Table 1 Levels of evidence and strength of recommendation modified after [25, 26].

1

Meta-analysis of randomized controlled trials, good quality randomized controlled trials or 'all or none' studies in which no treatment is not an option

2

'Low quality' randomized controlled trials (< 80% follow up), meta-analysis of good quality prospective 'cohort studies' or well designed controlled study without randomization

3

Good quality retrospective 'case-control'-studies or comparative studies

4

Consensus, usual practise, disease-oriented evidence, and/or expert opinion

Strength of recommendation

Explanation

A

Use of agent is recommended ("do it")

B

Use of agent should be considered ("probably do it")

C

Use of agent may be considered ("is not recommended")