Levels of evidence and recommendation grades
| Recommendation grade | Level of evidence | Description of evidence |
|---|---|---|
| Grade A | 1++ | Evidence obtained from at least one high quality meta-analysis, systematic review, or randomised controlled trial with a very low risk of bias, directly applicable to the target population, and showing overall consistency of results. |
| Grade B | 2++ | 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 change and a moderate probability that the relation is causal, directly applicable to the target population, and showing overall consistency of results. |
| 1++ | Extrapolated evidence obtained from at least one high quality meta-analysis, systematic review, or randomised controlled trial with a very low risk of bias. | |
| 1+ | Extrapolated evidence from well conducted meta-analyses, systematic reviews of randomised controlled trials, or randomised controlled trials with a low risk of bias. | |
| Grade C | 2+ | Evidence obtained from well conducted case-control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relation is causal, directly applicable to the target population, and showing overall consistency of results. |
| 2++ | 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 change and a moderate probability that the relation is causal. | |
| Grade D | 3 | Evidence from non-analytical studies, e.g. case reports, case series. |
| 4 | Evidence from expert opinion. | |
| 2+ | 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 relation is causal. |