Medical Subject Headings

Medical Subject Headings (MeSH) is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences; it can also serve as a thesaurus that facilitates searching. Created and updated by the United States National Library of Medicine (NLM), it is used by the MEDLINE/PubMed article database and by NLM's catalog of book holdings.

MeSH can be browsed and downloaded free of charge on the Internet through PubMed. The yearly printed version was discontinued in 2007 and MeSH is now available online only. Originally in English, MeSH has been translated into numerous other languages and allows retrieval of documents from different languages.

Structure of MeSH
The 2009 version of MeSH contains a total of 25,186 subject headings, also known as descriptors. Most of these are accompanied by a short description or definition, links to related descriptors, and a list of synonyms or very similar terms (known as entry terms). Because of these synonym lists, MeSH can also be viewed as a thesaurus.

Descriptor hierarchy
The descriptors or subject headings are arranged in a hierarchy. A given descriptor may appear at several places in the hierarchical tree. The tree locations carry systematic labels known as tree numbers, and consequently one descriptor can carry several tree numbers. For example, the descriptor "Digestive System Neoplasms" has the tree numbers C06.301 and C04.588.274; C stands for Diseases, C06 for Digestive System Diseases and C06.301 for Digestive System Neoplasms; C04 for Neoplasms, C04.588 for Neoplasms By Site, and C04.588.274 also for Digestive System Neoplasms. The tree numbers of a given descriptor are subject to change as MeSH is updated. Every descriptor also carries a unique alphanumerical ID that will not change.

Descriptions
Most subject headings come with a short description or definition. See the MeSH description for diabetes type 2 as an example. The explanatory text is written by the MeSH team based on their standard sources if not otherwise stated. References are mostly encyclopaedias and standard textbooks of the subject areas. References for specific statements in the descriptions are not given, instead readers are referred to the bibliography.

Qualifiers
In addition to the descriptor hierarchy, MeSH contains a small number of standard qualifiers (also known as subheadings), which can be added to descriptors to narrow down the topic. For example, "Measles" is a descriptor and "epidemiology" is a qualifier; "Measles/epidemiology" describes the subheading of epidemiological articles about Measles. The "epidemiology" qualifier can be added to all other disease descriptors. Not all descriptor/qualifier combinations are allowed since some of them may be meaningless. In all there are 83 different qualifiers.

Supplements
In addition to the descriptors, MeSH also contains some 139,000 Supplementary Concept Records. These do not belong to the controlled vocabulary as such and are not used for indexing MEDLINE articles; instead they enlarge the thesaurus and contain links to the closest fitting descriptor to be used in a MEDLINE search. Many of these records describe chemical substances.

Use in Medline/PubMed
In MEDLINE/PubMed, every journal article is indexed with some 10-15 headings and subheadings, with some of them designated as major and marked with an asterisk. When performing a MEDLINE search via PubMed, entry terms are automatically translated into (= 'mapped to) the corresponding descriptors with a good degree of reliability; it is recommended to check the Details tab in PubMed to see how a search formulation was 'translated'. By default a search will include all the descriptors that are located below the given one in the hierarchy.

Categories
The top-level categories in the MeSH descriptor hierarchy are:
 * Anatomy [A]
 * Organisms [B]
 * Diseases [C]
 * Chemicals and Drugs [D]
 * Analytical, Diagnostic and Therapeutic Techniques and Equipment [E]
 * Psychiatry and Psychology [F]
 * Biological Sciences [G]
 * Physical Sciences [H]
 * Anthropology, Education, Sociology and Social Phenomena [I]
 * Technology and Food and Beverages [J]
 * Humanities [K]
 * Information Science [L]
 * Persons [M]
 * Health Care [N]
 * Publication Characteristics [V]
 * Geographic Locations [Z]

For the full hierarchy, see List of MeSH codes.