LM has implemented the following changes for the treatment of chemical data in MEDLINE citations. Both changes are effective for the 2001 version of MEDLINE.

1. MEDLINE citations back through 1966 with a Medical Subject Heading (MeSH) Category D (Chemical) descriptor now also have an associated RN (CAS Registry/EC Number) field with the number and name of substance (NM). Previously, the RN and NM data existed only on citations created and indexed from mid-June 1980 forward. For Category D descriptors that name a group or class of compounds rather than an individual chemical, the RNs are zero.
   
   See the example below (Figure 1) for an illustration of how this change affects searching in PubMed^®.
   
    

   MeSH Category D Heading with Associated Registry Number (Acetaminophen) Acetaminophen has been a MeSH Heading since 1965. Prior to 2001, users could only search for this concept back through 1966 using the MH descriptor. Beginning in 2001, users can also search for this concept using the RN or NM field and get the citations back through 1966. A portion of the MeSH Browser for Acetaminophen is shown below. MeSH Heading Acetaminophen Tree Number D02.065.199.092.040 Registry Number 103-90-2 History Note 65 Using PubMed, the following searches all retrieve the same citations (1966 to the present): Acetaminophen [mh] Acetaminophen [nm] 103-90-2 [rn] Remember, if the chemical descriptor has indentations you would need to turn off automatic explosion 0by using the [mh:noexp] search tag (e.g., Ampicillin [mh:noexp]).

   Figure 1
   
   
   • For Supplementary Concept Records (SCR) that have been promoted to MeSH Heading (MH) descriptor status, the corresponding MH was retrospectively added to the relevant MEDLINE citations. Previously, only the RN/NM field for that specific chemical was on the citations that predated the promotion to MH status. (An MH occurrence for the heading the SCR mapped to was, and still is, on the citation. However, the MH that the SCR mapped to was likely a broader concept and therefore a different descriptor.) This change affects citations created since mid-June 1980, as that is when the RN/NM field was introduced for Supplementary Concepts.
     
     See the example below (Figure 2) for an illustration of how this change affects searching in PubMed.
     
     
    

   Supplementary Concept Record (SCR) Elevated to MH Status (Ketorolac) Ketorolac was an SCR from 1983 to 1999 and became an MH in 2000. Prior to 2001, users could only search for this concept back to 1983 using the RN or NM fields (or text words, of course). Beginning in 2001, users can also search on the MH field as well and get citations back to 1983. A portion of the MeSH Browser for Ketorolac is shown below. Editor's Note: Information in { } has been added for clarity. MeSH Heading: Ketorolac Tree Number D03.438.473.420.485 Registry Number: 66635-83-4 Previous Indexing: Tolmetin/analogs & derivatives (1983-1999) History Note: 2000; use KETOROLAC (NM) 1983-1999; {i.e., introduced as SCR in 1983; elevated to MH in 2000} Using PubMed, the following searches all retrieve the same citations back through the 1983 indexing year: Ketorolac [mh] Ketorolac [nm] 66635-83-4 [rn] Remember, if the chemical descriptor has indentations you would need to turn off automatic explosion by using the [mh:noexp] search tag (e.g., Antifreeze Proteins [mh:noexp]). Use text word searching or Previous Indexing headings for citations created prior to 1983. In this example, users should also see the Previous Indexing headings and History Note for Tolmetin.

   Figure 2
   
   

Why the Changes?
These changes make chemical searching in MEDLINE via PubMed more consistent. In the past, searchers had to use RN or NM in addition to MH to do a comprehensive search back to 1980. This was because searching MH alone did not retrieve chemicals with current SCR status, nor those chemicals having had SCR status before becoming a MeSH descriptor.

With these two changes in the treatment of chemical data in MEDLINE, users can now do a comprehensive search back to 1966 using RN or NM for MH status concepts. Furthermore, an MH search will retrieve back through the first year of introduction of that chemcial as an SCR without needing to "supplement" the strategy with an RN or NM. You'll still need to do text word searching or use Previous Indexing headings for the years prior to the introduction of the chemical into the controlled vocabulary.

Is This Reindexing?
On the surface, it may seem that NLM is reindexing existing citations with new concepts where SCRs have been promoted to MH status. In reality, it is more akin to a one-to-one maintenance action where the citation's information is simply being copied from one field (RN) to another (MH). The citations were originally indexed with the SCRs; therefore, we know the articles discuss these specific concepts. No additional examination of the articles is needed to determine if the MHs are appropriate. Given that the concepts are now MH status, it makes sense that users should automatically retrieve all citations indexed with those concepts regardless of the search tag used (MH or NM or RN).

Be advised that for this policy change neither the main point of the article nor applicable subheadings could be determined, so only the plain, corresponding MH was added to the citations retrospectively.

What About the History Note?
The History Note data in the MeSH Vocabulary File have not been updated to reflect the change in policy for the SCRs that have been elevated to MH status. The current history notes, while not wrong, do not account for the change in policy for 2001.

When searching for Ketorolac (see Figure 2) the History Note suggests using Ketorolac [nm] for the period 1983 through 1999. This is no longer necessary; Ketorolac [mh] will retrieve citations back to 1983. Furthermore, all three search strategies, Ketorolac [mh], Ketorolac [nm], and 66635-83-4 [rn] will retrieve the same results back through the 1983 indexing year. For citations created prior to 1983 (the date the concept of Ketorolac was first introduced to the controlled vocabulary), you will need to search with text words or the Previous Indexing headings back through time. In this example, users should also see the Previous Indexing headings and History Note for Tolmetin.

Why Does an RN That Should Be In PubMed Get No Postings?
Whenever an RN search yields zero retrieval, please try searching on the name of substance. NLM has not added Chemical Abstracts Service (CAS) registry numbers to MeSH for about 3 years. We are trying to address this deficiency. In the interim, use the MeSH Browser (from http://www.nlm.nih.gov/mesh/MBrowser.html) to ascertain if your substance of interest is in MeSH and then search on the name known to MeSH. This is one case where the new policy for MeSH heading chemicals that the MH, NM, and RN searching in PubMed should all retrieve the same results does not work. In summary, if an RN search fails, then try a name search. It would be best to qualify a name search with [nm] which would also retrieve chemicals that are SCRs and not MeSH descriptors. See Figure 3 for an example.

By Lou Knecht
Bibliographic Services Division
and
Vivian A. Auld
National Information Center on Health Services Research & Health Care Technology
and
Margaret A. McGhee
MEDLARS Management Section

Knecht L, Auld VA, McGhee M. Changes in the Treatment of Chemical Data in MEDLINE^® Citations. NLM Tech Bull. 2001 Nov-Dec;(323):e7.