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Scoping Reviews and Systematic Reviews: Is It an Either/Or Question?

What has always bothered me about the term scoping review is that, despite a clear role in knowledge synthesis, the name alone implies that it is not a systematic review. A systematic review has a clear definition and methods. Ascribing a different name to scoping reviews implies that they do not meet the definition of a systematic review. Although many groups have published on methods, without universally agreed-on standards for what constitutes a scoping review, regional variation and individual interpretation allow for a wide range of methods and approaches. An article in this issue may begin to change this. Using established methods developed by the EQUATOR (Enhancing the QUAlity and Transparency Of health Research) Network (1), Tricco and colleagues (2) engaged 26 experts across the globe and used a structured Delphi approach to examine and adapt the 27 PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) items for application to scoping reviews. After describing the purpose of scoping reviews and how they differ from other systematic reviews, the resulting extension (2) to the PRISMA reporting guidelines (3) defines and establishes scoping reviews as a type of systematic review. Although Tricco and colleagues focus on reporting recommendations rather than methodological standards, their work gives structure to the definition of a scoping review. The guidance document clearly established the important role and contribution of scoping reviews. Tricco and colleagues define a scoping review as a systematic approach to charting or mapping a broader question than a systematic review typically addresses. When researchers apply systematic review methods to broad questions that patients, clinicians, policymakers, and other decision makers ask, challenges arise. Systematic reviews are driven by PICO (population, intervention, comparison, outcome) elements that are clearly defined but rarely specified in the broad questions asked by decision makers. Researchers have much work to do before starting a systematic review. The Agency for Healthcare Research and Quality (AHRQ) Evidence-based Practice Center program has defined steps to follow after a topic is nominated (4, 5). The program recommends using a scoping review to map the existing research for potential duplication and feasibility, identify the main concepts (and determine the priority for investment), and define the PICO elements. The scoping process allows organizations to consider whether a potential topic is already covered by existing reviews, determine whether the evidence is too scarce to allow for a systematic review and where primary research is needed, and identify any areas within the broader topic where a systematic review may be appropriate. Another type of scoping review is the AHRQ Evidence-based Practice Center technical brief. Technical briefs systematically map evidence and concepts in newly emerging clinical areas for which the PICO elements have not been clearly elucidated or evidence is known to be limited. Instead of questions about what works, questions for a technical brief may include What interventions have been studied?, In what populations or settings have they been studied?, and What outcomes have been studied? The answers to these questions map the evidence to identify areas appropriate for further research or systematic review and allow readers to understand the current state of research and the context in which an emerging clinical area has been studied. For example, AHRQ published a technical brief on the different primary care models for medication-assisted treatment of opioid use disorder (6). Although evidence is too scarce to make general conclusions about what works, primary care practices found it helpful to know what models of care have been studied and for which outcomes. A technical brief on telehealth (7) was helpful to map the outcomes across different settings and identify areas where the evidence is already established, those where a systematic review may be helpful, and those where primary research is needed. The new reporting guideline shines light on the essential ways in which scoping reviews differ from other systematic reviews. When research questions are broad, a scoping review explores rather than summarizes the evidence. Consequently, scoping reviews do not include meta-analysis or assess the strength of evidence across studies. Instead, they chart concepts, themes, and the amount and type of evidence available. PRISMA guidelines recommend that systematic reviews report information on 27 items within 7 sections: title, abstract, introduction, methods, results, discussion, and funding. Tricco and colleagues judged 22 of these required items as relevant for scoping reviews, 5 as irrelevant, and 2 as optional. The differences reflect the focus on charting methods in scoping reviews versus outcomes in systematic reviews. For example, Tricco and colleagues recommend reporting methods for data charting rather than data collection in the abstract and methods (3). In scoping reviews, the results and discussion focus on charting concepts, themes, and types of evidence available rather than strength of evidence and quantitative findings. This shift in focus rendered the following 5 items from the original PRISMA irrelevant: summary measures, methods used to assess risk of bias across studies, methods used for any additional analyses, results of assessment of risk of bias across studies, and results of additional analyses. The 2 optional items relate to critical appraisal of sources of evidence, which may not be relevant when a scoping review does not assess strength of evidence across a body of evidence. Many scoping reviews provide new insights into complex questions, but unfortunately, many by groups like AHRQ are done to inform internal decisions and are never published. Having a clearer definition of scoping reviews and established elements for assessing quality may increase recognition of their value. By definition, scoping reviews address the topics on the minds of clinicians, consumers, and policymakers. Although they may not provide clear answers about what works, they help crystallize context, questions, and the extent of available evidence. Scoping reviews highlight areas where evidence is lacking and can help funders and researchers prioritize limited resources to address questions of greatest need.