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PubMed Tutorial from The Levy Library: Combining Terms


To add terms to your search you can use AND, OR or NOT, which should be capitalized to ensure best results.

Note: PubMed automatically puts an AND between concepts in keyword searches. So, heart attack prevention retrieves the same number of results as heart attack AND prevention

Using AND, OR and NOT PubMed Results
e-coli AND hamburger

Using AND will retrieve fewer results because both terms must be present in the reference.

Use AND to retrieve a set in which each citation contains all search terms.

anorexia OR bulimia

Using OR will retrieve more results because either term can be present in the reference.

Use OR when you want to pull together articles on similar topics.

mutation NOT frameshift

Use NOT to exclude a search term from your search.

Use NOT with caution because it may eliminate relevant results.

Use Parentheses for Longer Searches

PubMed processes ANDs, ORs and NOTs in a left-to-right sequence.

Try this: You want to research remedies for the common cold. Search for common cold AND vitamin C OR zinc. These results are suboptimal because PubMed retrieved all occurrences of "zinc" in the database, not only those in which the a term related to common cold appears. 

Using parentheses changes the order in which PubMed processes the search statement.

Now try this: Search for common cold AND (vitamin C OR zinc). Now your results will all pertain to either common cold and vitamin C or to common cold and zinc.  No results should be pertain to zinc, but not to common cold.

Searching for Genes

Automatic Term Mapping does not work for most gene names and symbols, even though a link to the Gene database may appear.

Example: Search for the gene SCGB2A2.

Notice a blue box appears above your results with a link to information about the gene, including synonyms.

BUT, scroll down to the Search Details box: this term does not map, meaning that synonyms like MGB1 and mammaglobin are not searched and papers like this one won't appear in your results list.

Best Practice

If you are looking for articles about a gene, keep in mind that different authors may refer to it with different names. You may need to combine the different names of the gene with an OR to create a more complete search, for example:SCGB2A2 OR MGB1 OR mammaglobin.

NCBI's Gene database is a good source for synonyms to build a more comprehensive PubMed search.