PMID (PubMed ID) is the unique identifier for a publication's abstract in PubMed.
PMCID (PubMed Central ID) is the unique identifier for a free complete article (once it is publicly available) in PubMed Central.
PubMed and PubMed Central, produced by the U.S. National Library of Medicine, are text archives of biomedical and life sciences literature.
A DOI (Digital Object Identifier) is a publisher’s unique identifier for a publication. The prefix http://dx.doi.org/ is presumed. Publishers may charge for access.
Vreeman DJ, Abhyankar S, McDonald CJ.Re: Unit conversions between LOINC codes Published June 19, 2017 [Letter]. J Am Med Inform Assoc. 2018 May 1;25(5):614-615. doi: 10.1093/jamia/ocx087.
Vreeman DJ, Abhyankar S, McDonald CJ.Unit conversions between LOINC codes Published June 19, 2017 [Letter]. Journal of the American Medical Informatics Association, 1–3. http://doi.org/10.1093/jamia/ocx087.
Zeng K, Bodenreider O.Experience in Integrating Large RDF-based Biomedical Knowledge Resources with Oracle [Abstract]. Proc 8th International Oracle Life Sciences User Group Meeting 2007:(electronic proceedings: http://www.olsug.org/wiki/images/d/d1/Kelly_Zheng.pdf)