The Levels of Evidence and Their Role in Evidence-Based Medicine

Abstract

As the name suggests, evidence-based medicine is about finding evidence and using that evidence to make clinical decisions. A cornerstone of evidence-based medicine is the hierarchical system of classifying evidence. This hierarchy is known as the levels of evidence. Physicians are encouraged to find the highest level of evidence to answer clinical questions. Several articles published in plastic surgery journals concerning evidence-based medicine topics have touched on this subject.1–6 Specifically, previous articles have discussed the lack of higher level evidence in Plastic and Reconstructive Surgery and the need to improve the evidence published in the Journal. Before that can be accomplished, it is important to understand the history behind the levels and how they should be interpreted. This article focuses on the origin of levels of evidence, their relevance to the evidence-based medicine movement, and the implications for the field of plastic surgery and the everyday practice of plastic surgery. HISTORY OF LEVELS OF EVIDENCE The levels of evidence were originally described in a report by the Canadian Task Force on the Periodic Health Examination in 1979.7 The report's purpose was to develop recommendations on the periodic health examination and base those recommendations on evidence in the medical literature. The authors developed a system of rating evidence (Table 1) when determining the effectiveness of a particular intervention. The evidence was taken into account when grading recommendations. For example, a grade A recommendation was given if there was good evidence to support a recommendation that a condition be included in the periodic health examination. The levels of evidence were further described and expanded by Sackett8 in an article on levels of evidence for antithrombotic agents in 1989 (Table 2). Both systems place randomized controlled trials at the highest level and case series or expert opinions at the lowest level. The hierarchies rank studies according to the probability of bias. Randomized controlled trials are given the highest level because they are designed to be unbiased and have less risk of systematic errors. For example, by randomly allocating subjects to two or more treatment groups, these types of studies also randomize confounding factors that may bias results. A case series or expert opinion is often biased by the author's experience or opinions, and there is no control of confounding factors.Table 1: Canadian Task Force on the Periodic Health Examination's Levels of EvidenceTable 2: Levels of Evidence from SackettMODIFICATION OF LEVELS Since the introduction of levels of evidence, several other organizations and journals have adopted variations of the classification system. Diverse specialties are often asking different questions, and it was recognized that the type and level of evidence needed to be modified accordingly. Research questions are divided into the following categories: treatment, prognosis, diagnosis, and economic/decision analysis. For example, Table 3 shows the levels of evidence developed by the American Society of Plastic Surgeons for prognosis9 and Table 4 shows the levels developed by the Centre for Evidence-Based Medicine for treatment.10 The two tables highlight the types of studies that are appropriate for the question (prognosis versus treatment) and how quality of data is taken into account when assigning a level. For example, randomized controlled trials are not appropriate when looking at the prognosis of a disease. The question in this instance is, “What will happen if we do nothing at all?” Because a prognosis question does not involve comparing treatments, the highest evidence would come from a cohort study or a systematic review of cohort studies. The levels of evidence also take into account the quality of the data. For example, in the chart from the Centre for Evidence-Based Medicine, a poorly designed randomized controlled trial has the same level of evidence as a cohort study.Table 3: Levels of Evidence for Prognostic StudiesTable 4: Levels of Evidence for Therapeutic StudiesA grading system that provides strength of recommendations based on evidence has also changed over time. Table 5 shows the Grade Practice Recommendations developed by the American Society of Plastic Surgeons. The grading system provides an important component in evidence-based medicine and assists in clinical decision making. For example, a strong recommendation is given when there is level I evidence and consistent evidence from level II, III, and IV studies available. The grading system does not degrade lower level evidence when deciding recommendations if the results are consistent.Table 5: Grade Practice RecommendationsINTERPRETATION OF LEVELS Many journals assign a level to the articles they publish, and authors often assign a level when submitting an abstract to conference proceedings. This allows the reader to know the level of evidence of the research, but