Definition
NCT Number
An NCT number is the unique identifier ClinicalTrials.gov assigns to each registered study, written as the letters NCT followed by eight digits, such as NCT01234567. It stays with the study for the life of the record.
1 min readReviewed September 14, 2026
Also known as: NCT ID, ClinicalTrials.gov identifier, NCT identifier, Trial registration number
Key facts
- Assigned by
- ClinicalTrials.gov, National Library of Medicine
- Format
- NCT plus 8 digits, for example NCT01234567
- Scope
- One number per registered study, covering all of its sites
- Cited in
- Journal articles, PubMed records, grant reports, some Medicare claims
What is an NCT number?
The NCT number is the primary key of ClinicalTrials.gov. When a sponsor or investigator registers a study through the Protocol Registration and Results System (PRS), the study receives an NCT number that appears in its public record address and stays the same through every later update.
A multisite trial has one NCT number, not one per site. The sponsor's internal protocol number, grant numbers and identifiers from other registries, such as the EU Clinical Trials Information System (CTIS), can be listed as secondary IDs on the same record.
Where NCT numbers are used
The identifier travels well beyond the registry itself:
- Journal publications: the International Committee of Medical Journal Editors (ICMJE) requires trials to be registered in a public registry before the first participant enrolls, and articles cite the registration number.
- PubMed: article records carry the NCT numbers of the trials they report, linking publications back to registrations.
- Medicare claims: CMS asks providers to report the clinical trial number on claims for items and services furnished in qualifying clinical trials.
- Funder reporting: NIH and other funders ask grantees to report the NCT numbers of the trials they support.
Why the NCT number matters
Trial names, acronyms and drug code names change and repeat across programs, but the NCT number does not. It is the reliable join key for connecting a trial registration to its publications, posted results, regulatory documents, press releases and site lists.
Analysts should still check for duplicates. A study can occasionally be registered twice, and one development program often spans several NCT numbers, such as a core study and its open-label extension. Counting NCT numbers is not the same as counting distinct research programs, and trials registered only outside ClinicalTrials.gov have no NCT number at all.