The National Library of Medicine (NLM) has announced that, as of December 1, 2015, NLM subject terms (MeSH) will be distributed in "deconstructed" form, rather than in the current combined form. Specifically, topical subjects will be recorded in 650 $a or 650 $a $x; geographic subjects will be recorded in 651 $a or 651 $a $x; and publication type/genre terms record will be in 655 $a. NLM encourages catalogers in other libraries that use MeSH to follow NLM practice in assigning their subjects.
In her announcement, posted to several electronic mailing lists, Diane Boehr of NLM gave the following example:
Old form (with string)
650 _2 $a Health Policy $z Africa $v Congresses
650 _2 $a Health Policy $z Caribbean Region $v Congresses
650 _2 $a Cross Cultural Comparison $z Africa $v Congresses
650 _2 $a Cross Cultural Comparison $z Caribbean Region $v Congresses
New form (deconstructed)
650 _2 $a Health Policy
650 _2 $a Cross Cultural Comparison
651 _2 $a Africa
651 _2 $a Caribbean Region
655 _2 $a Congresses
Because of NLM's recommendation that libraries follow suit if they want their data to be consistent with the NLM files, LTI is testing a profile option that would deconstruct these controlled headings in bibliographic records for libraries profiled to authorize MeSH headings. We expect testing to be completed in time for the upcoming 4th quarter run of Authority Update (AUP). When the profile option is available, we will send an updated message to the Users' List and will post it in the "News and Updates" section of our website.