PubMed E-utilities API Reference

PubMed E-utilities API Reference

Overview

The NCBI E-utilities provide programmatic access to PubMed and other Entrez databases through a REST API. The base URL for all E-utilities is:

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/

API Key Requirements

As of December 1, 2018, NCBI enforces API key usage for E-utility calls. API keys increase rate limits from 3 requests/second to 10 requests/second. To obtain an API key, register for an NCBI account and generate a key from your account settings.

Include the API key in requests using the &api_key parameter:

esearch.fcgi?db=pubmed&term=cancer&api_key=YOUR_API_KEY

Rate Limits

• Without API key: 3 requests per second
• With API key: 10 requests per second
• Always include a User-Agent header in requests

Core E-utility Tools

1. ESearch - Query Databases

Endpoint: esearch.fcgi

Purpose: Search an Entrez database and retrieve a list of UIDs (e.g., PMIDs for PubMed)

Required Parameters:

• db - Database to search (e.g., pubmed, gene, protein)
• term - Search query

Optional Parameters:

• retmax - Maximum records to return (default: 20, max: 10000)
• retstart - Index of first record to return (default: 0)
• usehistory=y - Store results on history server for large result sets
• retmode - Return format (xml, json)
• sort - Sort order (relevance, pub_date, first_author, last_author, journal)
• field - Limit search to specific field
• datetype - Type of date to use for filtering (pdat for publication date)
• mindate - Minimum date (YYYY/MM/DD format)
• maxdate - Maximum date (YYYY/MM/DD format)

Example Request:

esearch.fcgi?db=pubmed&term=breast+cancer&retmax=100&retmode=json&api_key=YOUR_API_KEY

Response Elements:

• Count - Total number of records matching query
• RetMax - Number of records returned in this response
• RetStart - Index of first returned record
• IdList - List of UIDs (PMIDs)
• WebEnv - History server environment string (when usehistory=y)
• QueryKey - Query key for history server (when usehistory=y)

2. EFetch - Download Records

Endpoint: efetch.fcgi

Purpose: Retrieve full records from a database in various formats

Required Parameters:

• db - Database name
• id - Comma-separated list of UIDs, or use WebEnv/query_key from ESearch

Optional Parameters:

• rettype - Record type (abstract, medline, xml, uilist)
• retmode - Return mode (text, xml)
• retstart - Starting record index
• retmax - Maximum records per request

Example Request:

efetch.fcgi?db=pubmed&id=123456,234567&rettype=abstract&retmode=text&api_key=YOUR_API_KEY

Common rettype Values for PubMed:

• abstract - Abstract text
• medline - Full MEDLINE format
• xml - PubMed XML format
• uilist - List of UIDs only

3. ESummary - Retrieve Document Summaries

Endpoint: esummary.fcgi

Purpose: Get document summaries (DocSum) for a list of UIDs

Required Parameters:

• db - Database name
• id - Comma-separated UIDs or WebEnv/query_key

Optional Parameters:

• retmode - Return format (xml, json)
• version - DocSum version (1.0 or 2.0, default is 1.0)

Example Request:

esummary.fcgi?db=pubmed&id=123456,234567&retmode=json&version=2.0&api_key=YOUR_API_KEY

DocSum Fields (vary by database, common PubMed fields):

• Title
• Authors
• Source (journal)
• PubDate
• Volume, Issue, Pages
• DOI
• PmcRefCount (citations in PMC)

4. EPost - Upload UIDs

Endpoint: epost.fcgi

Purpose: Upload a list of UIDs to the history server for use in subsequent requests

Required Parameters:

• db - Database name
• id - Comma-separated list of UIDs

Example Request:

epost.fcgi?db=pubmed&id=123456,234567,345678&api_key=YOUR_API_KEY

Response: Returns WebEnv and QueryKey for use in subsequent requests

5. ELink - Find Related Data

Endpoint: elink.fcgi

Purpose: Find related records within the same database or in different databases

Required Parameters:

