Table 1

Levels of evidence and grades of recommendations

Levels of evidence
IEvidence from at least one large randomized, controlled trial of good methodological quality (low potential for bias) or meta-analyses of well-conducted, randomized trials without heterogeneity.
IISmall randomized trials or large randomized trials with a suspicion of bias (lower methodological quality) or meta-analyses of such trials or of trials with demonstrated heterogeneity.
IIIProspective cohort studies.
IVRetrospective cohort studies or case–control studies.
VStudies without control group, case reports, experts' opinions.
Grades of recommendations
AStrong evidence for efficacy with a substantial clinical benefit, strongly recommended.
BStrong or moderate evidence for efficacy but with a limited clinical benefit, generally recommended.
CInsufficient evidence for efficacy or benefit does not outweigh the risk or the disadvantages (adverse events, costs, etc.), optional.
DModerate evidence against efficacy or for adverse outcome, generally not recommended.
EStrong evidence against efficacy or for adverse outcome, never recommended.