SNOMED CT (Systematized Nomenclature of Medicine -- Clinical Terms)

What is SNOMED CT (Systematized Nomenclature of Medicine -- Clinical Terms)?

SNOMED CT (Systematized Nomenclature of Medicine -- Clinical Terms) is a standardized, multilingual vocabulary of clinical terminology that is used by physicians and other healthcare providers for the electronic exchange of health information.

SNOMED CT, which is used in more than 80 countries, is maintained by the International Health Terminology Standards Development Organization (IHTSDO). The IHTSDO is a not-for-profit organization that acquired SNOMED CT in April 2007. The organization operates under the trading name of SNOMED International, which owns, develops, administers and distributes the SNOMED CT standard.

SNOMED CT was originally developed and maintained by the College of American Pathologists (CAP) in the United States. The standard was created through the convergence of CAP's original SNOMED Reference Terminology framework and the U.K.'s Clinical Terms Version 3, formerly known as the Read Codes.

SNOMED International releases SNOMED CT updates on the last day of each month. The U.S. National Library of Medicine (NLM), which is part of the National Institutes of Health, is the U.S. representative to SNOMED International. As such, NLM serves as the U.S. National Release Center for the SNOMED CT International Edition.

Like other countries, the U.S. also maintains and releases its own edition of SNOMED CT. The U.S. Edition is based on the International Edition but adds U.S.-specific content. The U.S. Edition is maintained by NLM, which provides SNOMED CT and other resources to licensees of the NLM Unified Medical Language System Metathesaurus. NLM releases updates of the U.S. version in March and September of each year.

Why is SNOMED CT used?

The SNOMED CT vocabulary encompasses the following types of clinical information:

  • Diseases.
  • Symptoms.
  • Signs.
  • Specimen types.
  • Living organisms.
  • Procedures (diagnostic, surgical, nursing).
  • Chemicals, including the chemicals used in drug preparations.
  • Drugs.
  • Human anatomy.
  • Physiological processes and functions.
  • Patients' occupations.
  • Patients' social contexts, e.g., religion and ethnicity).

By using numbers to represent the above medical concepts, SNOMED CT provides a standard by which medical conditions and symptoms can be referred to by medical and healthcare professionals, such as doctors and nurses, eliminating the confusion that may result from the use of regional or colloquial terms. The numbers may be entered into records by medical scribes working under the direction of doctors or nurses.

The numerical reference system also facilitates the exchange of clinical information among disparate healthcare providers and electronic health record (EHR) systems. SNOMED CT enables different systems to read disparate clinical terminologies with the help of a standardized vocabulary and consistent representation of clinical content. Originally, the standard was designed to enable the exchange of electronic health information (EHI) in the U.S. But, today, it is used in many other countries for the same purpose.

In addition to providing a common vocabulary for medical practitioners to share and access clinical content, SNOMED CT also facilitates clinical documentation and reporting. Through standardization and the use of codes and hierarchies, SNOMED CT removes the barriers that previously existed with respect to the exchange, reporting, and use of clinical and medical information.

How is SNOMED CT organized?

SNOMED CT provides a comprehensive vocabulary of medical terminology that helps to improve how EHI is recorded and shared. Its use can also lead to more informed medical decisions and better overall healthcare.

SNOMED CT is based on a logical model made up of multiple component types that, together, define a hierarchical structure for organizing the medical terminology. Three types of components lie at the heart of this structure: concepts, descriptions and relationships.

A concept is the representation of a unique clinical entity or characteristic. It might represent a diagnosis, medical procedure, anatomical structure, vital sign, organism, medical center or one of a variety of other items. The SNOMED CT vocabulary currently includes over 360,000 medical concepts, which can be used individually or combined to describe a complex condition.

Each concept is represented by a unique numerical identifier that distinguishes it from all other concepts. By using these identifiers, SNOMED CT provides an international standard for referring to medical conditions and symptoms. The numerical reference system also facilitates the exchange of clinical information among disparate healthcare providers and electronic medical record systems.

Each concept is also associated with a set of descriptions that provide human-readable terms for referencing the concept. There are two types of descriptions: fully specified name (FSN) and synonym. The FSN is a unique description that clearly distinguishes the concept from all other concepts. A concept can have only one FSN for each language or dialect, although it can have multiple synonyms.

A synonym is an alternative label for a concept. For each language or dialect, one of those synonyms is considered "preferred." The preferred description is the term commonly used in clinical settings to refer to the concept. The other synonyms can also be used, but they are labeled "acceptable" rather than preferred, which is merely a way to distinguish them from the preferred synonym.

