Evidence-Based Medicine, Systematic Reviews, and Guidelines in Interventional Pain Management: Part 6. Systematic Reviews and Meta-Analyses of Observational Studies

Observational studies provide an important source of information when randomized controlledtrials (RCTs) cannot or should not be undertaken, provided that the data are analyzed andinterpreted with special attention to bias. Evidence-based medicine (EBM) stresses theexamination of evidence from clinical research and describes it as a shift in medical paradigm,in contrast to intuition, unsystematic clinical experience, and pathophysiologic rationale. Whilethe importance of randomized trials has been created by the concept of the hierarchy ofevidence in guiding therapy, much of the medical research is observational. The reporting ofobservational research is often not detailed and clear enough with insufficient quality and poorreporting, which hampers the assessment of strengths and weaknesses of the study and thegeneralizability of the mixed results. Thus, in recent years, progress and innovations in healthcare are measured by systematic reviews and meta-analyses. A systematic review is definedas, “the application of scientific strategies that limit bias by the systematic assembly, clinicalappraisal, and synthesis of all relevant studies on a specific topic.” Meta-analysis usually is thefinal step in a systematic review.Systematic reviews and meta-analyses are labor intensive, requiring expertise in both the subjectmatter and review methodology, and also must follow the rules of EBM which suggests thata formal set of rules must complement medical training and common sense for clinicians tointegrate the results of clinical research effectively. While expertise in the review methods isimportant, the expertise in the subject matter and technical components is also crucial.Even though, systematic reviews and meta-analyses, specifically of RCTs, have exploded, thequality of the systematic reviews is highly variable and consequently, the opinions reachedof the same studies are quite divergent. Numerous deficiencies have been described inmethodologic assessment of the quality of the individual articles. Consequently, observationalstudies can provide an important complementary source of information, provided that thedata are analyzed and interpreted in the context of confounding bias to which they are prone.Appropriate systematic reviews of observational studies, in conjunction with RCTs, may providethe basis for elimination of a dangerous discrepancy between the experts and the evidence.Steps in conducting systematic reviews of observational studies include planning, conducting,reporting, and disseminating the results. MOOSE, or Meta-analysis of Observational Studies inEpidemiology, a proposal for reporting contains specifications including background, searchstrategy, methods, results, discussion, and conclusion. Use of the MOOSE checklist should improvethe usefulness of meta-analysis for authors, reviewers, editors, readers, and decision-makers.This manuscript describes systematic reviews and meta-analyses of observational studies. Authorsfrequently utilize RCTs and observational studies in one systematic review; thus, they should alsofollow the reporting standards of the Quality of Reporting of Meta-analysis (QUOROM) statement,which also provides a checklist. A combined approach of QUOROM and MOOSE will improvereporting of systematic reviews and lead to progress and innovations in health care.Key words: Observational studies, evidence-based medicine, systematic reviews, metaanalysis, randomized trials, case-control studies, cross-sectional studies, cohort studies,confounding bias, QUOROM, MOOSE