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Level of evidence of recommendations in the guidelines

Table 1: The current guidelines stratify the evidence into three categories, judging
by the type, quality and quantity of the referred studies.

Levels of Evidence: Description Levels of Evidence: Description
Category Sources of evidence Definition Code Quality of Definition
of proof Evidence
A Controlled and Great number of A High Further research is very unlikely to
randomized trials studies which imply a change our confidence in the estimate of
(CRT). substantial number of effect.
Large database. participants. Several high-quality studies with
consistent results
B Controlled and Studies include a In special cases: one large, high-quality
randomized trials. limited number of multi-centre trial.
Limited database. patients, analysis
post hoc or analysis B Moderate Further research is likely to have an
of CRT sub-groups, or important impact on our confidence in
metaanalysis of CRT. the estimate of effect and may change
The randomized trials the estimate.
are small, on various One high-quality study.
population groups, Several studies with some limitations.
with inconsistent C Low Further research is very likely to have
results. an important impact on our confidence
C Non-randomized trials. Proofs from non- in the estimate of effect and is likely to
Observational studies. controlled and non- change the estimate.
Expert consensus. randomized trials or One or more studies with severe
observational studies. limitations.
D Very low Any estimate of effect is very uncertain.
Expert opinion.
No direct research evidence.
One or more studies with very severe
limitations.

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