Table 1 Levels of evidence

1

Evidence obtained from:

• systematic review of randomized controlled trials;

• high-quality randomized controlled trials;

• high-quality prospective studies (e.g., all patients were enrolled at the same point of their disease with 80 % follow-up of enrolled patients); or

• testing of previously developed diagnostic criteria in series of consecutive patients.

2

Evidence obtained from:

• systematic review of level 2 studies or level 1 studies with inconsistent results;

• lesser quality randomized controlled trials (e.g., < 80 % follow-up, no blinding, or improper randomization);

• prospective comparative studies;

• retrospective studies;

• lesser quality prospective studies (e.g., patients enrolled at different points in their disease or <80 % follow-up); or

• development of diagnostic criteria on the basis of consecutive patients (with universally applied reference gold standard).

3

Evidence obtained from:

• systematic review of level 3 studies;

• case control studies;

• retrospective comparative studies; or

• study of nonconsecutive patients (without consistently applied reference gold standard).

4

Evidence obtained from:

• case series; or

• case control study with poor reference standard.

5

Evidence obtained from:

• expert opinion.