Use of MeSH in Indexing
The MeSH vocabulary is designed for use by NLM to index the MEDLINE database of journal citations and other media, and to search the MEDLINE data using PubMed. The following outlines how MeSH is used by indexing of the MEDLINE journal articles. For more detailed information see Bachrach and Charen1. See also Use of MeSH in Online Retrieval and Use Of Medical Subject Headings For Cataloging.
1. Combining specific MeSH Descriptors to index complex subjects: Co-ordination
Rather than creating a MeSH Descriptor for every subject that might discussed in a journal article, multiple MeSH Descriptors or Qualifiers are combined to index the desired subject. For example, jejunitis may be expressed by the use of the Descriptors Jejunum and Enteritis. Similarly, MeSH Qualifiers can be used in conjunction with appropriate Descriptors. For example, a deficiency of monoamine oxidase may be indexed by the Descriptor Monoamine Oxidase, combined with the Qualifier /Deficiency. This combining procedure is called "coordinated" indexing.
Note that not all complex subjects are indexed by co-ordination. When a particular complex subject occurs frequently, a "pre-coordinated" Descriptor may be created. For example, for a subject of arm injuries, instead of combining the Descriptor Arm with the Qualifier /Injuries, the single Descriptor Arm Injuries is used.
Articles may discuss multiple subjects in which case indexers supply Descriptors for all subjects. For example, "an article in an endocrinology journal does not relieve the indexer of covering the non-endocrine aspects of the text." (Bachrach and Charen, p. 25)
Indexers generally use the most specific MeSH subject heading available, rather than each of the broader subjects in which a searcher may be interested. For example, an article about pulmonary pathology is indexed under the Descriptor Lung rather than the more general Respiratory System. The reasons for indexing to the most specific are: (a) it records more information, and (b) it avoids multiple, redundant, indexing since the broader subjects are linked to the more specific subjects by the hierarchical MeSH Tree Structures. For example, in PubMed a search for Respiratory System by default includes articles indexed to Lung and other more specific Descriptors.
1Bachrach, C. A. and Charen, Thelma. "Selection of MEDLINE contents, the development of its thesaurus, and the indexing process." Med Inform (Lond). 1978 Sep;3(3):237-54. Note especially "Indexing principles, pp. 250ff.