The relationship between evidence and guidelines.

This reflection aims to soothe some frustrations, to cool current debates, and to encourage an understanding whereby the relationship between evidence and guidelines can be more productive in future. The relationship of evidence to guidelines, an important link between research and application, is not as simple and unidirectional as is often imagined. The recent emphasis on evidence of high quality and aggregation of evidence is appropriate but can be modified and extended. There are other important elements in guidelines besides the available evidence (which is centered on efficacy and effectiveness). Available evidence may be poor, or not be the most relevant to the decision at hand. To offset the limitations to which this predicament leads, effort is also appropriate to develop more rigorous principles for interpretation and evaluation of evidence, so as to make better use of what is available. The processes for generating guidelines from evidence emphasize reliability, generalizability of a limited kind, and freedom from bias to be achieved via systematic review. If the inclusion criterion for studies is too stringent, there may be little to say from evidence alone, or the evidence that remains may suffer limitations of relevance due to context, or it may be misapplied, simply because it has been endorsed as to quality. Knowledge-based medicine (KBM) would acknowledge a greater role for the scientific principles underlying particular pieces of evidence and deploy a fuller understanding of the health-care context. It should thus make guidelines more professionally accepted and useful in the future.