Table 1

Levels of evidence and grades of recommendation (adapted from the Infectious Diseases Society of America-United States Public Health Service Grading System*)

Levels of evidence
IEvidence from at least one large randomised, controlled trial of good methodological quality (low potential for bias) or meta-analyses of well-conducted randomised trials without heterogeneity
IISmall randomised trials or large randomised 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, expert opinions
Grades of recommendation
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
  • *By permission of the Infectious Diseases Society of America.2