| Levels of evidence |
| 1++ | High quality metaanalyses, systematic reviews of randomised controlled trials (RCTs) or RCTs with a very low risk of bias |
| 1+ | Well conducted metaanalyses, systematic reviews of RCTs or RCTs with a low risk of bias |
| 1- | Metaanalyses, systematic reviews or RCTs or RCTs with a high risk of bias |
| 2++ | High quality systematic reviews of casecontrol or cohort studies or high quality casecontrol or cohort studies with a very low risk of confounding, bias or chance and a high probability that the relationship is causal |
| 2+ | Well conducted casecontrol or cohort studies with a low risk of confounding, bias or chance and a moderate probability that the relationship is causal |
| 2− | Casecontrol or cohort studies with a high risk of confounding, bias or chance and a significant risk that the relationship is not causal |
| 3 | Nonanalytical studies (eg, case reports, case series) |
| 4 | Expert opinion |
|
| Grades of recommendations |
| A | At least one metaanalysis, systematic review or RCT rated as 1++ and directly applicable to the target population or a systematic review of RCTs or a body of evidence consisting principally of studies rated as 1+ directly applicable to the target population and demonstrating overall consistency of results |
| B | A body of evidence including studies rated as 2++ directly applicable to the target population and demonstrating overall consistency of results or extrapolated evidence from studies rated as 1++ or 1+ |
| C | A body of evidence including studies rated as 2+ directly applicable to the target population and demonstrating overall consistency of results or extrapolated evidence from studies rated as 2++ |
| D | Evidence level 3 or 4 or extrapolated evidence from studies rated as 2+ |