Table 1.

Grades of recommendation [12]

Grade of recommendations	Level of evidence	Type of study
A	1a	Systematic review of (homogeneous) randomized controlled trials
A	1b	Individual randomized controlled trials (with narrow confidence intervals)
B	2a	Systematic review of (homogeneous) cohort studies of “exposed” and “unexposed” subjects
B	2b	Individual cohort study/low-quality randomized control studies
B	3a	Systematic review of (homogeneous) case–control studies
B	3b	Individual case–control studies
C	4	Case series, low-quality cohort or case–control studies
D	5	Expert opinions based on non-systematic reviews of results or mechanistic studies