SNOMED CT

SNOMED (Systematized Nomenclature of Medicine), is a systematically organised computer processable collection of medical terminology covering most areas of clinical information such as diseases, findings, procedures, microorganisms, pharmaceuticals etc. It allows a consistent way to index, store, retrieve, and aggregate clinical data across specialties and sites of care. It also helps organising the content of medical records, reducing the variability in the way data is captured, encoded and used for clinical care of patients and research.

Purpose
Clinicians and organizations use different clinical terms that mean the same thing. For example, the terms heart attack, myocardial infarction, and MI may mean the same thing to a cardiologist, but, to a computer, they are all different. There is a need to exchange clinical information consistently between different health care providers, care settings, researchers and others (semantic interoperability), and because medical information is recorded differently from place to place (on paper or electronically), a comprehensive, unified medical terminology system is needed as part of the information infrastructure.

Design
SNOMED CT is a compositional concept system, which means that concepts can be specialised by combinations with other concepts. It is based on Description Logic and is designed so that content can be maintained as a dynamic resource.

Components

 * Concepts: Basic unit of meaning designated by a unique numeric code, unique name (Fully Specified Name), and descriptions, including a preferred term and one or more synonyms.
 * Descriptions: Terms or names (synonyms) assigned to a concept.
 * Hierarchies: 19 higher level hierarchies; each has sub-hierarchies
 * Relationships: Link concepts either within a hierarchy or across hierarchies
 * Subsets

Design fundamentals
Local extension of SNOMED CT is achieved by sanctioned specialisation, where new concepts are defined as controlled combinations of those already known. For example, a basic concept such as [burn] may be specialised / further qualified with a [severity] and a [body site], itself specialised, in order to define a new very detailed clinical concept such as a 'severe burn of the skin of the webspace between the left fourth and fifth toes':

284196006|Burn of skin|: 246112005|Severity|=24484000|severe, 363698007|Finding Site|= (113185004|Structure of skin between fourth and fifth toes|:272741003|Laterality|=7771000|left)

Such expressions are said to have been 'post-coordinated' by contrast with the set of entities already included in a given SNOMED CT release, which are normally collectively labelled 'pre-coordinated' concepts, even though the majority (85%) of these pre-coordinated concepts are currently in fact primitive entities.

NB: in other contexts or projects 'coordination' may refer not to the specification of a candidate new expression but rather to the subsequent operation of integrating one within the polyhierarchy (ie classifying it). In SNOMED CT terminology, saying that an expression has been 'post-coordinated' does not imply that it has also been classified.

Reliable analysis and comparison of any such post-coordinated expressions - with respect to both those concepts already within the SNOMED CT release dataset and any other ad hoc concepts created or yet to be created by its community of endusers - properly requires the application of an appropriate description logic classification algorithm. As of 2007, SNOMED CT content limits itself to a subset of the EL++ formalism, restricting itself to the following operators:

top, bottom primitive roles and concepts with asserted parent(s) for each concept definition and conjunction but NOT disjunction or negation role hierarchy but not role composition domain and range constraints existential but not universal restriction a restricted form of role inclusion axiom (xRy ^ ySz => xRz)

The logic may be extended in the near future to include General Concept Inclusion Axioms.

In theory, description logic reasoning can be applied to any new candidate post-coordinated expressions in order to assess whether it is a parent or ancestor of, a child or other descendent of, or semantically equivalent to any existing concept from the 370,000 pre-coordinated concepts which are already distributed worldwide. However, partly as the continuing fall-out from the merger with CTV3, SNOMED content in 2007 still contains undiscovered semantically duplicate primitive and defined concepts. Additionally, many concepts remain primitive whilst their semantics can also be legitimately defined in terms of other primitives and roles concurrently in the system. Because of these ommissions and actual or possible redundancies of semantic content, real-world performance of algorithms to infer subsumption or semantic equivalence will be unpredictably imperfect.

Features
Significant features of SNOMED CT by comparison with more traditional and familiar clinical terminologies and classifications (e.g. ICD) are :

(1) It contains roughly 3 times as many concepts 'out of the box' as most familiar clinical terminologies

(2) The set of entities (or 'concepts') within its list are organised as a polyhierarchy rather than the traditional (statistical) monohierarchy

(3) Clinical end users can dynamically and arbitrarily extend the set of concepts within its list so as to include those additionally required but not already included in a particular release.

Use
SNOMED CT is one of a suite of designated data standards for use in U.S. Federal Government systems for the electronic exchange of clinical health information.

Sample Computer Applications Using SNOMED CT

 * Electronic Medical Records
 * Computerized Provider Order Entry Such As E-Prescribing Or Laboratory Order Entry
 * Remote Intensive Care Unit Monitoring
 * Laboratory Reporting
 * Emergency Room Charting
 * Cancer Reporting
 * Genetic Databases

History
In January 2002, SNOMED CT was created by the merger, expansion, and restructuring of SNOMED RT (Reference Terminology) and the UK National Health Service (NHS) Clinical Terms Version 3 (also known as the Read Codes). The historical strength of SNOMED RT was its terminologies for specialty medicine and methods for distributed collaborative development, while the strength of Clinical Terms Version 3 was its terminologies for general practice. By combining these two systems, SNOMED CT is the most comprehensive clinical vocabulary available in any language, covering most aspects of clinical medicine with over 344,000 concepts. SNOMED CT cross maps to such other terminologies as ICD-9-CM, ICD-O3, ICD-10, Laboratory LOINC and OPCS-4. It supports ANSI, DICOM, HL7, and ISO standards. In April 2002, the SNOMED CT Spanish Edition was released, and in April 2003 the SNOMED CT German Edition was released.

The National Library of Medicine (NLM), on behalf of the U.S. Department of Health and Human Services, entered into an agreement with College of American Pathologists for a perpetual license for the core SNOMED CT (in Spanish and English) and ongoing updates. The contract provides to NLM a perpetual license to distribute SNOMED within the NLM’s Unified Medical Language System UMLS Metathesaurus for no cost use within the U.S. by both U.S. government (federal, state, local, and territorial) and private organizations. The contract also covers updates to SNOMED CT issued by the College of American Pathologists between June 30, 2003 and June 29, 2008.

In April 2007, SNOMED CT was acquired by IHTSDO, The International Healthcare Terminology Standards Development Organisation.