Adding up the evidence: systematic reviews and meta-analyses.

A systematic review aims to synthesize all available individual studies on a certain topic and uses explicit and reproducible methods for searching the literature, while a meta-analysis is a mathematical synthesis of the results of these individual studies. Because of the explosion of information in the scientific literature, these study designs can be useful tools to summarize the knowledge on a particular subject. In addition, combining individual studies in a meta-analysis increases statistical power, resulting in more precise effect estimates. Even though all parts of the specific methodology of systematic reviews include steps to minimize bias, both investigators and readers should be aware of potential biases like poor study quality and heterogeneity between studies. This paper explains how systematic reviews and meta-analyses should be performed and how to critically appraise them, based on an example from the nephrology literature.