PROSPERO is the international prospective register of systematic review protocols, hosted by the Centre for Reviews and Dissemination at the University of York on behalf of the National Institute for Health and Care Research. Launched in 2011 and free to use, it is the largest and most established protocol registry for systematic reviews of healthcare interventions, public health, social care, education, and clinical research.
A PROSPERO record is a structured, publicly searchable description of a planned systematic review, submitted before screening begins or at the latest before data extraction. Once approved, the review receives a unique PROSPERO identifier that journals, peer reviewers, and funders use to verify that the published review matches the registered plan. This guide covers what PROSPERO does, why it exists, who can use it, what review types are eligible, and how it fits alongside alternatives like the Open Science Framework and INPLASY.
Prepare your submission with a registration-ready protocol template.
Why Prospective Registration Reduces Bias in Systematic Reviews
Prospective registration solves a problem that took the systematic-review community two decades to articulate. Without registration, reviewers can change inclusion criteria, switch primary outcomes, or modify the analysis plan after seeing the data. These post-hoc changes look reasonable at the manuscript stage but can introduce outcome-reporting bias and inclusion bias that are invisible to peer reviewers. The mechanism is the same as in clinical trials, where the move to mandatory prospective registration on ClinicalTrials.gov reduced the gap between planned and reported outcomes.
PROSPERO addresses the analogous problem in evidence synthesis. By making the planned review question, eligibility criteria, search strategy, and synthesis methods publicly visible before any screening happens, the registry creates a credible record against which the finished review can be compared. Editors, methodologists, and peer reviewers can pull the original PROSPERO record and ask whether the manuscript matches the plan or has drifted from it without disclosure.
A second purpose of PROSPERO is deduplication. Hundreds of systematic reviews are started every week worldwide, and many overlap. A reviewer who searches PROSPERO before committing to a question can discover that a similar review is already underway and either join it, change scope, or wait for the existing review to publish. This avoids wasted effort and reduces redundant publications cluttering the literature.
What a PROSPERO Record Contains
A PROSPERO submission collects a fixed set of structured fields. The review question and objectives describe what the review aims to answer. The PICO elements, when applicable, define the population, intervention, comparator, and outcomes. The eligibility criteria specify which study designs, settings, and patient populations are included or excluded. The search strategy lists the databases and date ranges to be searched, with as much specificity as the reviewers can offer at registration. The methods of synthesis outline the planned approach to data extraction, risk of bias assessment, and quantitative or narrative synthesis. Since the platform relaunch in February 2025, the form also asks for a rationale for the review, explicit review objectives, the selection (screening) process, plans for reporting bias and certainty (GRADE) assessment, the peer review status of the protocol, and a designated review guarantor.
The record also captures administrative metadata: the review team and their institutional affiliations, the funder, conflicts of interest, the anticipated start and completion dates, and a contact author who is responsible for keeping the record current. The contact author is the named point of contact for editors and readers, and is also responsible for submitting amendments and recording deviations.
PROSPERO is publicly searchable. All approved records appear in the registry's search interface at crd.york.ac.uk/prospero. This visibility is part of the deduplication function: anyone planning a review can look up whether a similar protocol is already registered. It is also part of the accountability function: a finished review's PROSPERO ID lets any reader pull the original protocol and compare.
Eligible Review Types and the Scoping Review Exception
PROSPERO accepts a defined set of review types and explicitly excludes others. Systematic reviews of interventions, exposures, prognostic factors, prevalence, and methodology are eligible. Rapid reviews that follow systematic methodology with abbreviated steps are eligible. Umbrella reviews that synthesize existing systematic reviews are eligible. Diagnostic test accuracy reviews and prognostic reviews are eligible.
Scoping reviews are not eligible for PROSPERO. This is a deliberate scope decision: scoping reviews have a different methodological purpose (mapping a field rather than answering a focused question), and their protocols are not well-suited to the PROSPERO record structure. Scoping review protocols are typically registered on the Open Science Framework at osf.io instead, where the open structure of OSF accommodates the broader format. The systematic review fundamentals primer covers the conceptual distinction between systematic and scoping reviews.
Methodological reviews are eligible if they follow systematic-review methodology. Living systematic reviews are eligible and are tagged in the registry to alert readers that the record will be updated. Network meta-analyses are eligible. Individual participant data reviews are eligible.
