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NLM Classification 2017: Worldwide source of medical library classification

About the Classification Index    

How to Use the Index
     Relationship to MeSH®
     Cross References
          See References
               Pointing to a Single Index Term
               Indented under Index Terms
          General References
          See Also References
               Under Indented Terms
               To General Terms
     Drugs, Chemicals and Biological Agents


The Index to the National Library of Medicine Classification consists of Medical Subject Headings (MeSH). Some non-MeSH terms are used as cross references when no appropriate MeSH term is available. All MeSH entries in the Index are updated annually to be consistent with the latest edition of MeSH. The index terms are arranged in alphabetical order with Roman numerals filed as letters in this arrangement. Arabic numerals are found at the beginning of the Index.

The classification numbers assigned to the index terms are usually general numbers for the concept represented or numbers reflecting a medical view when that is more appropriate.

Indented terms represent more specific aspects of the subject or aspects of the subject to which a number different from the general number has been assigned. The indented terms are often elliptical and should be interpreted broadly. For example, when "Organic chemistry" is used as a term indented under the name of a chemical, the number following it is to be selected if the principal focus of the work being classified is the organic chemistry of the chemical. Some subheadings refer the user to another heading. General references or see also references are listed at the end of the alphabetical sequence of the indented terms under the index term.

Electrodes QD 571
     Biomedical engineering QT 36
     In electric stimulation therapy WB 495
     Used for special purposes, by subjects, e.g., in Urinalysis QY 185
     See also Microelectrodes QT 36, etc.

Library of Congress (LC) numbers are assigned to subjects that fall outside the scope of the NLM Classification. When a concept represented in MeSH has no exact equivalent in LC's schedules a number was selected that fit the concept most closely. Since NLM rarely uses LC's K (Law) schedules the numbers provided for index terms relating to laws are for the subject rather than the law.

How to use the Index


The Index is not a substitute for the main schedules. A user should always refer to the schedules for confirmation of the proper application of the number and its relationship to other numbers.

Many headings are assigned a range of numbers rather than a specific number. The schedules of the NLM Classification or the Library of Congress Classification are the only source of the meaning of specific numbers within the range.

The number assigned to a heading in this Index should not be used unless it represents the principal subject of the material being cataloged.

Relationship to MeSH

The Index represents only those MeSH terms that are linked to NLM or LC classification notation. The Index does not include all MeSH headings found in the Medical Subject Headings Annotated Alphabetic List or the MeSH Browser and it is not a substitute for these.

When assigning MeSH headings to a work the Annotated Alphabetic List or the MeSH Browser must always be consulted. The Index does not provide annotations nor does it show relationships between headings; these are found in the Annotated Alphabetic List and the Tree Structures, respectively, or the MeSH Browser. To view MeSH descriptor data, when available, click on the "tree" icon Mesh Browseron the left of the index term, which links to the term in the current Browser.


The number of indented terms under an index term varies greatly. The choice was dependent upon the needs that arose in the past. Therefore, the list is in no way exhaustive of the possibilities that can occur.

The Index contains over ten thousand index terms to which classification notations are assigned. Many terms are found only in the index and will not appear in the schedules. They refer to a number in the schedule where only a broader term or a related term appears.

Cross References

There are several types of cross references used in the index.

See References Point to a Single Index Term

A see reference points to a single index term or concept.

    Acquired, Immunity - see Immunity

References Indented Under Index Terms

See references that are indented under index terms link the concept to a more specific index term.

          Developing countries WA 395
          Mental see Mental Health WM 105, etc.
          Oral see Oral Health WU 113
          Public see Public Health WA

General References Under Index Terms

General references following all indented terms or concepts relate to the main index term or concept. Examples of general references are: "Used for special purposes, by subject, " " Specific types of [topic], by subject," etc.

          First aid WA 292
          Hospital programs see Disaster Planning WX 185
          Specific types of disaster, by subject
          See also Civil Defense UA 926-929, etc.

See also References Under Indented Terms

A See also reference provided under an indented term relates only to the indented term. When the See also reference is related to the main index term only it follows all indented terms and general references. The latter is represented in the Disasters example above (See also Civil Defense UA 926-929, etc.). The former is represented here.

          Alcoholic beverages HF 6161.L46
          Pharmaceutical QV 736
                See also Drug Labeling QV 835
          Tobacco HF 6161.T6
          Other special subjects, by business in HF 6161 or other appropriate number

See also References to General Terms

See also references lead the user from one index term to a related index term under which are listed indented terms that apply equally to both headings.

     Accidents, Occupational WA 485-491
          Of the eye WW 505-525
          See also special topics under Accidents
          First aid WA 292
          In anesthesia WO 288
          Medicolegal aspects
               Cause of death W 843
               Disability evaluation W 900-925
          See also Insurance, Accident W 100-250, etc.

Note when "etc." follows a number in any type of reference, it indicates that in addition to the numbers given in the reference, there are other numbers which also represent the index term. The user will find these other numbers listed under the index term. For example, the see also reference, under Accidents, to "Insurance, Accidents W 100-250, etc.," indicates that under the index term "Insurance, Accident" there are other class numbers in addition to the range W 100-250.

Drugs, Chemicals and Biological Agents

The numbers provided after index terms for drugs, chemicals and biological agents represent their biochemical, pharmacological or chemical properties. The index rarely gives a number for these agents when the material being cataloged discusses their use in the therapy of a particular disease or in a particular study. In such cases the material is classified with the disease or the subject of the study.