According to the widely discussed and generally accepted concept of evidence-based medicine (EBM), five steps are usually followed. Step 1 is to translate a clinical uncertainty into an answerable question. Step 2 consists of the systematic retrieval of the best evidence available. In step 3, this evidence is critically appraised for validity (closeness to the truth) and applicability (usefulness in clinical practice) and then applied, in step 4, in practice. Finally, in the last step, one evaluates the clinical results.
The most to least reliable types of evidence according to Sackett and co-workers:
1A = Systematic Review of Randomized Controlled Trials (RCTs)
1B = RCTs with Narrow Confidence Interval
1C = All or None Case Series
2A = Systematic Review Cohort Studies
2B = Cohort Study/Low Quality RCT
2C = Outcomes Research
3A = Systematic Review of Case-Controlled Studies
3B = Case-Controlled Study
4 = Case Series, Poor Cohort Case Controlled
5 = Expert Opinion
The hierarchical pyramid of evidence
The strongest evidence for any given therapeutic intervention is provided by the systematic review of randomized, (triple) blind, placebo-controlled trials. When they incorporate a high percentage of follow-up involving a homogeneous patient population and medical condition, they become even more reliable. This sort of evidence stands at the top of the hierarchical pyramid of evidence.
In contrast, case reports and expert opinion have little value as proof of efficacy because of the biases inherent in observation and the reporting of cases, difficulties in ascertaining who is an experienced reporter, and so on. Laboratory research, including valuable animal studies designed to learn more about the microscopic structure of living tissues, may provide impetus for new areas of research, but rarely— according to the principles of EBM— provide immediate guidance on how to treat patients on a day-to-day basis. Therefore, in vitro and animal studies are normally not included in the pyramid of evidence.
The most common definition of EBM is taken from Dr. David Sackett et al, a pioneer in evidence-based medicine. (See “Evidence based medicine: What it is and what it isn’t. Br Med J. 1996;312:71-72) It is “the conscientious, explicit and judicious use of current best evidence in making decisions about the care of the individual patient. It means integrating individual clinical expertise with the best available external clinical evidence from systematic research.”
However, the most reliable types, as outlined above, may not always be available for every field of clinical inquiry. A sham operation may not fall within the scope of good ethics, for example, or blinding may not always be feasible in every field of research. Sackett et al pointed out that “Evidence-based medicine is not restricted to randomized trials and meta-analyses. It involves tracking down the best external evidence with which to answer our clinical questions.” The TiUnite literature listed below falls into the upper range of the reliable types of evidence.