Table 1. Level of evidence and strength of recommendation. CDC classification system.

I	Evidence of at least one large randomised controlled trial (RCT), of good methodological quality (low potential for bias) or meta-analyses of well-conducted randomised clinical trials without heterogeneity
II	RCTsa small or large RCTsa with suspicion of bias (low methodological quality) or meta-analyses of these tests or tests with demonstrated heterogeneity
III	Prospective cohort studies
IV	Retrospective cohort studies or case–control studies
V	Studies without a control group, case reports and expert opinions

A	Solid evidence of efficacy with substantial clinical advantage – strongly recommended
B	Strong or moderate evidence, in terms of effectiveness, but with a limited clinical benefit – generally recommended
C	Insufficient evidence of effectiveness or benefit does not outweigh risks/disadvantages (i.e. adverse events or costs) – optional
D	Moderate evidence against effectiveness or evidence indicating adverse outcomes – generally not recommended
E	Strong evidence against effectiveness or indicating adverse outcomes – not recommended

^aRCTs, Randomised clinical trials