Table 1 Levels of evidence and grades of recommendations

Level of evidence

    1

Systematic review of all relevant randomized clinical trials or n-of-1 trials

    2

Randomized trial or observational study with dramatic effect

    3

Non-randomized controlled cohort/follow-up study (observational)

    4

Case series, case–control study, or historically controlled study

    5

Mechanism-based reasoning (expert opinion, based on physiology, animal, or laboratory studies)

Grades of recommendation

    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 troubling, inconsistent, or inconclusive studies of any level