Systematic Literature Reviews and Meta-Analyses

E very year, there is a great increase in the number of scientific publications. For example, the literature database PubMed registered 361 000 new publications in 1987, with 448 000 in 1997 and 766 000 in 2007 (research in Medline, last updated in January 2009). These figures make it clear how increasingly difficult it is for physicians in private practice, clinicians and scientists to obtain comprehensive current information on any given medical topic. This is why it is necessary to summarize and critically analyze individual studies on the same theme. Summaries of individual studies are mostly prepared when the results of individual studies are unclear or inconsistent. They are also used to study relationships for which the individual studies do not have adequate statistical power, as the number of cases is too low (1). The Cochrane Collaboration undertakes systematic processing and summary of the primary literature for many therapeutic topics, particularly randomized clinical studies (www.cochrane.org). They have published a handbook for the performance of systematic reviews and meta-analyses of randomized clinical studies (2). Cook et al. have published methodological guidelines for this process (3). Instructions of this sort help to lay down standards for the summary of individual studies. Guidelines have also been drawn up for the publication of meta-analyses on randomized clinical studies (4) and on observational studies (5). Publications on individual studies may be summarized in various forms (1, 6–10): Narrative reviews Systematic review articles Meta-analyses of published data Pooled reanalyses (meta-analyses with individual data). These terms are often not clearly allocated in the literature. The aim of the present article is to describe and distinguish these forms and to allow the reader to perform a critical analysis of the results of individual studies and the quality of the systematic review or meta-analysis.