| Grade A | Recommendation based on at least one high quality randomized controlled trial or at least one high quality meta-analysis of well-done randomized controlled trials. |
| Grade B | Recommendation based on high quality case-control or cohort studies OR a high quality systematic review. |
| Grade C | Recommendation based on non-analytical studies (case reports or case series). |
| Grade D | Recommendations based on expert opinion only. |
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