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:
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).
See the Systematic Reviews & Meta-Analysis page of this guide for infographics showing the systematic review process.