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Search Basics for the Health Sciences: Keywords vs. Subject Headings

A guide on basic searching skills that can be applied across databases used for the health sciences


Once you have your PICO(T) terms, you will need to enter them into whatever database or search engine you are using. Within databases, you have the option of using keywords or subject headings for the different parts of your PICO(T).

Each database has its own set of subject headings, designed specifically for the literature from the field(s) of study the database contains. Knowing the difference between keywords and subject headings, as well as the advantages and disadvantages for both of them, can help you perform better searches.


Keywords are:

  • natural language terms that describe your topic
  • able to be combined in any number of ways
  • lacking consistency in usage, definition, and sometimes spelling (e.g. GERD vs. GORD[U.K.])
  • either single words or phrases
  • used to search for matching words or phrases anywhere in the records the database contains (such as title, abstract, journal title)
  • used when no appropriate subject heading exists as an equivalent
  • sometimes either too broad or too narrow, resulting in either too many or too few results
  • reflective of recent phenomena in advance of when the subject headings are added

Examples of Keywords vs. Subject Headings

The chart below shows some examples of keywords and the equivalent subject headings in CINAHL and MEDLINE

Graph of differences between keywords and subject headings. Examples include the keyword "heart attack" versus the subject heading "myocardial infarction"

Note that the Subject Headings in CINAHL and MEDLINE are not always the same. In the last example--LGBT--note that while CINAHL has a subject heading for the term, MEDLINE requires that two different subject headings have to be combined to create an equivalent. For more information on how to combine related MeSH terms, see the next page.

Subject Headings

Subject Headings are:

  • “controlled” vocabulary used by an organization (e.g. the National Library of Medicine) to describe the concepts in the literature collected by that organization or database (such as MEDLINE or CINAHL).
  • Consistent in their definition across the records in the database.
  • Less flexible and must be chosen from the thesaurus used by the database; if the incorrect subject heading is selected, none of the results will be relevant.
  • Only searched for in the subject heading field of the record.
  • Helpful for retrieving a set of articles with fewer irrelevant results
  • Slow to change--this means that the most recent changes in knowledge--on diseases, drugs, devices, procedures, concepts--may not be reflected in the controlled vocabulary.

Which Should I Use?

The simple answer to the question of whether you should use keywords or subject headings is: it depends. Some basic guidelines are:

  • If the term or topic is very recent, keywords may be the best option
  • If no Subject Heading exists for your term, or seems inadequate, use a keyword
  • If the keyword is too vague or broad, a Subject Heading may help focus your search and eliminate too many results
    • e.g. neuroses would be a very broad keyword search
  • If you want a very comprehensive literature search, you should use both a keyword and a subject heading
    • e.g. Heart attack OR Myocardial Infarction