Release Date: January 10, 2014
Category: Scientific Writing
Author: Michelle S., Ph.D.
In the digital age, the organization of scientific literature is paramount. Fortunately, one forte of database management is the use of unique identifiers, or primary keys, to organize such a massive amount of information, offering a solution to the challenge of organizing scientific literature. A number of unique identifiers have been implemented in PubMed and other worldwide scientific search engines. These identifiers include PMID, PMCID, NIHMS ID, and DOI. With the implementation of NIH Public Access initiatives, and as more journals require the use of these identifiers for referencing, it is important to understand the purpose of these identifiers and where to find them. This article will cover each of these identifiers, what they are commonly used for, and where to find them while searching literature online.
PMID – The PubMed Indexing Number, or PMID, is assigned to each article as it is added to the PubMed database. This is a number used by PubMed to index the literature within MEDLINE, the U.S. National Library of Medicine's (NLM) premier bibliographic database (for more information on MEDLINE, click here). Generally, this number is NOT the one required by the NIH for grant proposals, applications, or reports. It is NOT the same number assigned to articles in PubMed Central, and will not meet the NIH guidelines. The PMID is generally visible in results displayed in Summary format of a PubMed online search. You will find the PMID listed directly below the author list, the article title, and the publisher information.
PMCID – In contrast to the PMID described above, the PubMed Central referencing number, or PMCID, is required in NIH grants proposals, applications, and reports. This number is assigned to an article when is it entered into PubMed Central, a free digital database of full-text scientific literature in biomedical and life sciences. Articles found in PubMed Central were submitted in compliance with the NIH Public Access Policy (for more information on PubMed Central, click here). To find the PMCID from a PubMed online search, you may do one of two things. First, you may follow the link for the article of interest to reach the Abstract display of that article. The PMCID and PMID are located in the same line of text below the abstract. If you wish to view the PMCIDs for all the articles displayed in a single PubMed search, you must change the format of the displayed results from Summary to AbstractPlus (select “AbstractPlus” from the drop-down menu under “Display.”) Once the format has updated, the PMCID will be right-justified in the same line of text as the PMID.
NIHMS ID – If an article has been submitted to PubMed Central, but it has not yet received a PMCID, it is given an NIH Manuscript Submission system reference number, or NIHMS ID. The NIHMSID is valid for 90 days after the publication date of an article. Once the Web version of the NIHMS submission is approved for inclusion in PMC and the corresponding citation is in PubMed, the article will also be assigned a PMCID. If you do not have a PMCID, an NIHMS ID will substitute during that 90-day period. To find an NIHMS ID, you must first have the PMID for the article of interest. Log in to NIHMS and use the search box in the upper right-hand corner. Select PubMed ID from the dropdown menu (the default is NIHMSID), insert the PMID, and click "Find Manuscript."
DOI – A DOI, or Digital Object Identifier, is a character string used to uniquely and permanently identify an electronic object (like a permanent URL). The DOI has applications that reach far beyond the realm of scientific literature; however, the organization of scholarly material is one primary function. The current APA style for referencing primary literature includes a DOI as a standard part of the citation whenever it is available. An increasing number of journals require authors to include the DOI for each article they cite in their manuscripts. If you use EndNote or other reference software to organize your reference library, you must ensure that the DOI is added to each entry in your library for it to be included in your bibliography. Fortunately, if MEDLINE has the DOI within the article information, it should import when you add the reference to your library. Keep in mind that not all articles have a DOI, as this is a fairly recent addition to citations and a product of the digital age. You can find a DOI, if it is available, from the results list of a PubMed search. It is in the same line of text as the journal title, issue and page number information for the article.
Keeping it all straight – With so many different identifiers used by different databases and online libraries, things can get confusing. Fortunately, NCBI has a website that allows you to extract all the identifiers linked to an article by entering a single identifier into the appropriate field on the webpage. Following the instructions provided on the webpage, enter your unique identifier and click “convert.” The tool will return all the unique identifiers associated with that article and display them in a format you choose. To reach this useful online tool, click here.
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