Written and maintained by CASRAI Editorial Board
Last updated
Short answer: PubMed is a database run by the U.S. National Library of Medicine (NLM) at the National Institutes of Health, but it is not a single quality tier. NLM’s own pages describe PubMed as three components, and only one of them, MEDLINE, comes with a published journal-selection process. How reliable a given PubMed record is depends first on which component it belongs to. This page shows how to tell, using NLM’s own documentation (checked 2026-10-29).
What PubMed contains
According to the PubMed About page (last updated March 11, 2025), PubMed holds more than 40 million citations and abstracts of biomedical literature. It does not contain full text itself; it links out to full text where available. The page names three components:
- MEDLINE, “the largest component of PubMed,” which “consists primarily of citations from journals selected for MEDLINE.”
- PubMed Central (PMC) citations, the second largest component.
- Books: citations for books and some individual chapters available on Bookshelf.
The About page does not say that PubMed as a whole is peer reviewed or endorsed for quality, so this page does not claim that either. For the difference between the PMC archive and PubMed itself, see PubMed vs PubMed Central.
How does NLM select journals for MEDLINE?
NLM’s MEDLINE journal selection page (last reviewed July 24, 2026) states that NLM decides “whether the scientific and editorial character and quality of a journal merit its inclusion in MEDLINE.” It also says NLM “views each journal comprehensively, rather than basing a decision on a defined list of criteria.” The quality assessment “focuses on 5 critical elements”:
- Scope and Coverage
- Editorial Policies and Processes
- Scientific Rigor/Methodological Rigor
- Production and Administration
- Impact
Within the editorial element, NLM asks whether the peer review process is explicit and sufficiently detailed, including information on the type of peer review, and whether ethical policies are findable, clearly stated and consistent with current best practices. Each journal is reviewed by at least two consultants, usually one scientist (a doctoral-level researcher or physician) and one medical librarian, and the review generally looks at articles from the last two years.
The NLM MEDLINE overview (last reviewed September 8, 2025) describes MEDLINE as NLM’s premier bibliographic database, with more than 31 million references to journal articles from more than 5,200 worldwide journals in about 40 languages. Its distinctive feature is that records are indexed with NLM Medical Subject Headings (MeSH). See CASRAI’s MEDLINE database entry and the guide to MeSH terms in PubMed.
Which PubMed records are not MEDLINE?
The PubMed help page (last updated September 30, 2026) states that “some citations are not indexed for MEDLINE, such as preprints, Online Ahead of Print citations.” The table below sets out only what NLM’s pages state about each type of record.
| Record type | What NLM’s pages state | What the pages do not state |
|---|---|---|
| Citation indexed for MEDLINE | From a journal selected for MEDLINE under NLM’s five-element review; indexed with MeSH | That every article in a selected journal was separately reviewed by NLM |
| PMC citation | Second largest component of PubMed | No journal-selection process is described on the About page |
| Bookshelf citation | Books and some individual chapters available on Bookshelf | No quality statement on the About page |
| Preprint | Not indexed for MEDLINE; can be excluded with a filter | Any peer-review status |
| Online Ahead of Print | Not indexed for MEDLINE | Whether the journal is itself MEDLINE-indexed |
How do I check a record’s status in PubMed?
The help page documents these controls (interface terms as NLM uses them):
- Open the PubMed help page and find the citation status section. NLM says the citation status “indicates the internal processing stage of an article in the PubMed database.”
- Search by status with the
[sb](subset) tag. The status terms documented arepublisher,inprocess,medlineandpubmednotmedline. - Use the MEDLINE filter, which “limits results to citations that are indexed for MEDLINE.”
- Use the Exclude preprints filter to remove preprint citations from a result set.
For building a full query, see the PubMed Advanced Search Builder guide.
Strengths and limits
What the sources support
- PubMed is maintained by NLM, and its MEDLINE component is defined by a published selection process with named elements and a two-consultant review.
- MEDLINE records carry MeSH indexing, which NLM identifies as the distinctive feature of MEDLINE.
- The status terms and filters let you separate MEDLINE records from the rest of PubMed.
Limits to keep in mind
- NLM’s selection page says it does not use a defined list of criteria, so inclusion is a judgment, not a checklist result.
- PubMed does not report a journal impact factor; see why PubMed does not report an impact factor.
- Appearing in PubMed is not by itself a statement about any single article’s validity; judging an individual study still takes reading its methods.
Alternatives and companion databases
Other databases are often searched alongside PubMed. CASRAI maintains side-by-side comparisons: PubMed vs Google Scholar, CINAHL vs PubMed and Embase vs MEDLINE. Each database should be assessed against its own documentation, in the same way as above.
FAQ
Is everything in PubMed peer reviewed?
NLM’s About page does not make that claim. It describes MEDLINE citations as coming from journals selected for MEDLINE, whose review includes how explicit the journal’s peer review process is, and the help page notes that some PubMed citations, such as preprints, are not indexed for MEDLINE.
Is MEDLINE the same as PubMed?
No. NLM describes MEDLINE as the largest component of PubMed. PubMed also includes PMC citations and Bookshelf citations.
Can I limit a PubMed search to MEDLINE only?
Yes. The MEDLINE filter limits results to citations indexed for MEDLINE, and the [sb] tag can be used with the status terms listed on the help page.
How can I leave out preprints?
Use the Exclude preprints filter, which removes preprint citations from your search results.
Sources
- PubMed, About PubMed, https://pubmed.ncbi.nlm.nih.gov/about/, checked 2026-10-29
- PubMed, PubMed User Guide, https://pubmed.ncbi.nlm.nih.gov/help/, checked 2026-10-29
- NLM, MEDLINE overview, https://www.nlm.nih.gov/medline/medline_overview.html, checked 2026-10-29
- NLM, MEDLINE journal selection, https://www.nlm.nih.gov/medline/medline_journal_selection.html, checked 2026-10-29
