Unified Medical Language System

About the Designer

Designed initially by Donald Lindberg, M.D., Director of the US National Library of Medicine in 1986, the Unified Medical Language System (UMLS) is a controlled compendium of many vocabularies which also provides a mapping structure between them. The UMLS is composed of three main knowledge components: Metathesaurus®, Semantic Network and SPECIALIST Lexicon.

The relationship between the various items below provides a logical understanding of the structure and purpose of these three components:

Metathesaurus ↔ concepts

  • Semantic Network ↔ categories
  • SPECIALIST Lexicon ↔ indices

The number of biomedical resources available to researchers is enormous. Often this is a problem due to the large volume of documents retrieved when the medical literature is searched. The purpose of the UMLS is to enhance access to this literature by facilitating the development of computer systems that understand biomedical language. This is achieved by overcoming two significant barriers: "the variety of ways the same concepts are expressed in different machine-readable sources & by different people" and "the distribution of useful information among many disparate databases & systems". Three main tools are used to accomplish this: Metathesaurus, Semantic Network, and SPECIALIST Lexicon.
The application of the UMLS Knowledge sources varies from the development stages to the end user stages. There are three UMLS Knowledge Sources: the Metathesaurus, the Semantic Network, and the SPECIALIST lexicon. Developers obtain the databases from the National Library of Medicine. System developers can then make modifications to the products to meet the needs of end users. System modifications can include changes to the construction of the databases, enhancements to how data is retrieved, and the overall structure and linkage of the data. Data sources affected by these changes include biomedical and health related data information as well as health informatics. Undertaking such efforts ensures that the end product will serve the needs of the end users.
Metathesaurus is organized by concept or meaning, and each concept has specific attributes that define the meaning. Identical or almost identical concepts are linked together with hierarchical context from the different vocabularies and relationships between the concepts are explained and represented.
The scope of the Metathesaurus is determined by the scope of the source vocabularies. The Metathesaurus itself is produced by the automated processing of a machine-readable version of the source vocabulary, followed by human intervention of editing and review. An individual can obtain a copy of the Metathesaurus, but it is intended to be primarily used by system developers because it is a multi-purpose base resource for the entire UMLS.
The Metathesaurus can supply information to a software program to create new data, answer a user’s inquiries, allow the user to refine his/her query questions and assist in converting the user’s vocabulary to those uniform vocabularies used by standardized classification systems. It can be used in those clinical applications to query clinical databases, can be linked to patient records and when used in concert with the Semantic Network and Specialist Lexicon, it gains further utility by better coherent data results and relational power.


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