Eutilities

Eutilities, short for Entrez Programming Utilities, are a set of network services provided by the National Center for Biotechnology Information (NCBI) that enable programmatic access to the NCBI Entrez databases. They are designed to support automated search, retrieval, and linking of data across resources such as PubMed, GenBank, RefSeq, GEO, and others, allowing researchers to incorporate NCBI data into software workflows.

The suite includes several utilities with distinct roles. EInfo provides database metadata and capabilities; ESearch queries databases and returns a list of identifiers; EPost posts lists of IDs to a WebEnv session for later retrieval; EFetch retrieves full records; ESummary returns concise summaries; ELink finds linked records across databases; ESpell performs spelling suggestions; EGQuery handles server-wide query translation. A common workflow is to use ESearch to obtain IDs, then EFetch or ESummary to retrieve data, and ELink to discover related records. Large result sets can be posted with EPost and accessed later using a WebEnv and a QueryKey.

Requests are made via HTTP to the base URL https://eutils.ncbi.nlm.nih.gov/entrez/eutils/ and can return data in formats such as XML or plain text; some endpoints also offer JSON via the retmode parameter depending on the utility. For higher throughput, NCBI provides API keys that raise rate limits (up to ten requests per second with a key, versus three per second without). Users are encouraged to include a tool name and contact email and to follow NCBI usage guidelines to ensure stable service for all users.

E-utilities are commonly used for automated literature searches, sequence and record retrieval, and integration of NCBI data into bioinformatics pipelines, making data access repeatable and scalable while requiring careful adherence to usage policies.