[Under the meta-scope: possibilities and limits of meta-analyses]

Critical review and statistical synthesis of the best evidence that is available on the benefits and risks of medical interventions is an important component of evidence-based medicine. There is, however, controversy regarding the merits and perils of this approach. In this article the potential and limitations of meta-analysis are illustrated and the problems of applying meta-analytical results to the individual patient are discussed. Whereas the classical narrative review article often reflects the personal opinion of the author, systematic reviews are characterized by clearly defined objectives and reproducible methods. If appropriate and feasible, individual studies are combined in a meta-analysis which will lead to enhanced precision of effect estimates. The potential of meta-analysis is illustrated by considering a patient who suffered a myocardial infarction in 1981. After consulting a misleading narrative review, this patient was discharged without beta-blockade for secondary prevention. Meta-analyses are, however, also liable to bias. For example, a meta-analysis of trials of magnesium infusions in acute myocardial infarction showed a substantial reduction in mortality. The beneficial effect of magnesium was not confirmed in the large ISIS-4 trial published in 1995. The occurrence of misleading meta-analyses is not surprising considering the selective publication of "positive" findings and the often inadequate quality of component studies. Misleading meta-analyses may also result from the inappropriate combination of heterogeneous studies. Every patient is unique regarding the exact localization and severity of the lesion, preexisting abilities, co-morbidity, social environment, personality and emotional response. The appropriate application of meta-analytic findings to the individual patient is difficult. It requires consideration of both quantitative and qualitative approaches to decision-making. Emphatic integration of these approaches characterizes the experienced, scientifically and socially competent physician.