Systematic reviewers increasingly must decide whether and how to incorporate existing systematic reviews into complex systematic reviews that are commissioned to support clinical guideline development or for other health policy uses. To date, however, this issue has been largely unexamined. Reviewers seeking to incorporate existing reviews into new reviews face a set of important questions: Can this practice adhere to systematic review principles? Will it save time? When should it be avoided? Will it produce valid results and be acceptable to users? Drawing from their collective experience, the authors outline a series of steps that can help reviewers reach reasoned decisions about the incorporation of existing systematic reviews and enumerate potential hazards to consider in doing so. They highlight issues surrounding the main steps reviewers must undertake, including locating existing reviews; assessing their relevance to the new review; assessing the quality of relevant reviews; determining how to incorporate high-quality, relevant existing systematic reviews; and clearly reporting the methods used and the results from this process. Further specification of methods, including the development of reporting standards for this approach, is needed.
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