Since 1975, NLM has included author-written abstracts from journals whose publishers gave the NLM permission for their use. These abstracts appear in the Abstract (AB) field. Over 50% of the documents in Ovid MEDLINE(R) contain abstracts. The abstract index contains all searchable words from the abstract.

The Abbreviated Source (AS) field includes a display of all the basic information needed to locate a citation, including the Journal Abbreviation (JA), the Volume(VO), Issue/Part (IP), Page (PG) and Date of Publication (DP).

The Authors (AU) field contains the authors of the article.  The format for authors is last name followed by one or more initials:  Smith JC or Smith J. Enter the last name, or if it is a common name, enter the last name and first initial. If you are unsure of the spelling of the last name (macdonald or mcdonald), enter one version and then scroll through the list of names to find the other.

Misspelled and corrected names are now included in the Author Index to ensure all possible matches. Only correct author names display, while incorrectly spelled names are indexed along with the correct spelling of the name.

This field is displayed in the Authors (AU) field.

This field is displayed in the Editors (EE) field.

This field is displayed in the Authors Full Name (FA) field.

This information displays as part of the Source (SO) field.

The Publisher Item Identifier (DI) field uniquely identifies individual published documents that are provided by the publisher for new citation submitted. The full PII appears as such e.g .S0036-36342008000600005.

Search all PII's by replacing punctuation with spaces such as "S0036 36342008000600005".di. for the displayed PII of S0036-36342008000600005.

The Editor (EE) field is phrase indexed.

The Electronic Date of Publication (EP) is the date the record was sent to NLM for inclusion in MEDLINE (R). This index appears in the format YYYYMMDD.

The ISSN Electronic (ES) field contains the Electronic International Standard Serial Number (EISSN) for the journal in which an article was published. It appears as an 8 digit number, separated by a hyphen: 0028-4793. To retrieve all of the articles for a journal, search by the Journal Name (JN).

Both Electronic and Print ISSNs can be searched in the ISSN Print (IS) field.

The Authors Full Name (FA) field contains the full name of the authors of the article when available.

The Editors Full Name (FE) field contains the full name of the editors of the article when available.

Subheadings are qualifiers added to MeSH subject headings to refine their meaning. Terms such as "etiology" or "therapy," when combined with a MeSH heading, give a precise idea of what an article covers.

The Floating Sub-Headings (FS) field contains both the 2-letter codes, such as "po," and the full heading, such as "poisoning." These are displayed following the corresponding MeSH Subject Headings.

Sometimes you may wish to retrieve every Floating Sub-Heading that includes a particular word; this is done by searching the single word in the Floating Sub-heading Word (FX) field.

This field displays as part of the Grant and Funding Information (GI) field.

This field displays as part of the Grant and Funding Information (GI) field.

This field displays as part of the Grant and Funding Information (GI) field.

The Gene Symbol (GS) field contains Gene Symbols as they are entered into Ovid MEDLINE(R) in the original document. They often, but not always, conform to standardized human gene nomenclature.  Ignore super or sub-scripts in entry of a gene symbol:  psub 1450 would be entered p1450. Enter hyphens as in the original symbol.

The Gene Symbol field was added to Ovid MEDLINE(R) in 1991.  Beginning in 1996, data will no longer be added to this field.

The Subject Heading Word (HW) index allows you to retrieve every MeSH subject headings and Publication Type that include a particular word. Do this by searching a single word in the Subject Heading Word (HW) field.

The Investigator Affiliation (IA) field contains author's full name and its affiliation to the investigator of the article.

The ISBN (IB) field appears as a 10-digit number and may contain 13-digit number if available.

The Indexing Method (IG) field was introduced by the NLM in September 2018. It will only be present in records if a value is specified. If Indexing Method is not present in status MEDLINE records, the indexing method is fully human indexed.

Values for Indexing Method (IG) are:

The ISSN Linking field will be used only for new records added during the 2008 production year (although NLM still does not know the timing when all will be in place to start to appear on records).

The Institution (IN) field contains the author's full name and its affiliation, which is usually the source for a reprint of the article. This information can be searched in most documents since 1988, but records from earlier years which were added to the database after 1988 may also contain institution information. Beginning with documents indexed in 1996, author's email address will also be included, if present in the journal.

The information is taken from the source document and is not standardized. Enter the single most descriptive word in an institution (harvard, not university). Consider both full spellings and abbreviations.

