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NLM Classification 2012: Worldwide source of medical library classification

About the NLM Classification

Scope of Revision
Historical Development
Structure of the NLM Classification
Relationship to MeSH®

Scope of Revision

This edition incorporates all 2013 additions and changes to the schedules (see Class Numbers Added and Canceled) and to the index, which has been newly edited and updated with appropriate MeSH concepts. One hundred and eight (108) new index entries were created of which forty (40) are from the 2013 MeSH; the remainder are MeSH terms from previous years. Numerous main index entries and cross references were modified to reflect changes in the MeSH vocabulary. Two-hundred and eighteen (218) index entries were deleted.

Although the broad nature of the NLM Classification schedules precludes complete correspondence with MeSH, new class numbers were added and existing class numbers were canceled as needed to reflect other changes in MeSH and the biomedical and related sciences literature cataloged.

The QU (Biochemistry. Cell Biology and Genetics) schedule was the major area of focus for the 2013 edition.

Background: With the 2005 edition of the NLM Classification, NLM established a new range of numbers in QU for cells (QU 300-400) and genetic concepts (QU 450-500). These topics were formerly scattered in various numbers in the Library of Congress general biology schedule, QH. When the genetic terms were brought into the NLM schedule, they were split between the QU schedule and the QZ (Pathology) schedule (for clinical aspects).

With the 2013 edition:

  • Most genetic terms have been consolidated in the QU schedule
  • Several new class numbers were created in the QU schedule for genetic terms
  • Genetic terms currently classed in QZ deal with genetic causes of diseases and genetic testing
  • Animal genetic terms are classed in LC QH schedule (e.g., Animals, Genetically Modified is classed at QH 442.6)

In addition:

Some notes at the beginning of the QU schedule were added or changed, for example:

  • Classify medical genetics in QZ 50 was changed to Classify genetic causes of disease in QZ 50.
  • Classify viral genetics in QW 160 was added.
  • Classify plant genetics in QK was added.

Some QU schedule outline headers were changed, for example:

  • QU 145-220 Vitamins was was changed to QU 145-220 Nutrition. Vitamins

Some QU class number captions and notes were revised to better reflect the scope of the number. For example:

  • At QU 34, the caption Biochemical phenomena (General or not elsewhere classified) was changed to Biochemical phenomena. Molecular biology (General or not elsewhere classified).
  • At QU 84, the caption Sugar acids and their salts and esters (General or not elsewhere classified) was changed to Sugar acids. Sugar alcohols. Sugar phosphates (General or not elsewhere classified).
  • At QU 130, the caption Inorganic substances (General or not elsewhere classified) was changed to Inorganic chemicals (General or not elsewhere classified).
  • At QU 220, the caption Other vitamins was change to Flavonoids.
  • At QU 107, the note was added: Classify works on growth hormone at WK 515.
  • At QU 375, the note was added: Classify physiology of specific cell with the cell.
  • At QU 500, the note was added: Classify works on genetics in general in QU 450.

Changes were made to other class schedules. For example:

  • At QZ 42, Adverse effects of drugs was removed from the caption.
  • At QZ 50, Heredity. Medical genetics was changed to Heredity. Genetic causes of disease.
  • At QZ 52, Genetic techniques was changed to Genetic testing.
  • At WG 141.5.A3, Angiocardiography. Angioscopy was changed to Angiography. Angioscopy.
  • At WG 500, Radiography was removed from the caption.
  • The schedule outline WR 650-670 Therapy was added.
  • At WX 162, the caption was changed from Patient care planning. Progressive patient care. Long-term care to Hospital care. Patient care planning. Long-term care.
  • Instructional notes were added or modified to clarify classification practices as needed. For example:
    • A note was added to QY 95: Classify techniques associated with a specific type of cell with the cell.
    • A note was added to WG 169: Classify works on cardiac anesthesia in WG 460.
  • Table G (geographic breakdown) is now permitted with W 76.

Changes were made to Table G (Geographic Notations).

One Table G number was added: GB1-- Balkan Peninsula

Historical Development

The genesis of the NLM Classification is a Survey Report on the Army Medical Library, published in 1944, which recommended that the "Library be reclassified according to a modern scheme," and that the new scheme be a mixed notation (letters and numbers) resembling that of the Library of Congress. Subsequently a classification committee was formed, chaired by Keyes D. Metcalf and including Mary Louise Marshall who compiled the schedules. Medical specialists acted as consultants to the committee. Based on the consultants' advice, that of the committee and of the NLM cataloging staff, Ms. Marshall produced a preliminary edition of the Library's Classification, which was issued in 1948.

The preliminary edition was revised by Frank B. Rogers and the first edition of the new classification was published in 1951 as the U.S. Army Medical Library Classification. It firmly established the current structure of the classification and NLM's classification practices. The headings for the individual schedules were given in brief form (e.g., WE - Musculoskeletal System; WG - Cardiovascular System) and together they provided an outline of the subjects that constitute the National Library of Medicine Classification . These headings were interpreted broadly as including the physiological system, the specialty or specialties connected with them, the regions of the body chiefly concerned and subordinate related fields. Within each schedule, division by organ usually has priority. All schedules, including some of their sections, are preceded by a group of form numbers representing publication types. These numbers, ranging generally from 1-39, are employed as mnemonic devices throughout the Classification.

Beginning with the 2002 edition, the National Library of Medicine Classification is published in electronic form and updated annually:

The online environment offers many advantages to users including hyperlinks between class numbers in the index and the schedules, and between terms within the index and direct links from these to the MeSH record itself under the MeSH Browser.

The online data creation and maintenance system of the Classification gives NLM the ability to update the classification annually in tandem with MeSH. Publication of printed editions ceased with the 5th revised edition, 1999. Beginning with the 2006 edition, the NLM Classification is also available in PDF (Portable Document Format) at

Structure of the NLM Classification

The National Library of Medicine Classification covers the field of medicine and related sciences, utilizing schedules QS-QZ and W-WZ, permanently excluded from the Library of Congress (LC) Classification schedules. The various schedules of the LC Classification supplement the NLM Classification for subjects bordering on medicine and for general reference materials. The LC schedules for Human Anatomy (QM), Microbiology (QR) and Medicine (R) are not used at all by the National Library of Medicine since they overlap the NLM Classification.

Relationship to MeSH

The schedules, with their special requirements for use with all types and forms of materials, preclude strict adherence to the hierarchical arrangement of the Medical Subject Headings (MeSH), the Library's thesaurus for indexing and cataloging. The schedules maintain their own character in order to provide for material, old as well as new, acquired for the Collection, including dictionaries, atlases, directories and other items, which are not suitable for the arrangements, found in MeSH. However, an effort was made to make schedule headings, subheadings and class number captions compatible with MeSH terminology. The MeSH Tree Structures were used extensively to determine the proper placement of a concept in a schedule and to relate index headings to one another. Since the representation of subjects in the schedules of the NLM Classification is intentionally broad, the captions do not enumerate all of the subordinate concepts that are to be classified in a given number. MeSH descriptors for these subordinate concepts do appear in the index, to the extent practical with appropriate references to the classification numbers.


The Index to the NLM Classification consists primarily of Medical Subject Heading (MeSH) concepts used in cataloging. It includes concepts first appearing in the latest edition of MeSH and other older concepts as warranted by the literature cataloged. It also provides access to classification numbers through these MeSH concepts. Index entries are updated annually to reflect additions and changes of the latest edition of MeSH. For additional information on the Index see About the Index.