EVIDENCE-BASED MEDICINE: Interpreting Studies and Setting Policy**

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Since the time of ancient Greece, medicine has been based on evidence. Conscientious physicians have always familiarized themselves with recent research and made their best efforts to keep abreast of journals. The modern term, evidence-based medicine (EBM), however, describes the effort to link both clinical practice and public policy to evidence that interventions improve outcomes of significance. Several aspects of this definition warrant attention.

First, the focus is on interventions. A large proportion of biomedical research concerns basic science and technology. Only some of this research investigates whether interventions in clinical practice or public policy improve health. Evidence-based medicine asks whether such interventions improve outcomes. Second, the emphasis is on outcomes of significance, those that patients and the public consider important. Intermediate outcomes and surrogate endpoints, such as pathophysiologic changes (as defined later), receive less weight. Third, EBM seeks to document the quality of the evidence for interventions and to make explicit the extent to which clinical practices and public policies are based on data or opinion. It is a misconception that EBM advocates withholding treatments for which evidence is lacking.69 Rather, it emphasizes the importance of disclosing the lack of evidence, to inform professionals and the public and to prioritize research better.

Current interest in EBM grew out of the nascent health care crisis of the late 1980s.23 Rising costs of care and increasing evidence of disturbing variations in clinical practice11 raised questions about the effectiveness and appropriateness of many tests and treatments.80 This concern gave rise to new government agencies and research initiatives, 66 closer attention to evidence by private insurers and health plans, 12 and a new domain of scholarly research that focuses on systematic reviews and meta-analyses (e.g., EBM journals and theme sections in journals, 25, 32, 59 Cochrane Library6). It promulgated a new approach to practice guidelines that emphasized a direct linkage between recommendations and evidence, making explicit disclosures when evidence was lacking, and giving lesser weight to opinion-based policies.22

The limitations and potential hazards of EBM are discussed later. Most of this article focuses on the two core elements of EBM: (1) reviewing evidence and (2) developing policy based on such reviews. Common to almost all EBM efforts is the task of gathering and evaluating relevant evidence from published (and sometimes from unpublished) literature. This comprehensive summary and critical appraisal of evidence, termed a systematic review, 14 characterizes the quality of the evidence but often stops short of making recommendations. Evidence-based practice guidelines and other quality improvement tools take the additional step of setting policy for patient care or public policy based on the systematic review. Each of these processes is discussed in this article.

Section snippets

EVALUATING THE QUALITY OF EVIDENCE

The first principle of systematic reviews is to gather all relevant evidence. Assessments of evidence that do not ensure a comprehensive search of the literature are vulnerable to bias from the selective collection of studies.58 Systematic reviews emphasize explicit search rules that clearly define the inclusion and exclusion criteria for admissible evidence, which keywords were used, which databases (e.g., Medline, Embase) were searched, and the number of articles included and excluded. They

SETTING POLICY

The endpoint of the process described in the previous section is to state what the evidence shows (science), not to prescribe what practitioners or policy makers should do (policy). The additional step of determining appropriate policy involves considerations that reach beyond data, either because firm scientific evidence is lacking or because other factors (e.g., clinical experience, standards of care, resource constraints, cost, public acceptability, politics) enter into the calculation of

SUMMARY

The ascendancy of EBM has been accompanied by a greater awareness of its shortcomings. It is increasingly evident from the cost, length, and difficulty of performing RCTs that studies cannot be launched to address every question in medicine. Good evidence is often lacking in medicine.60, 77 Epistomologists question the very notions of evidence and the suitability of current study designs and measurement tools to research the salient issues of concern to patients and others concerned with

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    Address reprint requests to Steven H. Woolf, MD, MPH, Department of Family Practice, Medical College of Virginia, 3712 Charles Stewart Drive, Fairfax, VA 22033

    **

    The practice guideline panel on idiopathic thrombocytopenic purpura described in this report was funded by the American Society of Hematology.

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