Definition of classes and levels of evidence used in AHA/ASA recommendations
| Class I | Conditions for which there is evidence for and/or general agreement that the procedure or treatment is useful and effective |
| Class II | Conditions for which there is conflicting evidence and/or a divergence of opinion about the usefulness/efficacy of a procedure or treatment |
| Class IIa | The weight of evidence or opinion is in favor of the procedure or treatment |
| Class IIb | Usefulness/efficacy is less well established by evidence or opinion |
| Class III | Conditions 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 A | Data derived from multiple randomized clinical trials or meta-analyses |
| Level of Evidence B | Data derived from a single randomized trial or non-randomized studies |
| Level of Evidence C | Consensus opinion of experts, case studies, or standard of care |
| Diagnostic recommendations | |
| Level of Evidence A | Data derived from multiple prospective cohort studies using a reference standard applied by a masked evaluator |
| Level of Evidence B | Data derived from a single grade A study or one or more case–control studies, or studies using a reference standard applied by an unmasked evaluator |
| Level of Evidence C | Consensus opinion of experts |
AHA/ASA, American Heart Association/American Stroke Association.