The Investigator (IR) field contains personal names of individuals (e.g., collaborators and investigators) who are not authors of a paper but rather are listed in the paper as members of a collective/corporate group that is an author of the paper. The format for authors is last name followed by one or more initials: Smith JC or Smith J.

The ISSN Print (IS) field contains the International Standard Serial Number (ISSN) for the journal in which an article was published. It appears as an 8 digit number, separated by a hyphen: 0028-4793. To retrieve all of the articles for a journal, search by the Journal Name (JN). Both Electronic and Print ISSNs can be searched in the IS index.

The Investigator NameID (IX) field is defined to contain a unique identifier associated with the name. The values in the Identifier attribute Source will designate the organizational authority that established the unique identifier.

The Journal Abbreviation (JA) field contains the abbreviation of the journal in which the article was published.

The Journal Name (JN) field contains the full name of the journal in which the article was published. Journal names are indexed as phrases, so enter enough letters of the journal name to locate the name in the index:  new engl (for New England Journal of Medicine). This field usually displays as part of the Source (SO) field.

If the first word of a Journal Name is "The", it has been stripped off.

The Keyword Heading Word (KF) index allows you to retrieve every Keyword Heading assigned by authors that include a particular word. Do this by searching a single word in the Keyword Heading Word (KF) field. 

The Keyword Heading Owner (KO) field contains an acronym that identifies the organization that provided the Keyword Heading (KW) data. The Keyword Heading Owner (KO) acronyms and their respective organizations are:

Acronym = Organization

KIE = Kennedy Institute of Ethics, Georgetown University
NASA = National Aeronautics and Space Administration
PIP = Population Information Program, Johns Hopkins School of Public Health
NLM = National Library of Medicine (used for the OLDMEDLINE records)
NOTNLM = (The journal publisher or other data provider. This is used when the Keyword Heading (KW) field includes author-supplied keywords. NLM began using this value in January 2013.)

The Keyword Heading (KW) field contains the Keyword Headings assigned by authors. Keyword Headings are the author keywords. To retrieve every Keyword Heading that includes a particular word, search for the word in the Keyword Heading Word (KF) field.

They appear for the first time in the 2023 MeSH.

The Name of Substance (NM) index contains single words from the CAS Registry/EC Number/Name (RN) field and is used to search portions of chemical names. This field displays as part of the CAS Registry/EC Number/Name (RN) field.

The Name of Substance (NM) field was added to Ovid MEDLINE(R) in 1980.

This field displays as part of the Grant and Funding Information (GI) field.

The Number of References (NR) field contains the number of bibliographic references listed in the review article.

Effective October 1, 2010 NLM discontinued the practice of including the number of bibliographic references listed in articles cited in MEDLINE. NLM had included the number of references for the following Publication Types: Review; Consensus Development Conference; Consensus Development Conference, NIH; Interactive Tutorial; Meta-Analysis. This change in policy is prospective only; we will not remove number of references data from existing citations.

This field displays in the Abstract (AB) and Title (TI) when available.

The General Note (NT) field contains general supplementary information about an item. The notes may include information on the availability of a bibliography, ordering information for dissertations or hearings, or general summaries of materials such as manuals or scripts. A series title may appear for a book or for audiovisual materials.

The NLM Journal Word (NW) index contains individual words from every journal name in the NLM Journal Name (NJ) field.

The Other Abstract (OA) field can include the Collaborating Partner, a brief abstract, Copyright and Language of the Other Abstract. The OA field is indexed in the Abstract (AB) field, and contains all searchable words from the Other Abstract.

Collaborating Partners include:

The Other ID (OI) is a unique alpha-numeric code. In particular, it will contain the PubMed Central Identifier (PMCID), where the alphabetical prefix 'PMC' precedes the identifier. If the publisher has embargoed access to the full text, the date of availability in PubMed Central will be noted. When the embargo is lifted, a revised record with the date of availability removed will be released.

Other ID's are entered into the index as phrases. In particular, PMC ID's should be searched as how the alpha-numeric code appears in PubMed Central (PMC).

The Organism Supplementary Concept (OS) field contains the Protocol Class 4 SCR terms that includes viruses and other organisms. The Organism Supplementary Concept (OS) terms are phrase indexed.

They appear for the first time in the 2018 MeSH and were made available as a means to index emerging viruses and other pathogenic organisms as they occur in the literature without delay

The Organism Supplementary Concept Word (OX) index allows the users to retrieve single terms from the Organism Supplementary Concept (OS) field.

