How to critically appraise systematic reviews and metaanalyses

Michael Bigby and Hywel Williams

A systematic review is an overview that answers a specific clinical question, contains a thorough, unbiased search of the relevant literature, explicit criteria for assessing studies and structured presentation of the results. Many systematic reviews incorporate a meta-analysis, that is, a quantitative pooling of several similar studies to produce one overall summary of treatment effect.1,2 Meta-analysis provides an objective and quantitative summary of evidence that is amenable to statistical analysis.1 Meta-analysis allows recognition of important treatment effects by combining the results of small trials that individually might have lacked the power to consistently demonstrate differences among treatments. Meta-analysis has been criticised for the discrepancies between the results of meta-analysis and results from large clinical trials.3-6 The frequency of discrepancies ranges from 10% to 23%.3 Discrepancies can often be explained by differences in treatment protocols, differences in study populations or changes that occur over time.3 Because of the importance of considering the need to have clear objectives, explicit criteria for study selection, an assessment of the quality of included studies and prior consideration of which studies to combine, meta-analyses that are not conducted within the context of a systematic review should be viewed with great caution.7

A systematic review can be viewed as a scientific and systematic examination of the available evidence. A good systematic review will have explicitly stated objectives (the focused clinical question), materials (the relevant medical literature) and methods (the way studies are assessed and summarised). The steps taken during a systematic review are shown in Box 8.1.

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