PubMed Literature Retrieval

Overview

Search and retrieve biomedical literature from PubMed using NCBI E-utilities REST API. Zero local dependencies — all calls are HTTP GET/POST to eutils.ncbi.nlm.nih.gov. Designed for cloud deployment and LLM tool integration.

When to Use

• Writing papers, grant proposals, or literature reviews that need citation support
• Searching PubMed by keywords, authors, date ranges, MeSH terms, or publication types
• Retrieving structured metadata (title, authors, abstract, DOI, etc.) for a set of articles
• Attempting to fetch full-text articles from PubMed Central (PMC)
• Building reference lists from search results

Base URL

All E-utility requests use:

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

API Key & Rate Limits

Include API key in every request: &api_key=YOUR_KEY

Always include &tool=your_tool_name&email=your@email.com in requests for NCBI compliance.

Core Workflow

digraph pubmed_flow {
    rankdir=TB;
    "Build Query" [shape=box];
    "ESearch" [shape=box, label="ESearch\n(get PMIDs)"];
    "Check Count" [shape=diamond];
    "EFetch Metadata" [shape=box, label="EFetch\n(get XML metadata)"];
    "Parse & Format" [shape=box];
    "Need Full Text?" [shape=diamond];
    "ELink to PMC" [shape=box];
    "EFetch PMC" [shape=box, label="EFetch PMC\n(full text XML)"];
    "Return Results" [shape=doublecircle];

    "Build Query" -> "ESearch";
    "ESearch" -> "Check Count";
    "Check Count" -> "EFetch Metadata" [label="≤200 results"];
    "Check Count" -> "ESearch" [label=">200: paginate\nwith retstart"];
    "EFetch Metadata" -> "Parse & Format";
    "Parse & Format" -> "Need Full Text?";
    "Need Full Text?" -> "ELink to PMC" [label="yes"];
    "Need Full Text?" -> "Return Results" [label="no"];
    "ELink to PMC" -> "EFetch PMC";
    "EFetch PMC" -> "Return Results";
}

Step 1: ESearch — Search PubMed

URL Pattern

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&term=<QUERY>&retmax=<N>&retstart=<OFFSET>&usehistory=y&sort=relevance

Query Syntax — Field Tags

Boolean Operators

Use uppercase: AND, OR, NOT

(breast cancer[tiab] OR breast neoplasms[mh]) AND immunotherapy[tiab] AND 2020:2024[dp] AND Review[pt]

Publication Type Filters

Sort Options

ESearch Parameters

Example ESearch Call

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&term=CRISPR+AND+cancer[tiab]+AND+2023:2024[dp]+AND+Review[pt]&retmax=50&sort=pub_date&usehistory=y

ESearch Response (XML)

Key fields to extract:

• <Count> — total matching records
• <IdList><Id> — list of PMIDs
• <QueryKey> and <WebEnv> — for History Server (when usehistory=y)

Step 2: EFetch — Retrieve Article Metadata

URL Pattern (by PMID list)

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pubmed&id=PMID1,PMID2,PMID3&rettype=xml&retmode=xml

URL Pattern (by History Server)

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pubmed&query_key=KEY&WebEnv=WEBENV&rettype=xml&retmode=xml&retstart=0&retmax=100

EFetch Parameters

Parsing EFetch XML — Key XPaths

Extract these fields from each <PubmedArticle>:

Step 3 (Optional): Full-Text Retrieval via PMC

Tier 1: Check PMC availability from EFetch

If the EFetch XML contains <ArticleId IdType="pmc">PMCxxxxxxx</ArticleId>, the full text is available in PMC.

Tier 2: EFetch from PMC

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pmc&id=PMC_ID&rettype=xml

This returns JATS XML with the complete article body, including sections, figures, tables, and references.

Tier 3: PMC Open Access PDF

https://www.ncbi.nlm.nih.gov/pmc/articles/PMCxxxxxxx/pdf/

Tier 4: Fallback via DOI

If not in PMC, construct: https://doi.org/<DOI> — may lead to publisher paywall.

ELink Alternative (batch PMC lookup)

To find PMC IDs for multiple PMIDs at once:

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&db=pmc&id=PMID1&id=PMID2&id=PMID3

LLM-Friendly Output Format

After parsing XML, format results as structured text:

--- Article 1 of N ---
PMID: 12345678
Title: Example Article Title Here
Authors: Smith J, Zhang Y, Wang L, et al.
Journal: Nature Medicine (Nat Med)
Date: 2024 Jan
Volume: 30, Issue: 1, Pages: 45-52
DOI: 10.1038/s41591-024-xxxxx
Abstract: [Full abstract text here...]
MeSH: Neoplasms; Immunotherapy; CRISPR-Cas Systems
Type: Review
PMC: PMC9876543 (full text available)
URL: https://pubmed.ncbi.nlm.nih.gov/12345678/
---

Pagination for Large Result Sets

When <Count> exceeds retmax:

1. Use usehistory=y in ESearch to get WebEnv and query_key
2. Loop EFetch with incrementing retstart:
   • Batch 1: retstart=0&retmax=200
   • Batch 2: retstart=200&retmax=200
   • Continue until retstart >= Count
3. Respect rate limits between requests

Common Search Patterns

Find recent reviews on a topic

<TOPIC>[tiab] AND Review[pt] AND 2020:2024[dp] AND English[la]

Find clinical trials by intervention

<INTERVENTION>[tiab] AND Randomized Controlled Trial[pt] AND humans[filter]

Find articles by specific author in a journal

<AUTHOR>[au] AND <JOURNAL>[ta] AND 2020:2024[dp]

Find free full-text articles

<TOPIC>[tiab] AND free full text[sb]

Find systematic reviews and meta-analyses

<TOPIC>[tiab] AND (Systematic Review[pt] OR Meta-Analysis[pt])

MeSH-based precise search

<MESH_TERM>[majr] AND <SUBHEADING>[sh] AND 2020:2024[dp]

Common Mistakes

Quick Reference