Much like a research study protocol, a review protocol is the rationale, hypothesis and planned methods for the review. It is ideally registered before the review begins, to allow for peer review of the methods. This in turn helps to reduce bias in the review when it happens. Protocols are typically developed for systematic and scoping reviews. They are not commonly seen for integrative reviews.
A protocol is also one of the elements of the PRISMA statement checklist. While not every journal requires that a systematic/scoping review have a published protocol, more and more journals are requiring it. Taking the time to register a protocol will you give you extra points when you want to publish your review.
There are several places where you can register a review protocol. You only need to register a protocol once.
