Study Identifier

A study identifier is the unique code a registry assigns to a specific research study — a clinical trial, a systematic review, or another registered research project — to identify that study itself. It is a distinct concept from an identifier assigned to one of the study’s resulting outputs (a DOI on a results paper or dataset) or to an individual researcher (an ORCID iD on the principal investigator). A study identifier’s format, prefix, and issuing authority are registry-specific — there is no single universal ‘study identifier’ scheme, only a family of registry-specific ones.

Why a study identifier is not a DOI, an ORCID iD, or a grant number

These four identifier types answer four different questions, and a single research project routinely has all four attached to different parts of it at once:

• Study identifier — which registered study is this? Assigned once per registry, at registration.
• DOI — which specific output (article, preprint, dataset, protocol paper) is this? A single study can produce several DOI-bearing outputs over its lifetime, each with its own DOI, while the study itself keeps one identifier per registry.
• Researcher identifier (e.g. ORCID iD) — which person is this? Identifies an investigator, not the study they run.
• Grant award number — which funding award is this? A registry record may list the grant number that funded a study without that number being the study’s own identifier.

Conflating these is a common source of citation and metadata errors — for example, citing a trial’s NCT number as if it were a publication identifier, or citing a results paper’s DOI as if it uniquely identified the underlying trial (it doesn’t: the same trial can generate a protocol paper, an interim-results paper, and a final-results paper, each with its own DOI, all pointing back to one NCT number).

Major study identifier formats

Each primary clinical trial or study registry issues its own identifier format:

• NCT number — issued by ClinicalTrials.gov, the registry operated by the U.S. National Library of Medicine. Format: ‘NCT’ followed by an eight-digit number, e.g. NCT00000419.
• ISRCTN number — issued by the ISRCTN registry, a primary registry in the WHO International Clinical Trials Registry Platform (ICTRP) network, owned by a non-profit ISRCTN company and administered by BioMed Central (part of Springer Nature). Format: ‘ISRCTN’ followed by an eight-digit number, e.g. ISRCTN12345678.
• PROSPERO ID — issued by PROSPERO, the International Prospective Register of Systematic Reviews, maintained by the Centre for Reviews and Dissemination (CRD) at the University of York. Format: ‘CRD42’ followed by digits, e.g. CRD42020162679. Unlike the other formats here, PROSPERO registers systematic review protocols rather than interventional trials.
• Other primary registries — the WHO ICTRP network recognizes additional national and regional primary registries with their own identifier formats, including the Australian New Zealand Clinical Trials Registry (ANZCTR) and the EU’s Clinical Trials Information System (CTIS). The WHO also issues a voluntary Universal Trial Number (UTN) that is not itself a registry identifier but exists specifically to tie together records for the same trial across multiple national registries.

Why one study can carry several identifiers

It is normal, not an error, for a single study to have more than one study identifier attached to it:

• Cross-registration. A trial run in the UK with US sites might register with both ISRCTN and ClinicalTrials.gov to satisfy different national or funder requirements, producing two valid, permanently linked identifiers for the same underlying study.
• Registry migration or re-registration. Older studies occasionally carry a legacy identifier from a since-superseded system alongside their current one.
• Different registry types for different study stages. A systematic review of trials on a topic will have its own PROSPERO identifier for the review protocol, entirely separate from the NCT or ISRCTN numbers of the individual trials it analyzes.
• The UTN as a linking mechanism. Where a UTN has been requested, it functions specifically to connect a study’s multiple registry-specific identifiers as referring to one underlying study, without replacing any of them.

Because of this, a complete, unambiguous citation to a study should generally reference the specific registry and identifier being cited (e.g. ‘ClinicalTrials.gov identifier NCT00000419’) rather than a bare number, since the same string of digits has no fixed meaning outside its issuing registry’s namespace.

Related CASRAI resources

For the compliance mechanics around trial registration deadlines and results-reporting obligations, see the clinical trial registration term and CASRAI’s clinical trial registration and reporting compliance guide. For how study identifiers fit into the broader persistent-identifier ecosystem alongside DOIs, ORCID iDs, and ROR IDs, see Persistent Identifier Federation Across Research Infrastructure. For the protocol-registration step that typically produces a study identifier in the first place, see Preregistration of a Study Protocol.