A relationship defines how two concepts are linked together. SNOMED CT defines multiple relationship types, but the most common is the |Is a| relationship. (Note that the SNOMED CT documentation encloses component names, such as concepts or relationships, in vertical bars.)

The |Is a| relationship type is the one used to define the SNOMED CT's hierarchical structure. For example, |myocardial infarction| has an |Is a| relationship to |myocardial necrosis|, which means that |myocardial infarction| is a subtype of |myocardial necrosis| in the larger hierarchical structure, as shown here.

SNOMED CT screenshot
A search engine from West Coast Informatics provides a way to find information about SNOMED CT concepts and their relationships.

This is an online browser that is available through West Coast Informatics. The browser provides a search engine for finding information about SNOMED CT concepts and their relationships. It is one of many such browsers available for finding SNOMED CT information. In this case, the search results are for the concept |myocardial infarction|, which has a unique identifier of 22298006.

As shown, the concept is named "myocardial infarction," which is the concept's preferred synonym. Other synonyms include |cardiac infarction|, |heart attack| and |myocardial infarct|. However, the concept's official FSN is |myocardial infarction (disorder)|.

The hierarchical relationship shown is determined by the |Is a| relationships that exist between the different concepts. As indicated, |myocardial infarction| has an |Is a| relationship to |myocardial necrosis|, and |myocardial necrosis| has an |Is a| relationship to |myocardial lesion|.

At the top of the hierarchy is the main |SNOMED CT| concept, which incorporates the entire SNOMED CT vocabulary. Directly beneath this is the |clinical finding| concept, which is one of the standard's 19 children concepts. Other parent concepts include |procedure|, |body structure|, |pharmaceutical/biologic product|, |environments and geographical locations|.

The |Is a| relationship is not the only type of relationship used in the SNOMED CT vocabulary. For example, the |finding site| relationship links a condition to a body site, the |causative agent| relationship associates a causative agent with a disease, and the |procedure device| relationship specifies the device associated with a procedure.

A single concept is often linked to multiple other concepts through the various types of relationships. This multilink pattern is repeated throughout the SNOMED CT vocabulary, resulting in a web of interconnectivity between concepts. As a result, the vocabulary's structure is sometimes referred to as a polyhierarchy, rather than a basic hierarchy, such as the type used to represent an operating system's file system.

The SNOMED CT vocabulary also includes other types of components, such as the reference set, which provides a flexible mechanism for adding customizations or enhancements to the standard. However, it is still the three main component types -- concepts, descriptions and relationships -- that define the overall structure and how the various concepts fit together to form the polyhierarchy.

Using SNOMED CT with other standards

The SNOMED CT standard can be used with other international standards, including the following:

  • ICD-9-CM (International Classification of Diseases, Ninth Revision, Clinical Modification). A standard for assigning codes to diagnoses and procedures in the U.S.
  • ICD-10 (International Classification of Diseases, Tenth Revision). An update on ICD-9 -- it provides an internationally comparable and usable format to collect, process, classify and present mortality statistics.
  • CPT (Current Procedural Terminology). A set of standardized codes for medical services and procedures to enable medical reporting, care reviews and administrative management.

Mapping SNOMED CT to these other standards further simplifies medical information access and reporting for medical professionals. It also eliminates confusion and makes it easier to identify the codes in different standards, thus improving the efficiency and productivity of these providers.

More on history and development of SNOMED CT

Development on SNOMED CT started in the 1960s with the publication of the Systematized Nomenclature of Pathology (SNOP) by CAP. The SNOP nomenclature described terms related to morphology (the study of the arrangement of parts of organisms in the context of their functions) and anatomy (the study of the body of organisms and its structure).

In 1975, SNOP was expanded to form SNOMED. However, it was only in 2002 that clinical terms were added to SNOMED to form SNOMED CT. In 2007, IHTSDO was established by nine countries: Australia, Canada, Denmark, Lithuania, Sweden, the Netherlands, New Zealand, the U.K. and the U.S. The international not-for-profit organization is based in the U.K.

After its formation, IHTSDO purchased the intellectual property rights to all versions of SNOMED from CAP. Since then, IHTSDO and its nine charter member countries have promoted the international distribution, adoption and use of SNOMED CT. IHTSDO also manages new updates to the terminology in what is designed to be a transparent and fair manner, and it administers the rights to the standard.

Learn about the role EHRs play in clinical informatics, and see how health information exchanges can support public health and equity.

This was last updated in January 2024

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