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Fact Sheet
Medical Subject Headings (MeSH®)

The Thesaurus

MeSH is the National Library of Medicine's controlled vocabulary thesaurus. It consists of sets of terms naming descriptors in a hierarchical structure that permits searching at various levels of specificity.

MeSH descriptors are arranged in both an alphabetic and a hierarchical structure. At the most general level of the hierarchical structure are very broad headings such as "Anatomy" or "Mental Disorders." More specific headings are found at more narrow levels of the thirteen-level hierarchy, such as "Ankle" and "Conduct Disorder." There are 27,883 descriptors in 2016 MeSH with over 87,000 entry terms that assist in finding the most appropriate MeSH Heading, for example, "Vitamin C" is an entry term to "Ascorbic Acid." In addition to these headings, there are more than 232,000 Supplementary Concept Records (SCRs) within a separate file. Generally SCR records contain specific examples of chemicals, diseases, and drug protocols. They are updated more frequently than descriptors. Each SCR is assigned to a related descriptor via the Heading Map (HM) field. The HM is used to rapidly identify the most specific descriptor class and include it in the citation.

MeSH Applications

The MeSH thesaurus is used by NLM for indexing articles from 5,400 of the world's leading biomedical journals for the MEDLINE®/PubMED® database. It is also used for the NLM-produced cataloging database that includes bibliographic descriptions of books, documents, databases, and audiovisuals acquired by the Library. Each bibliographic reference is associated with a set of MeSH terms that describe the content of the item. Similarly, search queries use MeSH vocabulary to find items on a desired topic.

Establishing and Updating MeSH

The Medical Subject Headings Section staff continually revise and update the MeSH vocabulary. Staff subject specialists are responsible for areas of the health sciences in which they have knowledge and expertise. In addition to receiving suggestions from indexers and others, the staff collect new terms as they appear in the scientific literature or in emerging areas of research; define these terms within the context of existing vocabulary; and recommend their addition to MeSH. Professionals in various disciplines are also consulted regarding broad organizational changes and close coordination is maintained with various specialized vocabularies.

MeSH Data

MeSH, in machine-readable form, is provided at no charge via electronic means. The MeSH Web site is the central access point for additional information about MeSH and for obtaining MeSH in electronic form. You can also obtain MeSH in XML and other formats from the MeSH download page

An RDF format version of MeSH is also available.

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