01 · The Question
If You Already Have a Protocol, Do You Also Need to Register the Review?
You have written the protocol for your systematic or scoping review. The question, eligibility criteria, search strategy, screening procedures, and synthesis plan are documented. Is that enough, or should you also preregister the review publicly?
The two practices serve related but different purposes.
A protocol specifies what you intend to do. Preregistration places a time-stamped record of that plan in a public or externally maintained location before the completed results are known. That can make the sequence of methodological decisions more transparent and can help other researchers discover reviews already underway.
Registration is therefore worth considering for structured evidence syntheses, but where and how you register depends on the review type and the eligibility rules of the registry you intend to use.
03 · What You Need to Know
What Preregistration Adds to a Review Protocol
Protocol and Registration Are Not the Same Thing
A protocol is the methodological plan for your review. It can specify the review question, eligibility criteria, information sources, search methods, screening procedures, data extraction or charting, appraisal, synthesis, and other methodological decisions relevant to the chosen review design.
Registration takes at least part of that planned information and places it in a registry or another persistent public record. The registration is usually time-stamped, making it possible to establish that the plan existed at a particular stage of the project.
You can therefore have a protocol without registering it. You can also register a review while maintaining a substantially more detailed protocol elsewhere.
Protocol
The prospective methodological plan describing what the review will do and how it will be conducted.
Preregistration
A time-stamped public or externally recorded declaration of planned review information made before the relevant results are known.
If you have not yet established the protocol itself, resolve whether and how your literature review needs a protocol before treating registration as the main methodological task.
Why Register a Systematic Review?
Prospective registration can make several aspects of the research process more transparent. Readers can compare the completed review with the registered plan and determine whether important elements changed. That does not mean changes are automatically problematic. It means the sequence of decisions becomes more visible.
Registration can also make ongoing reviews discoverable. Before beginning a new systematic review, researchers can search relevant registries alongside published literature to identify similar reviews already in progress. This may reveal unnecessary duplication or opportunities to refine the new question.
PRISMA-P explicitly includes reporting of registration information when a systematic review protocol has been registered.
PROSPERO Is Important, but It Is Not a Universal Registry for Every Review
PROSPERO, maintained by the Centre for Reviews and Dissemination at the University of York, is a well-known prospective register for eligible systematic reviews. Researchers should check its current scope and inclusion criteria before assuming their project can be registered there.
This matters particularly for scoping reviews. JBI's current scoping-review guidance notes that scoping reviews and literature reviews are not eligible for PROSPERO registration and points researchers toward alternatives such as the Open Science Framework or Figshare, as well as protocol publication.
Registry policies can change. Verify the current eligibility requirements directly before designing your dissemination plan around a particular platform.
Scoping Reviews Still Need Prospective Planning
The inability to use a particular registry does not mean a scoping review should be planned retrospectively. JBI states that an a priori protocol must be developed before undertaking a scoping review. The protocol should predefine objectives, methods, eligibility criteria, relevant data, extraction, and presentation, with deviations subsequently identified and explained.
PRISMA-ScR likewise asks authors to report whether a protocol exists, where it can be accessed, and registration information when available.
The practical implication is that protocol development and public registration should be treated as related but separate decisions.
Registration Helps Distinguish Prespecified Decisions From Later Decisions
Imagine that a systematic review initially plans to examine three primary outcomes. After examining the included studies, the reviewers discover that a fourth outcome produces especially striking findings and elevate it to primary status.
That change may or may not have a defensible methodological rationale. A prospective record allows readers to see that the outcome hierarchy changed and evaluate the explanation.
The same logic applies to eligibility criteria, subgroup analyses, synthesis methods, and other consequential decisions. Preregistration does not prevent changes. It makes undisclosed rewriting of the original plan more difficult.
Registration Does Not Lock the Review Forever
Evidence syntheses regularly encounter circumstances that could not have been predicted perfectly at protocol development. Searches retrieve unexpected evidence. Outcomes are reported differently from anticipated. A planned meta-analysis proves inappropriate. Definitions need clarification.
A registered review can still change. The appropriate response is to document substantive amendments, state when they occurred, and explain why they were necessary.
Transparency is more important than pretending that a complex research project followed its first draft without modification.
Registration Does Not Prove Methodological Quality
A registered review can still have a weak search strategy, poorly justified eligibility criteria, inappropriate appraisal methods, flawed synthesis, or unsupported conclusions. Registration records what was planned; it does not certify that the plan was good.
Similarly, a review should not be considered methodologically worthless solely because it was not registered. The implications depend on the methodology, timing, field, journal requirements, and reasons for nonregistration.
Registration is one transparency mechanism within a much larger review process.
Preregistration and Protocol Publication Also Differ
A registry record is often relatively structured and concise. A published protocol can provide considerably more methodological detail and may undergo editorial assessment or peer review, depending on the venue.
Researchers may use both. A systematic review might have a registry entry plus a published protocol. A scoping review might publicly post or register its protocol on an appropriate platform and subsequently publish the protocol in a journal.
The important issue is not accumulating as many badges of transparency as possible. It is creating a sufficiently detailed and accessible prospective record for the type of review being conducted.
| Practice |
What It Provides |
What It Does Not Provide |
| Internal protocol |
Prospective methodological plan for the review team |
Necessarily a public or independently time-stamped record |
| Registry entry |
Discoverable, time-stamped record of planned review information |
A substitute for full methodological planning |
| Public protocol repository |
Persistent access to a detailed prospective protocol where supported |
Automatic methodological peer review |
| Published protocol |
Detailed public methodological account, potentially with editorial or peer review |
A guarantee that the eventual review will be conducted correctly |
| PRISMA or PRISMA-ScR reporting |
Guidance for transparent reporting of the completed review |
A registry or review methodology |
Timing Matters
The methodological value of preregistration comes partly from recording intentions before researchers know the results that those decisions could affect. Registering after study selection, extraction, or synthesis may still create a useful public record, but it should not be represented as though it were fully prospective.
Registries and journals may also impose their own timing rules. Check these before the review advances too far.
Watch Out
Do not retrospectively create or substantially rewrite a registration and present it as evidence that all reported methods were prespecified. If registration occurred after important review stages had begun, report the timing accurately.
Registration Is Not the Same as PRISMA Compliance
PRISMA is a reporting framework. PRISMA 2020 provides guidance for reporting systematic reviews, while PRISMA extensions address particular review types and circumstances. PRISMA-P addresses systematic-review protocols.
Completing a PRISMA checklist does not register a review, and registering a review does not establish that the eventual manuscript has been reported according to PRISMA.
07 · A Quick Checklist
Before Registering Your Review
Before preregistration, check:
Develop the review protocol before creating the registration entry.
Verify that the registry or platform accepts your specific type of systematic, scoping, or other evidence synthesis.
Check the registry's current timing and eligibility requirements directly rather than relying on an old paper or example.
Ensure that the registered question, eligibility criteria, outcomes, and planned synthesis correspond to the detailed protocol.
Record the date and review stage at which registration occurred.
Retain the full protocol even when the registry captures only a subset of its methodological detail.
Update or document important amendments when the registration platform permits or requires this.
Report registration information and protocol availability in the final review according to applicable reporting guidance.
