eSearch

ESearch is a web-based search utility that forms part of the Entrez system operated by the National Center for Biotechnology Information (NCBI). It functions as the primary search component of the Entrez Programming Utilities (E-utilities), providing a programmatic way to query various NCBI databases such as PubMed, Gene, Nucleotide, and others. The main purpose of ESearch is to return a list of identifiers (UIDs) that match a specified query, which can then be used to retrieve full records through other E-utilities like eFetch or eSummary.

ESearch accepts a range of parameters to control the scope and format of results. The essential parameters include db, which specifies the target database; and term, which contains the search query. Other common parameters include retmax to limit the number of IDs returned, retstart for pagination, and retmode to choose the output format (for example, xml or json). A notable capability is usehistory, which stores results on the NCBI server and returns webenv and querykey values that enable subsequent requests to fetch complete records without repeating the query.

In typical workflows, a user first submits a query to esearch.fcgi and receives a response containing an idlist of matching UIDs, along with a count of total matches. If usehistory is enabled, the response also provides a webenv and querykey, allowing subsequent requests to eFetch or eSummary to retrieve detailed records or summaries for the identified items. This separation helps manage large result sets and supports efficient data retrieval.

ESearch is widely used in bioinformatics and systematic literature searches to programmatically locate relevant records, enabling automated workflows, data mining, and integration with other analysis tools. It is important to consult the official NCBI documentation for up-to-date parameters, formats, and usage limits.