Table 1 Definition of the level of evidence and class of recommendation

 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.

 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.