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The Institute of Medicine (IOM) defines clinical practice guidelines as "statements that include recommendations, intended to optimize patient care, that are informed by a systematic review of evidence and an assessment of the benefits and harms of alternative care options."
Based on this definition, guidelines have two parts:
The foundation is a systematic review of the research evidence bearing on a clinical question, focused on the strength of the evidence on which clinical decision-making for that condition is based.
A set of recommendations, involving both the evidence and value judgments regarding benefits and harms of alternative care options, addressing how patients with that condition should be managed, everything else being equal.
Important: When there is little evidence on a topic, a clinical practice guideline may be based on expert consensus rather than a systematic review of the evidence. Guidelines based on well-done systematic reviews are considered the highest level of evidence (at the top of the pyramid), but guidelines based on expert consensus are considered expert opinion (lower on the pyramid).