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LEVEL (QUALITY ) OF EVIDENCE‡
LEVEL A
◼ High-quality evidence
‡
from more than 1 RCT
◼ Meta-analyses of high-quality RCTs
◼ One or more RCTs corroborated by high-quality registry studies
LEVEL B-R (Randomized)
◼ Moderate-quality evidence
‡
from 1 or more RCTs
◼ Meta-analyses of moderate-quality RCTs
LEVEL B-NR (Nonrandomized)
◼ Moderate-quality evidence
‡
from 1 or more well-designed, well-executed
nonrandomized studies, observational studies, or registry studies
◼ Meta-analyses of such studies
LEVEL C-LD (Limited Data)
◼ Randomized or nonrandomized observational or registry studies with limitations
of design or execution
◼ Meta-analyses of such studies
◼ Physiological or mechanistic studies in human subjects
LEVEL C-EO (Expert Opinion)
Consensus of expert opinion based on clinical experience
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 B only),
studies that support the use of comparator verbs should involve direct comparisons of the
treatments or strategies being evaluated.
‡ The method of assessing quality is evolving, including the application of standardized,
widely used, and preferably validated evidence grading tools; and for systematic reviews,
the incorporation of an Evidence Review Committee.
COR indicates Class of Recommendation; EO, expert opinion; LD, limited data; LOE, Level
of Evidence; NR, nonrandomized; R, randomized; RCT, randomized controlled trial.
