Table 1 Definition of the levels of evidence and the grade of recommendations [1]

EVIDENCE LEVEL

 I = Evidence obtained from several controlled, randomised clinical trials and/or systematic revisions of randomised trials

 II = Evidence obtained from a single randomised trial with an appropriate design

 III = Evidence obtained from cohort studies with controls or their meta-analysis

 IV = Evidence obtained from case-control retrospective studies or meta-analysis

 V = Evidence obtained from case studies (series of cases) without a control group

 VI = Evidence based on the opinion of qualified experts or a committee of experts as indicated in the guidelines, in consensus conferences, or based on the opinions of the members of the work group responsible for these guidelines

GRADES OF RECOMMENDATION

 A = the performance of that particular procedure or diagnostic test is highly recommended (indicated by a special recommendation supported by good quality scientific evidence, even though not necessarily of Types I or II)

 B = there are some doubts about the fact that this particular procedure/operation must always be recommended, however it is considered that its performance must be physical examined

 C = there is substantial uncertainty in favour of or against the recommendation to perform this procedure or operation

 D = the performance of this procedure is not recommended

 E = the performance of this procedure is strongly advised against