EDLINE Unique Identifiers (UI) in their current 8-digit form will continue to be added to MEDLINE records through the current 2001 production year. Beginning in the 2002 production year, the National Library of Medicine (NLM) will cease to generate the 8-digit UIs and begin to generate 9-digit UIs. It is better to make such a change at the end of a processing year which for MEDLINE is at the end of October. This is a change to previous announcements that NLM would cease creating and using the UI.

NLM decided to adopt the PubMed Unique Identifier (PMID) as the single journal citation accession number for managing and disseminating MEDLINE data. This was prompted by two conditions: (1) user feedback told us that having two accession numbers in the PubMed displays was confusing, and (2) the development of the new Data Creation and Maintenance System (DCMS) for MEDLINE data allowed NLM to record the PMID so that communication and data exchange with the PubMed retrieval system could be straightforward.

For transition purposes, the UI remains searchable in PubMed. It displays on the MEDLINE and XML formats of PubMed and is included in the data sent to licensees. NLM has not yet determined how long it will support the PubMed searchability of the UI or the export of the UI data to licensees. If the decision is made to discontinue searchability or export of the UI, that announcement will be made in advance and would occur at the end of a processing year. There is no plan to discontinue searchability or export of the UI for the 2002 production year. Note that a UI of 0 (zero) now means one of three things: (1) the citation has just been added to PubMed via the XML electronic input stream directly from the publishers and is awaiting assignment of a UI and elevation to an in-process status; (2) the citation is from a non-MEDLINE journal added via the XML electronic input stream and will never be assigned a UI; or, (3) the citation is from a back issue of a MEDLINE journal that predates when indexing began for that title.

Again, the 8-digit UI format will cease to be assigned to MEDLINE citations at the end of the 2001 production year; for the 2002 production year, UIs will be 9 digits long. The PMID is the unique number for all records in PubMed, and users should make the transition to using the PMID for identifying citations, for example, in bibliographies, links to PubMed, and for DOCLINE or Loansome Doc needs. The PMID should be used for all citations, those indexed during the upcoming 2002 production year (and beyond) as well as retrospective citations.

Lou Knecht
Bibliographic Services Division

Knecht L. Future of the MEDLINE Unique Identifier. NLM Tech Bull. 2001 May-Jun;(320):e2.