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Clinical Trial Registry Numbers in MEDLINE®/PubMed® Records

A clinical trial is a research study using human volunteers to answer specific health questions. The International Committee of Medical Journal Editors (ICMJE) has required that all clinical trials be publicly registered before papers reporting the results of the clinical trial are submitted and accepted for publication. See the ICMJE Web site for their editorial and updates on clinical trial registration. is an ICMJE-acceptable public registry, offering up-to-date information for locating clinical trials for a wide range of diseases and conditions. The U.S. National Institutes of Health (NIH), through its National Library of Medicine® (NLM®), developed this site in collaboration with the Food and Drug Administration (FDA), as a result of the FDA Modernization Act, which was passed into law in November 1997.

In July 2005, NLM began to include the registry number in the MEDLINE record when the number is published as part of the original paper. The format for the registry number is “NCT” followed by an 8-digit number, e.g.: NCT00000419.

In PubMed, registry numbers are included in the Secondary Source ID (SI) field, and searchable using the [si] tag, e.g.,

nct00000419 [si]

To retrieve all records in PubMed that list trial registration in, search: [si]

Other identification numbers may be assigned to a trial by the institute, agency, or organization sponsoring the trial, e.g., NEI-63. These additional numbers are part of the registry databank and may be searched directly in that databank to discover the “NCT” number; these additional numbers will not be carried in the MEDLINE record.

ISRCTN Register

The ISRCTN Register is another registry that meets the ICMJE requirements. In May 2006, NLM began to include International Standard Randomised Controlled Trial Number (ISRCTN) data in the MEDLINE record when the number is published in the article. The basic format for the ISRCTN is “ISRCTN” followed by an 8-digit number, e.g., ISRCTN80764175.

In PubMed, these registry numbers are searchable using the [si] tag, e.g.,

isrctn80764175 [si]

To retrieve all records in PubMed that list trial registration in the ISRCTN Register, search:

isrctn [si]

Records created prior to May 2006 with an ISRCTN in the article title or abstract of the online citation have been edited to create an SI field.

XML Data

In the MEDLINE XML format, the DataBankList set of elements (DataBank and AccessionNumber elements can be multiply-occurring) contains the registry number data, e.g.,

<DataBankList CompleteYN="Y">

For more information about clinical trials data in MEDLINE, see the following NLM Technical Bulletin articles:

Williams RJ, Tse T. New Look for NLM Tech Bull. 2007 Sep-Oct;(358):e2.
Knecht LS. ISRCTN to be Added to MEDLINE®/PubMed® Data. NLM Tech Bull. 2006 Mar-Apr;(349):e9.
Ripple A, Tse T, Zarin DA. Scope Expanded. NLM Tech Bull. 2005 Jul-Aug;(345):e6.
Tybaert S, Knecht LS. Identifier to be Added to MEDLINE/PubMed Data. NLM Tech Bull. 2005 May-Jun;(344):e13.

For more about MEDLINE data, see the MEDLINE/PubMed Data Element (Field) Descriptions and the MEDLINE/PubMed Resources Guide.