Under the metascope: potential and limits of meta-analyses

Critical review and statistical synthesis of the best evidence that is avai lable on the benefits and risks of medical interventions is an important co mponent of evidence-based medicine. There is, however, controversy regardin g the merits and perils of this approach. In this article the potential and limitations of metaanalysis are illustrated and the problems of applying m eta-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 defi ned objectives and reproducible methods. If appropriate and feasible, indiv idual studies are combined in a meta-analysis which will lead to enhanced p recision of effect estimates. The potential of meta-analysis is illustrated by considering a patient who suffered a myocardial infarction in 1981. Aft er consulting a misleading narrative review, this patient was discharged wi thout beta-blockade for secondary prevention. Meta-analyses are, however, also liable to bias. For example, a meta-analys is 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 pub lication of <<positive>> findings and the often inadequate quality of compo nent studies. Misleading meta-analyses may also result from the inappropria te combination of heterogeneous studies. Every patient is unique regarding the exact localization and severity of th e lesion, preexisting abilities, co-morbidity, social environment, personal ity and emotional response. The appropriate application of meta-analytic fi ndings 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, scientifical ly and socially competent physician.