Unified Medical Language System (UMLS)

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Unified Medical Language System (UMLS) is a major medical dictionary which links the major international terminologies into a common structure, allowing for efficient translation and interoperability.


The Unified Medical Language System (UMLS) is an effort to combine different medical vocabularies and coding systems into a unified vocabulary. Over the years many vocabularies have been developed to address specific needs of groups within the health care system. Examples include the CPT-4 for procedure coding, MeSH for litrature indexing, Systematized Nomenclature Of Medicine (SNOMED) for patient specific information and many others. The UMLS groups the different vocabularies under common concepts and defines relationships between these concepts and the terms within concepts. The UMLS currently includes vocabularies form about 140 different sources.

Knowledge sources

The UMLS has three knowledge sources. The Metathesaurus is the result of combining the different vocabularies. The Symantic Network defines 135 symantic types and 54 relationships for these types. Every concept in the Metathesaurus is given one or more type. This helps to develop relationships between concepts in the Metathesaurus. The third knowledge source is the SPECIALIST lexicon. This is a lexicon of common English words and words accruing in the biomedical field. The SPECIALIST lexicon is used for natural language processing by the SPECIALIST natural language processing system (NLP). This lexicon also comes with a group of programs, which help with full text processing. These tools are capable of using free text, HTML and Medline abstracts as input and have a variety of outputs ranging from individual words, terms, multiword terms, phrases, sentences, and sections. There are also tools to normalize words and reorder phrases to help with indexing and matching. Also, tools are available to map text to UMLS concepts and concepts to text. These tools are available with a web-based interface, command line programs and Java APIs.

The UMLS and its tools are available from the National Library of Medicine at no cost for developers and researchers after signing a license agreement.

Other controlled vocabularies

The final category of ANA recognized terminologies that has been recognized is that of Reference Terminologies. These two classifications represent links between the interface terminologies above and terminologies used by other disciplines. They can be used for the exchange of information within and across EHRs, or to create queries that can combine data from many sources. They are the Logical Observation Identifiers Names (LOINC) and the Systemized Nomenclature of Medicine – Clinical Terminology (SNOMED – CT). LOINC was originally developed to standardize terminology surrounding lab test results, but has since incorporated additional kinds of information that represent clinical observations (Regenstrief Institute, 2010). SNOMED – CT, owned by the International Health Terminology Standards Development Organization, is a comprehensive clinical terminology (US National Library of Medicine, 2010). These reference terminologies represent acknowledgement by the ANA that nursing data must map to clinical data from other disciplines to be able to compare and contrast care provision and outcomes across the collaborative health care team.

List of controlled vocabularies