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| Levels of evidence |
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| 1 ++ | High-quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of bias |
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| 1 + | Well-conducted meta-analyses, systematic reviews, or RCTs with a low risk of bias |
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| 1 − | Meta-analyses, systematic reviews, or RCTs with a high risk of bias |
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| 2++ | High-quality systematic reviews of case control or cohort studies
High-quality case control or cohort studies with a very low risk of confounding or bias and a
high probability that the relationship is causal |
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| 2+ | Well-conducted case control or cohort studies with a low risk of confounding or bias and a
moderate probability that the relationship is causal |
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| 2− | Case control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal |
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| 3 | Nonanalytic studies, for example, case reports and case series |
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| 4 | Expert opinion |
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| Grades of recommendation |
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| Note: The grade of recommendation relates to the strength of the evidence on which the recommendation is based. It does not reflect the clinical importance of the recommendation. |
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| A | At least one meta-analysis, systematic review, or RCT rated as 1++ and directly applicable to the target population; or a body of evidence consisting principally of studies rated as 1+, directly applicable to the target population and demonstrating overall consistency of results |
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| 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+ |
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| 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++ |
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| D | Evidence level 3 or 4; or extrapolated evidence from studies rated as 2+ |
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