Table 1.

ACC/AHA guideline recommendation system: applying class of recommendation and level of evidence to clinical strategies, intervention, treatments, or diagnostic testing in patients care.* Adapted from Halperin et al. [10].

Table 1A
Class (strength) of recommendation (COR)	Description	Benefit vs risk
Class I - Strong	-Is recommended, indicated and useful -Should be performed -Is indicated, effective and beneficial - Comparative-effectiveness phrases** • Treatment/strategy A is recommended/indicated in preference to B • Treatment A should be chosen over treatment B	Benefit ≫ Risk
Class IIa – Moderate	-Is reasonable -Can be effective and beneficial - Comparative-effectiveness phrases** • Treatment/strategy A is probably recommended/indicated in preference to B • It is reasonable to choose treatment A over treatment B	Benefit ≫ Risk
Class IIb – Weak	-May be reasonable but usefulness may be unclear or not well established	Benefit ≥ Risk
Class III – No benefit	-Is not recommended -Not indicated -Should not be performed	Benefit = Risk
Class III - Harm	-Potentially harmful -Should not be performed	Benefit < Risk

Table 1B
Level (quality) of evidence (LOE)	Description	Supporting evidence
Level A	- High quality evidence*** from >1 RCT - Meta-analysis of high quality RCTs -One or more RCTs corroborated by high quality registry studies	RCTs/meta-analysis/registries
Level B – R	- Moderate quality evidence*** from 1 or more RCT - Meta-analysis of moderate quality RCTs	RCTs/meta-analysis
Level B - NR	- Moderate quality evidence*** from 1 or more well designed, well executed non-randomised, observational or registry studies - Meta-analyses of such studies	Non-randomised clinical trials
Level C - LD	- Randomised or non-randomised, observational or registry studies with limitations of design or execution - Meta-analyses of such studies - Physiological mechanistic studies	RCT or non-RCT but with limited data
Level A – EO	Consensus expert opinion based on clinical experience	Non-randomised clinical trials

COR and LOE are determined independently (any COR may be paired with any LOE).

A recommendation with LOE C does not imply that the recommendation is weak. Many important clinical questions addressed in guidelines do not lend themselves to clinical trials. Although RCTs are unavailable, there may be a very clear clinical consensus that a particular test or therapy is useful or effective.

*The outcome or result of the intervention should be specified (an improved clinical outcome or increased diagnostic accuracy or incremental prognostic information).

**For comparative-effectiveness recommendations (COR I and IIa; LOE A and only), studies that support the use of comparator verbs should involve direct comparisons of treatments or strategies being evaluated.

***The method of assessing quality is evolving, including the application of standardised, widely used, and preferably validated evidence grading tools; and for systematic reviews, the incorporation of an Evidence Review Committee.

RCT: randomised clinical trial; COR: class of recommendation; LOE: level of evidence; NR: non-randomised; R: randomised; LD: limited data; EO: expert opinion.