer is present now, and in this patient.
      • Family history of breast cancer refers to breast cancer occurring in a family member of a patient. 
      • Past history of breast cancer indicates that the breast cancer occurred in the patient, at some time in the past, and it is not necessarily present now.

Not only are the differences significant relative to a patient's health record, but they are also important to population-based data retrieval; e.g. it is incorrect to retrieve those who have a family history of breast cancer when searching for patients with a diagnosis of breast cancer.

Default context

When a SNOMED CT concept appears in an EHR without any explicitly stated context, that concept is considered to have a default context.  However, the information in the health record structure or information model, can override the default context.

Default context for a Clinical finding concept implies that the finding is present (vs. being absent), that it applies to the subject of the record (the patient), and that it is current (or at a specified time in the past, linked to the concept). 

Default context for a Procedure concept implies that the procedure was completed, that it was performed on the subject of the record (the patient), and that it was done at the present time (or at a specified time in the past, linked to the concept).

Explicit context

Concepts in the Situation hierarchy (given the appropriate record structure) have explicit context and can represent Clinical findings and Procedures that:

Have not yet occurred

For example, 

Refer to someone other than the patient

For example, 

Have occurred at some time prior to the time of the current entry in the record

For example,

Attributes

These attributes are used to represent Clinical finding and Procedure concepts within the Situation hierarchy:


Clinical FindingProcedure
AttributesAssociated findingAssociated procedure
Finding contextProcedure context
Subject relationship contextSubject relationship context
Temporal contextTemporal context

Expressing context

Context typically alters the meaning of a concept, i.e. the resulting concept is no longer a subtype of the original concept. 

Precoordinated expression. Clinical context is specified in the description and entered into a field in a patient's EHR.

For example,

Postcoordinated expression. Clinical context is specified by combining concepts.

For example,

Concept or expression in an EHR field. A concept is placed in a field with a predefined meaning in an electronic health record. The meaning is conveyed by the context in which it is recorded. 

For example, 

    • Hip replacement planned might be represented as 397956004 | Prosthetic arthroplasty of the hip (procedure)|  within a section of a patient's health record called Planned actions. A planned hip replacement is not a kind of hip replacement, so the Planned actions record section modifies the context
    • 2004005 | Normal blood pressure (finding)|  might be placed in a field labeled as Goal in a patient's EHR. A goal of normal blood pressure is not a kind of Normal blood pressure (finding), so the Goal field in the EHR modifies context.

Concepts in medical records

When a user places a concept from SNOMED CT in a patient's health record, it transforms the concept from a theoretical representation of a clinical notion to an actual instance of the concept.

For example, 

The placement of a concept in an EHR field may:

  • Affect the quality of the meaning, but not the instance. The placement of 194828000 | Angina (disorder)| in a field labeled Current problems, Past medical history, or History of indicates that an instance of angina has occurred in the patient. The specific field affects the quality of the meaning, but not the instance. The adopted context is compatible with the default context.
  • Critically affect the meaning and the instance. The placement of 49049000 | Parkinson's disease (disorder)| in a Family history field or 41339005 | Coronary angioplasty (procedure)| in a Planned procedures field does not indicate that an instance of the disorder or the procedure has occurred in the patient. The adopted context is incompatible with the default context (In these circumstances, the electronic health application programmer needs to identify the appropriate context values from a authoritative list and link them to the concepts placed in the fields to substitute for their default contexts). 

When a Situation with explicit context concept is used in an EHR, it should contain all of the context attributes and applicable values in order to guarantee accurate meaning if that concept (plus context) is subsequently transferred to another record environment.

Elaboration: changing concept meaning

Elaboration in SNOMED CT refers to any addition to or change of the meaning of a concept that may be brought about when it is embedded in a clinical situation. Embedding a concept in a clinical situation may elaborate the semantic interpretation of a concept in one of the following ways:

  1. Subtype qualification
  2. Axis modification
  3. Affirmation or Negation
  4. Combination

Subtype qualification

A subtype qualification refines the meaning of a concept.  Subtype qualification is elaboration that results in a concept that is a subtype of the original unelaborated focus concept. A focus concept is the part of a SNOMED CT expression that represents a clinical finding, observation, event, or procedure. It may be given context by a surrounding context wrapper and may be made more specific by a refinement.

