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| A = consistent level 1 studies |
| B = consistent level 2 or 3 studies or extrapolations* from level 1 studies |
| C = level 4 studies or extrapolations from level 2 or 3 studies |
| D = level 5 evidence or troublingly inconsistent or inconclusive studies of any level |
*“Extrapolations” are where data is used in a situation that has potentially clinically important differences than the original study situation.
