Evidence level of individual studies: A proposed framework for surgical research

E medicine is ‘‘the conscientious, explicit, and judicious use of current best evidence in making decisions about the care of individual patients. ’’ This becomes complicated when busy health care providers are faced with the task of summarizing ‘‘the current best evidence. ’’ Systematic reviews, such as those published by the Cochrane Collaboration, serve this purpose by appraising and distilling the daunting amount of information available. These reviews are commonly attached to a level of evidence, which gauges the confidence of estimates reported by existing studies. Thus, levels of evidence pertain to the knowledge generated by the summative collection of research on a specific topic. Although the evidence base can come from a single study, more often it is the final step of a long scientific journey, in which experts collect, appraise, and summarize the findings of several individual studies using a specific, standard methodology. Different systems to define hierarchy of evidence have been proposed by renowned groups including the pioneer Cochrane Collaboration, the Oxford Centre for EvidenceBased Medicine (OCEBM), the US Preventive Task Force, and the Evidence-Based Practice Center (EPC) program of the US Agency for Healthcare Research and Quality. The recently launched Grades of Recommendation, Assessment, Development, and Evaluation (GRADE) system follows a detailed stepwise process to rate evidence and to determine the strength of recommendations in systematic reviews, health technology assessments, and clinical practice guidelines. According to the GRADE group Web site, more than 50 organizations have endorsed their system, including the World Health Organization, the American College of Physicians, the American College of Chest Physicians, the American Endocrine Society, the American Thoracic Society, the Canadian Agency for Drugs and Technology in Health, and the UK’s National Institute for Health and Clinical Excellence. The British Medical Journal encourages authors of clinical guidelines to use the GRADE system. The Cochrane Collaboration has also adopted the principles of the GRADE system for evaluating the quality of evidence for outcomes reported in systematic reviews. Yet, a systematic review is not always available; thus, how will busy health care providers manage the formidable volume of information that becomes available everyday? ‘‘How does the article I read today change (or not) what I will recommend to my patients tomorrow? ’’ To address this imperative, several scientific journals have recently adapted grading systems to assess the level of evidence of individual articles in an effort to provide guidance to their readers. This year, The Journal of Trauma joined the discourse by requiring authors to assign levels of evidence to their own clinically oriented studies. As detailed previously, the existing grading systems (e.g., GRADE) were originally designed to rate the summative body of evidence and not the level of evidence in individual articles. The grading of evidence in individual studies is a middle step in determining the hierarchy of evidence and comprises a judgment regarding the confidence and uncertainty emanating from a particular study. Several aspects of the investigation are examined, including, but not limited to appropriate design to address well-formulated research questions, appropriately measured outcomes, assessment of inferential error, risk of bias, and control of confounding. The results of this appraisal will inform the reader about the level of uncertainty of the study’s findings and how much it adds to the existing knowledge in that topic. As part of the process of assessing the overall level of evidence, the GRADE system rates the evidence from individual studies into one of four categories ranging from high to very low (Table 1). Study design is the GRADE’s critical measure to classify the quality of the evidence: for therapeutic studies, randomized clinical trials (RCTs) always start as High and observational studies as Low. From this starting point, evidence may be downgraded or upgraded through the evaluation of several specific domains as follows: (1) risk of bias, (2) imprecision, (3) inconsistency, (4) indirectness, (5) publication bias, (6) effect size, (7) existence of dose-response pattern, and (8) effect of plausible confounding on findings (Table 2). When the study addresses diagnostic accuracy, however, a slightly different classification applies. GRADE is a somewhat complicated system, which, as its own authors recognize, involves an element of subjectivity. In addition, there is the negative connotation created by classifying a study as low quality, a term which could be CURRENT OPINION