The Primary Author (PA) index contains the first author listed for the article. This allows users to search for articles by any primary author. Author names display in Authors (AU) and Authors Full Name (FA) fields. Searching for smith rh.pa. will return results where Smith RH was listed as the main author of the article.

The Publisher (PB) index contains publisher information for books, monographs or any non-journal publications. This information is taken from the source document as it appears, so variant forms of a publisher are possible. This field is usually displayed as part of the Source (SO) field.

The Page (PG) field consists of the inclusive pagination of a journal article. Often a journal title combined with the beginning page number is enough to locate a citation. You can search for pages with alphanumeric characters, such as "a1.pg." This field displays as part of the Source (SO) field.

The Publishing Model (PI) field provides the model of publishing of the original document described in the record. The field indicates if the article cited is available in Print or Electronic format. It also indicates if the citation was converted from a Print or Internet source. This field is display only; you cannot search this field.       

The Publication History Status (PH) field contains information from the publisher regarding important events in the publishing process. It contains a date, and one of the following values for each date in the publication history:

[received] - date manuscript received for review
[accepted] - accepted for publication
[revised] - article revised by publisher or author
[aheadofprint] - published electronically, to be followed by the print

Other ID's are entered into the index as phrases. In particular, PMC ID's should be searched as how the alpha-numeric code appears in PubMed Central (PMC).

The Pubmed Central Release (PQ) field contains the embargo date associated with the availability of the published article in PMC.

The Protocol Supplementary Concept (PS) field contains the Protocol Class 2 SCR terms. These terms were previously exported in CAS Registry/EC Number/Name of Substance (RN) in MEDLINE/PubMed. The PS terms are phrase indexed.

With the 2011 reload, Protocol Supplementary Concept (PS) were loaded separately only for new records added during 2011 production year.

The Protocol Supplementary Concept Word (PX) index allows the users to retrieve single terms from the Protocol Supplementary Concept (PS) field.

In addition to indexing documents with Medical Subject Headings, the indexers at the National Library of Medicine assign terms to describe the article type. Publication Types include such classifications as reviews, clinical trials, directories and letters. These terms appear in the Publication Type (PT) field.

To view the index, enter enough letters to distinguish a publication type (jour for "journal article"); the entire index is quickly scrollable to determine all possible publication types.

With the 2005 reload, Publication Types are also loaded into the MeSH. Because some Publication Types are also MeSH terms, the syntax to enter the Tree is "Randomized controlled trial [publication type]". To post the results from the Tree without entering the Tree, the syntax is: "Randomized controlled trial [publication type]"/ or Randomized controlled trial.pt.

The RN field can contain the following:
- The unique 10-digit Unique Ingredient Identifiers (UNIIs) assigned by the Food and Drug Administration (FDA) Substance Registration System (SRS)
- The 5- to 9-digit Chemical Abstracts Service (CAS) for Registry Numbers
- The number EC derived from Enzyme Nomenclature to designate a particular enzyme

To find UNII number 8R78F6L9VO (Sumatriptan) search for: 8R78F6L9VO Sumatriptan.rn. or: 8R78F6L9VO*.rn.
To find CAS number 54-16-0 (Hydroxyindoleacetic Acid)), search for: 54-16-0 Hydroxyindoleacetic Acid.rn.
To find EC number (EC 1-14-14-1 (Aryl Hydrocarbon Hydroxylases)), search for: ec-1-14-14-1 aryl hydrocarbon hydroxylases.rn.

A zero (0) is a valid value when an actual number cannot be located or is not yet available. E.g. search for: 0 Pulmonary Surfactants.rn

Note: NLM has not added Chemical Abstracts Service (CAS) registry numbers to MeSH since 1998, therefore only registry numbers that were previously available are displayed in the MEDLINE record.

The Record Owner (RO) field contains the owner of the record.

The Report Number (RR) field contains the primary identification number of a report. This is normally the number given to the report by the issuing organization (not the US Government Clearing House Number). Inspec treats IEEE Standards as reports.

The Rare Disease Supplementary Concept (RS) field contains the Protocol Class 3 SCR terms. The RS terms are phrase indexed.

Beginning with 2011 MEDLINE/PubMed data, Rare Disease Class 3 SCRs are also going to be exported.

The Rare Disease Supplementary Concept Word (RX) index allows the users to retrieve single terms from the Rare Disease Supplementary Concept (RS) field.