• dbfrom - Source database
• db - Target database (can be same as dbfrom)
• id - UID(s) from source database

Optional Parameters:

• cmd - Link command (neighbor, neighbor_history, prlinks, llinks, etc.)
• linkname - Specific link type to retrieve
• term - Filter results with search query
• holding - Filter by library holdings

Example Request:

elink.fcgi?dbfrom=pubmed&db=pubmed&id=123456&cmd=neighbor&api_key=YOUR_API_KEY

Common Link Commands:

• neighbor - Return related records
• neighbor_history - Post related records to history server
• prlinks - Return provider URLs
• llinks - Return LinkOut URLs

6. EInfo - Database Information

Endpoint: einfo.fcgi

Purpose: Get information about available Entrez databases or specific database fields

Parameters:

• db - Database name (optional; omit to list all databases)
• retmode - Return format (xml, json)

Example Request:

einfo.fcgi?db=pubmed&retmode=json&api_key=YOUR_API_KEY

Returns:

• Database description
• Record count
• Last update date
• Available search fields with descriptions

7. EGQuery - Global Query

Endpoint: egquery.fcgi

Purpose: Search term counts across all Entrez databases

Required Parameters:

• term - Search query

Example Request:

egquery.fcgi?term=cancer&api_key=YOUR_API_KEY

8. ESpell - Spelling Suggestions

Endpoint: espell.fcgi

Purpose: Get spelling suggestions for queries

Required Parameters:

• db - Database name
• term - Search term with potential misspelling

Example Request:

espell.fcgi?db=pubmed&term=cancre&api_key=YOUR_API_KEY

9. ECitMatch - Citation Matching

Endpoint: ecitmatch.cgi

Purpose: Search PubMed citations using journal, year, volume, page, author information

Request Format: POST request with citation strings

Citation String Format:

journal|year|volume|page|author|key|

Example:

Science|2008|320|5880|1185|key1|
Nature|2010|463|7279|318|key2|

Rate Limit: 3 requests per second with User-Agent header required

Best Practices

Use History Server for Large Result Sets

For queries returning more than 500 records, use the history server:

1. Initial Search with History:

esearch.fcgi?db=pubmed&term=cancer&usehistory=y&retmode=json&api_key=YOUR_API_KEY

2. Retrieve Records in Batches:

efetch.fcgi?db=pubmed&query_key=1&WebEnv=MCID_12345&retstart=0&retmax=500&rettype=xml&api_key=YOUR_API_KEY
efetch.fcgi?db=pubmed&query_key=1&WebEnv=MCID_12345&retstart=500&retmax=500&rettype=xml&api_key=YOUR_API_KEY

Batch Operations

Use EPost to upload large lists of UIDs before fetching:

# Step 1: Post UIDs
epost.fcgi?db=pubmed&id=123,456,789,...&api_key=YOUR_API_KEY

# Step 2: Fetch using WebEnv/query_key
efetch.fcgi?db=pubmed&query_key=1&WebEnv=MCID_12345&rettype=xml&api_key=YOUR_API_KEY

Error Handling

Common HTTP status codes:

• 200 - Success
• 400 - Bad request (check parameters)
• 414 - URI too long (use POST or history server)
• 429 - Rate limit exceeded

Caching

Implement local caching to:

• Reduce redundant API calls
• Stay within rate limits
• Improve response times
• Respect NCBI resources

Response Formats

XML (Default)

Most detailed format with full structured data. Each database has its own DTD (Document Type Definition).

JSON

Available for most utilities with retmode=json. Easier to parse in modern applications.

Text

Plain text format, useful for abstracts and simple data retrieval.

Support and Resources

• API Documentation: https://www.ncbi.nlm.nih.gov/books/NBK25501/
• Mailing List: utilities-announce@ncbi.nlm.nih.gov
• Support: vog.hin.mln.ibcn@seitilitue
• NLM Help Desk: 1-888-FIND-NLM (1-888-346-3656)