Evidence based medicine overviews, bulletins, guidelines, and the new consensus

Abstract

Evidence to support a clinical action or decision may be drawn from many sources. Epidemiologists have described for a number of years a “hierarchy of evidence”, ranging from clinical experience, case report or anecdote through structured observational studies, like the case-control study, to randomised controlled trials.1 Some have recently added the statistical overview, or “meta-analysis”. It would be naive to assume that “experienced clinical opinion” is based wholly on a clinician's own experience: it will invariably incorporate a wider knowledge drawn from medical school (the influence of a charismatic teacher may last a lifetime), relevant texts, and from selected reading of more recent literature and may well reflect a current “consensus view”. It would be equally naive to regard one clinical trial, even a large well designed trial, as providing the definitive answer on its own, because a trial is often restricted to a selected subset of patients under atypically controlled circumstances and with insufficient numbers to evaluate side effects. The trial results need to be interpreted in context and the guideline for good clinical practice lies in the synthesis of results of repeated trials, findings of relevant observational studies, and knowledge of the underlying biological, biochemical, or pharmacological mechanisms. That synthesis was previously to be found in textbooks, but books are increasingly being replaced by reviews, bulletins, and guidelines.