There are currently two identifiers unique to citations found in PubMed: the PubMed Identifier (PMID) and the PubMed Central Identifier (PMCID). A third identifier, the Digital Object Identifier (DOI), is often found in PubMed citations, though it does not originate with PubMed.
The PMID is a "1- to 8-digit accession number with no leading zeros" (MEDLINE) assigned sequentially to every new citation added to PubMed from a MEDLINE journal. PMIDs are assigned to research articles as well as other documents published in indexed journals, including letters to the editor and opinion pieces. You will find the PMID at the bottom of each PubMed record (highlighted in orange in the example here) .
The PMID of 1 is associated with an article from 1975, but older articles are assigned later PMIDs as they are digitized and added to PubMed.
For PubMed citations that are also included full-text in PubMed Central, you will see a second identifier, the PMCID, listed after the PMID and beginning with the letters PMC (highlighted in red).
DOIs are digital identifiers of objects such as articles and data sets assigned by publishers to provide persistent links to those objects. A citation's DOI is found alongside its PMID, and PMCID if applicable (highlighted in blue).
To find an article using the PMID, add the search filter [uid] or [PMID] in the PubMed search bar.
The PubMed search will also infer the [uid] search filter if you do a simple keyword search for a PMID number.
To find an article using a PMCID, put the whole number, including the PMC letters, in the search bar, without adding [uid]:
To find an article using a DOI (digital object identifier), put the whole number, again without the DOI part, in the search bar: