Table 1.

Levels of evidence and recommended grades

		Definition
Level of evidence
A	There is good research-based evidence to support the recommendation. Data derived from at least 1 randomized controlled trial, meta-analysis or systematic review
B	There is fair research-based evidence to support the recommendation. Data derived from at least 1 well designed cohort or case-control study without randomization
C	There is fair evidence but it is insufficient to support the recommendation. Data derived from case series or observational studies
D	The recommendation is based on expert opinion and panel consensus. Consensus of expert opinions based on experience
Recommended grade
Class I	The recommendation is supported by level of evidence A. Clinically important outcomes and the study population is representative of the population in the recommendation.	Is recommended
Class IIa	The recommendation is supported by level of evidence B. Clinically important or validated surrogate outcomes.	Should be considered
Class IIb	The recommendation is supported by level of evidence C or D. The outcome is an unvalidated surrogate condition but clinically important outcomes.	May be considered
Class III	The recommendation is supported by level of evidence C or D. Outcome is an unvalidated surrogate for clinically important population, or the applicability of the study is irrelevant.	Is not recommended

Data from Schünemann et al.³