We at NLM in Focus do our best to inform you about the people, products, and programs of the world’s largest medical library. However, there are always more questions to answer and realms to explore.
In this occasional feature, we answer questions on Wednesdays that we think might be on your mind and invite you to submit your own. If you have something you’ve always wondered about the National Library of Medicine, please let us know by using the “Leave a Reply” box at the foot of this page.
Today, we’re asking “How many national libraries are there in the United States?”
For most countries, the answer is usually one.
But the United States actually has five.
In addition to the National Library of Medicine, the other national libraries are the Library of Congress, the National Agricultural Library, the National Library of Education, and the National Transportation Library.
National libraries or libraries established by the government of a country serve as preeminent repositories of information. Often, they include numerous rare, valuable, or significant works and play an important role in the preservation of their countries’ cultures and intellectual traditions.
National libraries are commonly open to the public for research, and while members of the public may not directly check out items, they may be able to obtain them through interlibrary loan. National libraries are also increasingly making their collections available online, as copyright and digitization projects allow.
NLM, the Library of Congress, and the National Agricultural Library coordinate collection development activities and work together to keep collecting duplication to the minimum necessary to serve their diverse user populations. They have made cooperative statements on veterinary science, human nutrition, and AIDS literature. NLM and the Library of Congress also work together on several community cataloging and metadata endeavors including the development of BIBFRAME.
What are you wondering about?
Let us know below.
[Image credit: Books image created by Abdo from the Noun Project / modified]
NLM in Focus 
If PubMed just posted its 25-millionth citation, why are the PubMed IDs not up to 25-million digits and beyond?
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Thanks for the suggestion. We’re looking into it.
You asked, “If PubMed just posted its 25-millionth citation, why are the PubMed IDs not up to 25-million digits and beyond?”
We understand your question to be asking about the discrepancy between the number of citations in PubMed, which is about 28 million, and the PMIDs, which now extend beyond 30 million.
PubMed identifiers, a/k/a PMIDs, are automatically assigned to new PubMed records, and they simply go up sequentially with every new record. So yes, there is a record with a PMID of 1, and we’re now at 30,071,571 (and rising).
As unique identifiers, PMIDs do not change and they are not reused, which is how we end up with a higher PMID number than we have actual records in the system. Over the 20-plus years we have been generating PMIDs, many have been used during system testing, while others were assigned to records that were deleted for some reason, such as duplicate citations or submission errors.