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This JAMA Guide to Statistics and Methods describes how ACC/AHA guidelines are formatted to rate class (denoting strength of a recommendation) and level (indicating the level of evidence on which a recommendation is based) and summarizes the strengths and benefits of this rating system in comparison with other commonly used ones.

As the volume of information from clinical studies expands, clinical guidelines offer a means of disseminating the most up-to-date and evidence-based practices. To help readers understand the quality of supporting evidence, most clinical guidelines include a rating system intended to summarize the strength of evidence for each clinical recommendation. A 2018 systematic review identified 17 rating systems that were currently in use, each with its own evaluation schema, nomenclatures, and assessment approaches.1 Some of the commonly used rating systems in clinical practice guidelines include the United States Preventive Services Task Force rating system, the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach, and the American College of Cardiology (ACC)/American Heart Association (AHA) rating system.2 Authors select a rating system based on considerations that include applicability to the data and the familiarity of the rating system to the intended consumers of the guideline.

As an example, guidelines developed by the ACC/AHA follow a prescriptive format, whereby major topics begin with specific recommendations accompanied by the ACC/AHA rating system2 to summarize the strength and quality of the evidence informing each recommendation. In JAMA Cardiology, Arnett et al3 summarized a portion of the 2019 ACC/AHA guideline on the primary prevention of cardiovascular disease,4 which developed recommendations using this rating system.


The ACC/AHA recommendation system uses 2 different ratings denoted by the terms class and level. Class refers to the strength of the recommendation, while level refers to the level of evidence on which the recommendation is based.

Class of recommendation is rated I through III. A class I recommendation implies that an intervention should be performed or prescribed in most situations, and that the benefits of the intervention far exceed any potential harm. Class II recommendations are used for interventions for which expected benefits still outweigh expected harms but to a lesser degree than class I recommendations. Class II recommendations are further subdivided into IIa (moderate) and IIb (weak) recommendations. If an intervention is not recommended, either because no evidence of benefit exists or there is evidence of harm, then a class III (no benefit) or class III (harm) recommendation is assigned.

The level of evidence corresponds with the confidence the authors have in their recommendation based on the quality of studies available. Level of evidence is graded A through C. An A rating is given when evidence comes from multiple randomized clinical trials with concordant results or from high-quality meta-analyses. A recommendation drawn from moderate-quality trials or a ...

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