For example,

    • A past history of replacement of the left hip may be represented by a SNOMED CT expression in which the focus concept, hip replacement, is refined by laterality, left and enclosed in a context wrapper representing past history.

Subtype qualification

Subtype qualification has also been called a qualifier (e.g. ENV136060, GEHR, CTV3) or a secondary status term (e.g. NHS Context of Care). In SNOMED CT, the term subtype expresses more clearly the distinctive property of a qualifier.  This is helpful because the meaning of modify and qualify are synonymous in many dictionaries and by some International Organization of Standardization (ISO) authorities.

Axis modification

The attributes used to define situation concepts permit explicit (rather than default) representation of various contexts. These attributes can change the meaning of a Clinical finding or Procedure concept in a way that changes the hierarchy (or axis) of the concept from Clinical finding or Procedure to Situation with explicit context. The resulting modified meaning is not a subtype of the original meaning of the concept, and therefore the axis-modifying attributes are not used to qualify the concept, but instead are used to qualify a Situation concept.

For example, 

Axis modification

Axis modification is not the same as affirmation (present) or negation (not present) of a concept, where the essential characteristics of the concept are unchanged.

Affirmation and Negation

Depending on perspective, affirmation and negation may simply be viewed as the inversion of meaning of an unelaborated concept that represents a Clinical finding. A concept may be stated in the negative in a clinical situation (e.g. meningism not present). This creates the potential for a concept to represent two meanings, one of which is the inverse of the other. However, the effects of negation on interpretation are far-reaching and distinct from other elaborations.

Negation, like axis modification, results in a concept that is not a subtype of the unelaborated concept. However, negation explicitly rules out the unelaborated concept.

For example,

    • Family history of myocardial infarction does not imply that a patient had a myocardial infarction.
    • No headache implies that patient has headache is untrue. A negative statement may expand further in the opposite direction of a positive statement. If headache is a subtype of pain then patient has headache implies patient has pain. However, patient has no headache does not imply patient has no pain. Conversely, patient has headache does not imply patient has occipital headache, but patient has no headache implies patient does not have occipital headache.

Negation

The representation of negation within SNOMED CT that arises from restrictions imposed by the existing description logics results in the hierarchy being inverted e.g., coronary heart disease not present is NOT properly a subtype of "Heart disease not present".  An initial attempt was made to move negated content into the situation hierarchy so that the content remained available, but SNOMED International recommends handling negation outside of SNOMED CT by the EHR vendor rather than try and represent it within the terminology.


A concept may be stated to be possible in a clinical situation. Statements that explicitly indicate uncertainty can be considered in two possible ways:

  • Somewhere between affirmation and negation
  • As a type of elaboration

Combination

Two or more concepts may be embedded in a clinical situation in a way that links them together. Linkages may include:

  • Simple combination of concepts
  • Combination of a concept that is present and another that is absent

Context shift

Once a concept has context-shifted and become context-dependent, it should not be used in an expression that once again shifts context. In other words, when one context attribute is given an axis modifying value, the other context attributes are fixed.

For example,

    • The model for 430679000 | Family history of diabetes mellitus type 2 (situation)|  IS A Situation with explicit context (situation) with:
      • Subject relationship context of Person in family of subject (person)
      • Associated finding of Diabetes mellitus type 2 (disorder)
      • Finding context of Known present (qualifier value)
      • Temporal context of Current or past (actual) (qualifier value)

Even though the Family part of the concept results in an explicit axis shift of the Subject relationship context only, SNOMED CT requires default values for Finding context and Temporal context, rather than allowing them to be unspecified. 

To negate a concept with Finding context (attribute) of Known present (qualifier value), the Finding context (attribute) should instead have a value of Known absent (qualifier value).

For example,

    • The concept 160273004 | No family history: Hypertension (situation)|  negates 160357008 | Family history: Hypertension (situation)|  by changing the value of Finding context (attribute) to Known absent (qualifier value) with Temporal Context (attribute) of All times past (qualifier value). The parent IS A Situation with explicit context (situation) with:
      • Temporal context of All times past (qualifier value)
      • Associated finding of Hypertensive disorder, systemic arterial (disorder)
      • Finding context of Known absent (qualifier value)
      • Subject Relationship Context of Person in family of subject (person)
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