Why do I get a “Identifier not found in PMC” error message when trying to convert a valid DOI that has a PMID?

Question

I read on https://pubmed.ncbi.nlm.nih.gov/30265128/:

• PMID: 30265128 (PMID=PubMed identifier)
• DOI: 10.1164/rccm.201806-1083IM

However when trying to convert the DOI 10.1164/rccm.201806-1083IM ot its PMID on https://www.ncbi.nlm.nih.gov/pmc/pmctopmid/ I get "Identifier not found in PMC":

Same issue with:

• PMID: 29794847; DOI: 10.1097/SLA.0000000000002825
• PMID: 30601258; DOI: 10.1097/SLA.0000000000003154
• PMID: 31725892; DOI: 10.1093/eurheartj/ehz824

Why do I get a "Identifier not found in PMC" error message when trying to convert a valid DOI that has a PMID?

(Note that DOI names are case insensitive).

Answer 1

From understanding this is because these papers aren't in PMC but only in PubMed. Which raises the question: How can I programmatically get the PMID from the DOI of a paper that isn't in PMC (i.e.. that doesn't have a PMCID)?

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Background reading: PMID vs PMCID: What’s the Difference?

PubMed is an index of the biomedical literature. A PMID, also known as the PubMed reference number, is a number assigned by the NIH National Library of Medicine to papers indexed in PubMed. PubMed Central is an archive of full-text journal articles. The National Library of Medicine assigns a PMCID, also known as a PMC Identifier, to each full-text paper in PubMed Central.