A priori writing and registration of a systematic review protocol is a key step for conducting a rigorous systematic review. This is important to mitigate a variety of biases during the review process. A pre-defined protocol also helps to structure the review process. Such a systematic review protocol pre-specifies the employed review methodology. Concretely, the protocol should predefine the following parameters:
- Research question: What is the rationale and PICO of your study?
- Search strategy: Which key words or (preliminary) search queries will you employ? Which databases are you going to be search?
- Inclusion and exclusion criteria of articles: How will you deal with grey literature or reviews? Which tools/software will be used to screen articles?
- Data extraction: Which data are you going to extract? How many raters will extract data?
- Risk of bias assessment (sometimes also referred to as quality assessment): Which tool are you going to use? How many raters will assess risk of bias?
- Data synthesis strategy: How will you summarize the data? Do you plan to conduct a meta-analysis? Do you plan to assess publication bias?
- Status of review while drafting the protocol, e.g., search completed, abstract screening started.
- Impact on 3R: Although this is not mandatory, we recommend to also included a statement in your protocol how your systematic review can impact 3R (Replacement, Reduction and Refinement) measures to foster humane animal research.
Different online platforms can be used to register such a protocol; the most commonly used resource is the International Prospective Register of Systematic Reviews (PROSPERO). PROSPERO has currently registered almost 200’000 systematic review protocols. Submitted protocols are reviewed by PROSPERO editors and feedback needs to be implemented prior to protocol publication. Alternatives include the Open Science Framework (OSF) which does not provide a feedback loop but does enable defining an embargo period for online protocol publication. Of note, PROSPERO and OSF entries should be checked for systematic review protocols addressing similar questions to avoid unnecessary duplication of work. PROSPERO provides entry masks for protocols of both clinical and preclinical systematic reviews.
Download the protocol template here.
Checking previously published protocols can also be of help during the drafting of the protocol. Here is an example.
Pitfalls
A common pitfall during protocol drafting is to be insufficiently specific on certain items, e.g., in- and exclusion criteria or outcome measures to extract. Going in detail through every step helps to avoid this issue: Are conference abstracts included? How to deal with unstandardized reporting (e.g., mean versus median)? Of note, although the protocol should be abided by, protocol pledges can be adjusted during the review process – if sufficiently justifiable. However, such protocol deviations should be reported in the registered protocol and in the published manuscript.