esearchfcgi

esearch.fcgi, often accessed as the esearch endpoint of the NCBI E-utilities, is a web API that implements the ESearch function. It enables programmatic searching across NCBI databases such as PubMed, Nucleotide, Protein, and Gene, returning a list of matching record identifiers and related metadata. The endpoint is typically used to perform a query, retrieve a count of results, and obtain either a short list of IDs or a full history of results for later retrieval.

The interface accepts standard query parameters, including the database to search (db) and the search expression (term). Additional parameters control the number of results returned (retmax), the starting point for paging (retstart), and the format of the response (retmode), which can be XML or JSON in common usage. A key feature is the option to use a history server by setting usehistory=y, which stores results on the server and returns a WebEnv and optional QueryKey values. These values can be used in subsequent calls to efetch, esummary, or related utilities to retrieve detailed records without repeating the search.

Typical workflows involve an initial esearch call to obtain IDs and, if desired, to establish a history, followed by subsequent requests to efetch or esummary to fetch full records or summaries by passing the WebEnv/QueryKey. This separation supports efficient data retrieval for large result sets and batch processing.

Usage should comply with NCBI’s guidelines, including providing a valid tool and contact information, and observing rate limits to avoid undue load on the service.