Table 2. Definition of level of evidence.

Level A

There exists high standard meta-analysis or several high-standard randomised clinical trials that give consistent results

Level B

There exist good quality evidence from randomised trials (B1) or prospective or retrospective studies (B2). The results are consistent when considered together

Level C

The methodology of the available studies is weak or their results are not consistent when considered together

Level D

Either the scientific data do not exist or there is only a series of cases

Expert agreement

The data do not exist for the method concerned, but the experts are unanimous in their judgement