Table 2

Class of recommendation and level of evidence definitions used by major guideline bodies

Classes of recommendationsDefinition
Class IEvidence and/or general agreement that a given treatment or procedure is beneficial, useful, effective.
Class IIConflicting evidence and/or a divergence of opinion about the usefulness/efficacy of the given treatment or procedure.
 Class IIaWeight of evidence/opinion is in favour of usefulness/efficacy.
 Class IIbUsefulness/efficacy is less well established by evidence/opinion.
Class IIIEvidence or general agreement that the given treatment or procedure is not useful/effective, and in some cases may be harmful.
Levels of evidenceDefinition
Level of evidence AData derived from multiple randomised clinical trials or meta-analyses.
Level of evidence BData derived from a single randomised clinical trial or large non-randomised studies.
Level of evidence CConsensus of opinion of the experts and/or small studies, retrospective studies, registries.