Table 1.

Definition of the level of evidence and class of recommendation

Criteria	Definition
Level of evidence
A	Data derived from multiple randomized controlled trials or meta-analyses.
B	Data derived from a single randomized controlled trial or nonrandomized clinical trials.
C	Experts’ opinion or data derived from limited evidence.
Class of recommendation
I	Evidence and/or general agreement that a given treatment or procedure is beneficial, useful, and effective. Therefore, it is recommended.
IIa	Conflicting evidence and/or a divergence of opinion about the usefulness/efficacy of the given treatment or procedure exists. However, in general, weight of evidence/opinion is in favor of usefulness/efficacy. Therefore, it is reasonable to be performed.
IIb	Usefulness/efficacy less well established by evidence/opinion. Therefore, it may be considered.
III	Evidence or general agreement that a given treatment or procedure is not beneficial and may be harmful in some case. Therefore, it is not recommended.