Recommendation grades and levels of evidence
| Grade | Evidence |
|---|---|
| RCT, randomised controlled trial. | |
| A | At least one high quality meta-analysis, systematic review, or RCT with a very low risk of bias, and directly applicable to the target population; or a systematic review of RCTs or a body of evidence consisting principally of studies with a low risk of bias, directly applicable to the target population, and demonstrating overall consistency of results |
| B | A body of evidence including high quality systematic reviews of case–control or cohort studies, or high quality case–control or cohort studies with a very low risk of confounding, bias, or chance and a high probability that the relationship is causal, directly applicable to the target population, and demonstrating overall consistency of results; or extrapolated evidence from at least one high quality meta-analysis, systematic review, or RCT with a very low risk of bias, or from a systematic review of RCTs or a body of evidence consisting principally of studies with a low risk of bias, and demonstrating overall consistency of results |
| C | A body of evidence including well conducted case–control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal, directly applicable to the target population and demonstrating overall consistency of results; or extrapolated evidence from high quality systematic reviews of case–control or cohort studies, or high quality case–control or cohort studies with a very low risk of confounding, bias, or chance and a high probability that the relationship is causal |
| D | Non-analytic studies—for example, case reports, case series—or expert opinion; or extrapolated evidence from well conducted case–control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal |