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.