Table 2.

Definition of Classes and Levels of Evidence Used in AHA/ASA Recommendations

Class IConditions for which there is evidence for and/or general agreement that the procedure or treatment is useful and effective.
Class IIConditions for which there is conflicting evidence and/or a divergence of opinion about the usefulness/efficacy of a procedure or treatment.
  Class IIaThe weight of evidence or opinion is in favor of the procedure or treatment.
  Class IIbUsefulness/efficacy is less well established by evidence or opinion.
Class IIIConditions for which there is evidence and/or general agreement that the procedure or treatment is not useful/effective and in some cases may be harmful.
Therapeutic recommendations
  Level of Evidence AData derived from multiple randomized clinical trials or meta-analyses
  Level of Evidence BData derived from a single randomized trial or nonrandomized studies
  Level of Evidence CConsensus opinion of experts, case studies, or standard of care
Diagnostic recommendations
  Level of Evidence AData derived from multiple prospective cohort studies using a reference standard applied by a masked evaluator
  Level of Evidence BData derived from a single grade A study or 1 or more case-control studies, or studies using a reference standard applied by an unmasked evaluator
  Level of Evidence CConsensus opinion of experts