ackground:

The Medical Subject Headings (MeSH®) controlled vocabulary contains Supplementary Concept Records (SCRs) in addition to Main Headings (also called Descriptors) and Subheadings (also called Qualifiers). SCRs are typically chemicals that indexers have seen in the biomedical literature cited in MEDLINE. Such a substance is not a Main Heading, but is established as an SCR in MeSH so that the preferred form of the substance name can be controlled and added to MEDLINE citations as part of the regular indexing process. Other information about the substance may also be listed in the SCR record in the MeSH Browser such as a corresponding registry number (Chemical Abstracts Service or Enzyme Commission Nomenclature number) and entry vocabulary, consisting of synonyms and closely related names (e.g., trade names and lab numbers) for the substance.

There are over 130,000 SCRs in MeSH. Unlike the Main Headings and Subheadings that are changed on an annual basis, new SCRs are added and existing ones edited on a daily basis. The results of this daily maintenance in the MeSH authority files are passed along to the MEDLINE citations, so that the updated information is available (Tuesday - Saturday) in the MEDLINE citation in PubMed where the substance name is searched as [nm] and the registry number as [rn]. Both entry vocabulary and related registry numbers are also part of the PubMed translation tables for searching SCRs. SCR Names of Substance and Registry Numbers were added to the online MEDLINE citations beginning mid-year 1980.

Each SCR has an associated Main Heading(s), which is its Heading Mapped-to (HM) in the MeSH Browser. (See Figure 1.) Every time an indexer chooses an SCR as relevant to an article, the DCMS (Data Creation and Maintenance System) also adds the SCR's Heading Mapped-to value(s) to the MEDLINE citation as an MH, along with the asterisk designation if the indexer has indicated that the SCR is the main point of the article.

What Has Changed:
As science progresses and the MeSH vocabulary is reviewed, these Heading Mapped-to values may change. Until now, the NLM has not done maintenance to the MEDLINE citations when the HM changes in the MeSH authority file. As a result, the literature indexed in MEDLINE may not have a consistent MH throughout time that corresponds to a particular SCR.

As part of the annual year-end processing cycle this year, NLM performed maintenance to add the current HM value as an MH for every SCR Name of Substance in a MEDLINE citation whenever the current value was absent. The 2003 MeSH data was used for this sweep against the entire MEDLINE file. Then the 2004 MeSH data were examined for specific HM changes since the 2003 MeSH, and those additional changes were also added to the MEDLINE citations as appropriate.

This HM maintenance is also now part of the SCR updating routine, so these changes will be reflected in MEDLINE in PubMed throughout the year, not just annually.

NLM chose to add the current HM value as an MH to the MEDLINE citations rather than attempt to replace the old value with the current value for two reasons:

1. It is impossible to tell if the HM value was placed in the MEDLINE citation originally from the Heading Mapped-to routines, or if the indexer independently chose that MH for another reason pertinent to that article, and
2. the HM value may have changed more than once over time and/or have more than one value at a time, making it difficult to know with certainty which MH should be replaced.

One situation is of particular note in this new maintenance procedure: the MH that is added to the MEDLINE citation is the exact string as stored in the HM element of the MeSH Browser. When an asterisk is part of that string (see Figure 1), the asterisk is added to the MEDLINE citation, even though the SCR may not have been a main point of that particular article. The NLM is investigating the significance and feasibility of altering that procedure.

NLM expects that these additional, current MH access points will improve search retrieval for those searchers choosing to use MeSH in their strategies. Examples 1 and 2 demonstrate the maintenance activity and the resulting changes in retrieval.

Example 1:
As seen in Figure 1, the SCR cadmium acetate has two Heading Mapped to values: *Acetates and *Cadmium. In 2003 PubMed:

cadmium acetate [nm] has 49 postings

cadmium acetate [nm] NOT cadmium [mh:noexp] has 0 postings

cadmium acetate [nm] NOT acetates [mh:noexp] has 31 postings

The Heading Mapped-to maintenance added *Acetates as a MeSH Heading to the 31 citations. No action was taken for the *Cadmium value because all 49 citations with the SCR already had that MH present. Evidently at some time in the past, this SCR had only the one HM value with *Acetates being added as the second HM value later.

Effective with the 2004 maintained PubMed data, an MH search on either Acetates or on Cadmium will include all 49 of these citations with cadmium acetate as a name of substance.

Example 2:
The SCR paldimycin has two Heading Mapped to values in the 2004 MeSH Browser: Acetylcysteine/analogs & derivatives and Disaccharides. (See Figure 2.)

In 2003 PubMed:

paldimycin [nm] has 12 postings

paldimycin [nm] NOT acetylcysteine/aa [mh:noexp] has 12 postings

paldimycin [nm] NOT disaccharides [mh:noexp] has 12 postings

The Heading Mapped-to maintenance added both Acetylcysteine/analogs & derivatives and Disaccharides to all 12 citations having paldimycin [nm]. In the past this SCR was probably mapped to Glycopeptides as all 12 citations have this MH present.

Effective with the 2004 PubMed maintained data, an MH search on either Acetylcysteine or on Disaccharides will include all 12 of these citations with paldimycin as a name of substance.

Lou Knecht
Bibliographic Services Division

Knecht L. Heading Mapped-to Maintenance: for Supplementary Concept Records' Names of Substance. NLM Tech Bull. 2003 Nov-Dec;(335):e4.