Trial registration creates a public record of all clinical trials that researchers are planning and what they intend to do, e.g. which intervention(s) they will be testing, the comparator(s) they will use, and what outcomes they will be measuring. See the WHO Trial Registration Data Set for a list of the minimum amount of data that should be included for a trial to be considered fully registered.
A clinical trial is any study which meets the World Health Organisation (WHO) definition: "any research study that prospectively assigns human participants or groups of humans to one or more health-related interventions to evaluate the effects on health outcomes". You can find more information about trial registration and what counts as a “health-related intervention” and a “health outcome” on the International Committee of Medical Journal Editors (ICMJE) website.
If you are carrying out a clinical trial, you must register it in an approved clinical trial registry before you enrol the first patient. Details of suitable registries are listed on the ICMJE website and include the primary registries in the WHO network as well as clinicatrial.gov.
You should include the trial registration number (also known as a clinical trial number) in the abstract of any manuscripts that report the results of that clinical trial.
Why should clinical trials be registered?
Trial registration makes the research process more transparent.
Registering clinical trials helps make sure that researchers try to publish all research that is carried out, even if the outcome was non-significant or ‘negative’. This helps to prevent publication bias (publication of only some of the research that has taken place) and means that those making clinical decisions to make them based on all of the evidence.
It also prevents outcome selection bias which is when researchers change the outcomes of their study or just report some of the results. Selection bias can also distort the evidence on which clinical decisions are made.
Knowing what studies are currently taking place also avoids another group of researchers starting the same study at the same time.
What if I have started a clinical trial but haven’t registered it?
If you have already started a clinical trial, but you have not registered it yet, you should register it now. Some trial registries will allow you to register at this late stage (retrospectively). Some journals will still consider your manuscript for publication, but may ask you why you did not register it before you started. In future you should always register clinical trials before the first patient is enrolled. Some journals will not consider manuscripts that report clinical trials that were not registered before they started.
If you are carrying out a systematic review, it is also good practice to register it. PROSPERO is currently the only public registry for systematic reviews. As with clinical trials, you should include the registration number in your abstract when you write your manuscript. PROSPERO does not accept retrospective registration.