- A clinical trial citation names the sponsor, the year, the trial title, the registry identifier and the URL of the registry record.
- The NCT number, assigned by ClinicalTrials.gov at registration, is the element that lets anyone retrieve the trial.
- ClinicalTrials.gov retired its classic website on June 25, 2024: record addresses now follow the format clinicaltrials.gov/study/NCTXXXXXXXX.
- For a trial still in progress, give an open date range and the date you consulted the record.
- In the EU, trials under the Clinical Trials Regulation are registered in CTIS, mandatory for new applications since January 31, 2023.
To cite a clinical trial, name the sponsor or principal investigator, the year of registration, the trial title, its registry identifier (the NCT number on ClinicalTrials.gov) and the URL of the registry record. The citation style sets the order of these elements, and APA and AMA are the two most common in clinical research. Citing the registry entry, and not only the publication, lets a reader check the protocol, the endpoints and the status of the study.
Accurate referencing makes it possible to:
- Formally identify the study
- Verify consistency between the protocol and publication
- Avoid publication bias
- Ensure reproducibility
How to cite a clinical trial in APA or AMA style?
The citation style depends on the publication context. Here are the two most common formats:
APA style (American Psychological Association)
APA has no dedicated rule for clinical trials. University library guides recommend citing the registry record with the sponsor as author and the identifier after the title:
Sponsor or Investigator. (Year, Month Day). Title of the trial (Identifier NCTXXXXXXXX). ClinicalTrials.gov. https://clinicaltrials.gov/study/NCTXXXXXXXX
AMA style (American Medical Association)
Name P. Study title. ClinicalTrials.gov. NCT Identifier: NCTXXXXXXXX. Updated [date]. Accessed [date].
In both cases, the NCT identifier, the unique number assigned by the ClinicalTrials.gov registry, is essential. It allows the trial to be retrieved at any time and its data to be verified in real time.
ClinicalTrials.gov retired its classic website on June 25, 2024. Study records now sit at clinicaltrials.gov/study/ followed by the NCT number. Links in the old ct2/show format still redirect, but a new citation should use the current address.
How do you cite a trial that is still ongoing?
Give the start date followed by an open range, for example (2025, March – ), and add the date you consulted the record. The sponsor updates a registry entry during the trial (status, enrollment, sometimes the endpoints), so the access date tells your reader which version you read.
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Where can reliable information on clinical trials be found?
Several official registries provide access to structured and verified information on ongoing or completed clinical trials:
- ClinicalTrials.gov: the reference registry, managed by the NIH, which lists thousands of studies worldwide.
- CTIS (Clinical Trials Information System): the EU portal for trials under the Clinical Trials Regulation. Its use became mandatory for new applications on January 31, 2023, and ongoing trials had to move there by January 30, 2025.
- EU Clinical Trials Register: trials authorized under the former Clinical Trials Directive, identified by a EudraCT number.
- ISRCTN Registry: an international registry recognized by the WHO, used in particular for public health studies.
- WHO ICTRP: the international platform of the World Health Organization, which aggregates data from several national registries.
These platforms provide the following information for each trial: title, objectives, type of intervention, expected number of participants, and inclusion criteria.
What are the best practices for registering a clinical trial?
Prospective registration is now a regulatory and editorial requirement.
1. Register before enrolling the first patient
Required by most scientific journals and regulatory authorities. The ICMJE asks journal editors to require registration at or before the first patient's consent.
2. Enter the key structuring elements
- Primary objective
- Primary endpoint
- Sample size
- Intervention plan
- Target population
3. Ensure document consistency
The registry must be aligned with:
- The protocol
- The statistical analysis plan
- The final publication
Inconsistencies are frequently detected during peer review.
4. Update the status
- Recruiting
- Active, not recruiting
- Completed
- Terminated
Sources
- National Library of Medicine, Technical Bulletin, May-June 2024: retirement of the classic ClinicalTrials.gov website on June 25, 2024. https://www.nlm.nih.gov/pubs/techbull/mj24/mj24_Modern_Clinical_Trials_Website.html
- European Medicines Agency, Clinical Trials Information System. https://www.ema.europa.eu/en/human-regulatory-overview/research-development/clinical-trials-human-medicines/clinical-trials-information-system
- ICMJE, Clinical Trial Registration. https://www.icmje.org/recommendations/browse/publishing-and-editorial-issues/clinical-trial-registration.html
- Singapore Institute of Technology Library, citing clinical trials in APA 7. https://libguides.singaporetech.edu.sg/citation/clinicaltrials
How do you cite a clinical trial from ClinicalTrials.gov?
Name the sponsor or investigator, the registration date, the trial title, the NCT number and the record's URL, in the order your style requires. In APA: Sponsor. (Year, Month Day). Title of the trial (Identifier NCTXXXXXXXX). ClinicalTrials.gov. https://clinicaltrials.gov/study/NCTXXXXXXXX. Add the date you consulted the record if the trial is still running.
What is an NCT number?
The identifier ClinicalTrials.gov assigns to a trial when it is registered: the letters NCT followed by eight digits. It stays the same for the life of the record, which makes it the most reliable element of a citation. The ICMJE recommends that journals publish it at the end of the abstract.
How do you cite an ongoing clinical trial?
Use the start date with an open range, for example (2025, March – ), and give the date you accessed the record. Status, enrollment and sometimes endpoints change while a trial runs, so the access date fixes the version you read.
How do you cite a trial registered outside ClinicalTrials.gov?
Keep the same structure and use that registry's identifier and URL: the EU CT number for a trial in CTIS, the EudraCT number for an older European trial, the ISRCTN number on the ISRCTN registry. Each record on the WHO ICTRP portal keeps the identifier of its primary registry.
Should you cite the registry entry or the published results?
Both, once results are published. The publication reports the findings. The registry entry shows what the protocol planned before the first patient, so your reader can compare the planned endpoints with the reported ones.
With over 10 years of experience working in CROs, Khalil brings deep expertise in clinical trials and a clear understanding of the daily challenges faced by research professionals. His insights are grounded in real-world operations, making his perspective both practical and strategic.
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