Table 1:

Levels of evidence for studies of the accuracy of diagnostic tests^{^a}

Levels of Evidence	Type of Evidence
Ia	Systematic review (with homogeneity)^{^b} of level-1 studies^{^c}
Ib	Level-1 studies^{^c}
II	Level-2 studies^{^d}
	Systematic reviews of level-2 studies
III	Level-3 studies^{^e}
	Systematic reviews of level-3 studies
IV	Consensus, expert committee reports or opinions and/or clinical experience without explicit critical appraisal; or based on physiology, bench research, or “first principles”

↵a Adapted from The Oxford Centre for Evidence-Based Medicine Levels of Evidence (2001) and the Centre for Reviews and Dissemination Report Number 4 (2001). Copyright National Institute for Health and Care Excellence February 2004.
↵b Homogeneity means there are no or minor variations in the directions and degrees of results between individual studies that are included in the systematic review.
↵c Level-1 studies are studies:
- • that use a blind comparison of the test with a validated reference standard.
- • in a sample of patients that reflects the population to whom the test would apply.
↵d Level-2 studies are studies that have only 1 of the following:
- • narrow population (the sample does not reflect the population to whom the test would apply).
- • use a poor reference standard (defined as that where the “test” is included in the “reference,” or where the “testing” affects the “reference”).
- • the comparison between the test and reference standard is not blind.
- • case-control studies.
↵e Level-3 studies are studies that have at least 2 or 3 of the features listed above.