The Secondary Source AN (SA) index contains the unique identifiers of outside resources discussed in MEDLINE articles. The display will include the Secondary Source ID (SI) and the unique identifier separated by a forward slash (GENBANK/S78411). If you wish to search all works from a particular databank use the Secondary Source ID (SI) index: refseq.si.

The Journal Subset (SB) field identifies the subset for MEDLINE records from certain journal lists on specialized topics. The NLM discontinued the Journal Subset (SB) indexing for MEDLINE/PubMed with the 2022 reload, with the exception of "index medicus.sb." and "oldmedline citations.sb.". Ovid replaced the corresponding Journal subset limits new journal selections based on NLM Catalog searches, using matching MeSH terms and title words to create new lists of titles for the below journal subsets. We strongly recommend that you review your search strategies if you used the Journal Subset (SB) field, or limits in search strategies prior to the 2022 reload.

Do not confuse these journal subsets with the Topics subsets available on PubMed's Limits screen, which are based on different search strategies.

The Season (SE) field contains the month or season that the citation article was published.

The field usually consists of a three-character alphabetic abbreviation of the month or season.

The MeSH Subject Headings (SH) field contains the Medical Subject Headings used by indexers at the National Library of Medicine (NLM) to describe the content of an article. NLM's MeSH terms are organized in a hierarchy, or "tree" structure.

MeSH headings are entered into the index as phrases and should be searched as they appear in the NLM MeSH books or in our Tree display.

The Secondary Source ID (SI) identifies secondary source databanks with unique identifiers of outside resources discussed in MEDLINE articles. The field is composed of the source acronym: GENBANK.

The Secondary Source ID (SI) formerly known as the Molecular Sequence (MS) field was added to Ovid MEDLINE(R) in 1988.

The Status (ST) field contains the status of the citation.

In-Data-Review records have passed the first step in quality control and will either be reissued as In-process records or go directly to PubMed-not-MEDLINE as the final record status.

In-Process records are electronically submitted and have gone through the second citation-level review.

Medline records have passed all steps in quality control and have been indexed.

Publisher - Citations recently added to PubMed via electronic submission from a publisher, and are soon to proceed to the next stage, PubMed - in process. This tag is also on citations received before late 2003 if they are from journals not indexed for MEDLINE, or from a journal that was accepted for MEDLINE after the citations' publication date. These citations bibliographic data have not been reviewed.

PubMed-not-MEDLINE records have gone through the second citation-level review, but are from non-Medline journals or precede the date a journal was selected for MEDLINE indexing but are now being submitted for inclusion.

The Synonyms (SY) field contains a synonymous name for generic drug names that are not actual trade names.

The Title Comment (TC) field pertains to and contains citations to associated journal publications, e.g., comments, errata, retractions, or cited references. These data enable links between the record at hand and its associated citations.

This field begins with the indication of comment or correction type (see Comments (CM) fields for a full list of types) and then displays the Journal Abbreviation (JA), Date of Publication (DP), Volume (VO), Issue/Part (IP) and Page (PG).

The Text Word (TW) index in Ovid MEDLINE(R) includes Title (TI) and Abstract (AB).

In order to perform a free-text search in all of the relevant fields that contain text words appropriate for a subject search, you are advised to use the multi-purpose field (MP), e.g. heart.mp.

The Unique Identifier (UI) field contains the PMID (PubMed identifier or PubMed unique identifier) that is the unique number assigned to each PubMed record.

PMIDs do not change over time or during processing and are never reused.

They appear for the first time in the 2023 MeSH.

NLM does not expect many journals will publish versions, thus the incidence of versioned citations will be small.

The Exploded Subheadings (XS) field displays the two-letter codes for the pre-exploded subheadings.

Sometimes multiple MeSH Subheadings can be logically grouped together. Such related groups of subheadings are "pre-exploded" under one broad subheading. This means that all the grouped subheadings can be retrieved together by searching on the "pre-exploded" version of the broad subheading. These pre-exploded MeSH Headings are displayed in the MeSH Subject Headings (SH) field. Their complete list can be found in https://www.nlm.nih.gov/mesh/subhierarchy.html. The CAPITAL two letter codes in the left column are the Exploded Subheadings. Terms under the exploded subheading are also searched when you use the exploded subheading.

For example, the Ovid search: an.xs retrieves any records that contain the subheadings, Analysis, Blood, Cerebrospinal Fluid, Isolation and Purification or Urine.