Table 1.

Classes of recommendations and levels of evidence according to the ESC and ACC/AHA guidelines¹,²

Classes of recommendations	ESC	ACC/AHA
Class I	Evidence and/or general agreement that a given treatment or procedure is beneficial, useful and effective	Benefit >>> Risk Procedure/treatment should be performed
Class II	Conflicting evidence and/or divergence of opinion about the usefulness/efficacy of a given treatment procedure	–
Class IIa	Weight of evidence/opinion is in favor of usefulness/efficacy	Benefit >> Risk Additional studies with focused objectives needed It is reasonable to perform procedure/administer treatment
Class IIb	Usefulness/efficacy is less well established by evidence/opinion	Benefit ≥ Risk Additional studies with broad objectives needed; additional registry data would be helpful Procedure/treatment may be considered
Class III	Evidence or general agreement that the given treatment or procedure is not useful/effective and in some cases may be harmful	Risk ≥ Evidence No additional studies needed Procedure/treatment should not be performed/administered since it is not helpful and may be harmful
Levels of evidence
A	Data derived from multiple randomized clinical trials or meta-analyses	Multiple (3–5) population risk strata evaluated General consistency of direction and magnitude of effect
B	Data derived from a single randomized clinical trial or large nonrandomized studies	Limited (2–3) population risk strata evaluated
C	Consensus of opinion of the experts and/or small studies, retrospective studies, registries	Very limited (1–2) population risk strata evaluated

Abbreviations: ESC, European Society of Cardiology; ACC/AHA, American College of Cardiology/American Heart Association.