Eligible Protocol Stage: When You Must Register
PROSPERO accepts registrations at any point before data extraction begins. The strongest case for accountability is registration before any screening, when no studies have been read in full text and the team cannot have been influenced by what was found. Registration at title-and-abstract screening or even at full-text screening is still accepted, but later registrations are less protective against bias and the record will indicate the stage at which registration occurred.
Registration after data extraction begins is not accepted. This rule exists because at that point the team has read the included studies in detail and could be influenced by what they have seen when defining the analysis. A protocol registered after data extraction cannot credibly claim to be prospective.
Reviewers occasionally try to register after the review is well underway, sometimes after a journal editor asks for a PROSPERO ID at peer review. In those cases the registry will accept the submission only if data extraction has not started, will record the actual stage at registration, and will note that registration was not fully prospective. Reviewers in this situation are usually better served by registering on the Open Science Framework, which has no eligibility stage requirement, and explaining the timing in the methods section.
How Registration Works, in Brief
Registration is a structured online form. Once submitted, the record goes through automated system checks, every named author approves it, and it is then published with a unique CRD number that journals and readers use to find it. The field-by-field walkthrough, with weak and strong example answers, is in our step-by-step PROSPERO registration guide.
Amendments After Registration
PROSPERO distinguishes between amendments to the protocol, which are changes made before the relevant work has started, and deviations from the protocol, which are departures discovered during the conduct of the review. Both are expected at some level in any honest review: real-world data extraction often reveals issues the protocol could not have anticipated. The registry's expectation is that changes are documented transparently rather than concealed.
Amendments are submitted through the PROSPERO interface and result in a new dated version of the record. Previous versions remain visible: a reader can always see the original record and every subsequent amendment. This version history is part of the accountability mechanism. A reviewer who quietly changes the primary outcome a week before manuscript submission will leave a trail that editors and methodologists can find.
Deviations are described in the methods section of the published manuscript rather than in the PROSPERO record itself, because by the time deviations are recognized the review is usually well into the synthesis stage. The relevant PRISMA 2020 item asks authors to describe any departures from the protocol and their reasons. A manuscript that lists three or four well-documented deviations from a registered protocol reads as honest; a manuscript that lists no deviations and conspicuously changes the primary outcome reads as suspicious.
How Journals and Funders Use PROSPERO IDs
Major medical journals routinely require a PROSPERO ID at the manuscript submission stage for systematic reviews of healthcare interventions. The ID is reported in the abstract, the methods section, and sometimes in the title page metadata. Editors use the ID to pull the original protocol when assigning peer reviewers and to check the registered plan against the submitted manuscript.
Peer reviewers, when given access to the PROSPERO record, frequently use it to spot scope creep, outcome switching, or post-hoc subgroup analyses. A reviewer who sees that the registered protocol specified two primary outcomes but the manuscript reports only one, with the other relegated to a sentence in the discussion, will flag the discrepancy. The same applies to subgroups: subgroup analyses that appear in the manuscript without appearing in the registered protocol will draw a request for justification.
Funders also use PROSPERO for post-award monitoring. A funded systematic review that does not appear in PROSPERO within a reasonable time after the start date raises a flag, as does a review that registers but then makes substantial changes to the question or methods. Some funders also require PROSPERO registration as a condition of funding, written into the grant agreement.
Searching PROSPERO Before Starting a Review
Before committing to a systematic review topic, every team should search PROSPERO for similar protocols already registered. The registry's search interface accepts free-text queries, MeSH-style keyword combinations, and structured filters by review type, status, and registration date. A few minutes spent searching can prevent months of wasted work. PROSPERO only lists registered protocols, so pair it with a check of what is already published: the free research gap finder for systematic reviews counts existing reviews in PubMed and trials still running on ClinicalTrials.gov for your topic.
Useful search patterns include the disease or condition combined with the intervention or exposure, the outcome of interest when it is reasonably specific, and the review type filter to focus on the relevant methodology. The status filter distinguishes between ongoing reviews, completed reviews, and reviews that have been published. An ongoing review with a registration date in the past six months is a different kind of overlap than a review that has been registered for three years without progress, and the implications for your team's planning are different.
If a closely related review is already registered, the practical options are to change scope (different population, different outcome, different time period), adopt a different review type (umbrella review of the topic instead of a fresh systematic review), or contact the registered team about collaboration. Continuing with a duplicative protocol is rarely the right choice. The comprehensive search strategy guide covers the broader literature-search workflow that complements a PROSPERO search.
