Table 1.4Levels of evidence for studies of the accuracy of diagnostic tests

LevelType of evidence
IaSystematic review (with homogeneity)a of level-1 studiesb
IbLevel-1 studiesb
IILevel-2 studiesc; systematic reviews of level-2 studies
IIILevel-3 studiesd; systematic reviews of level-3 studies
IVConsensus, expert committee reports or opinions and/or clinical experience without explicit critical appraisal; or based on physiology, bench research or ‘first principles’

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.


Level-1 studies are studies that use a blind comparison of the test with a validated reference standard (gold standard) in a sample of patients that reflects the population to whom the test would apply.


Level-2 studies are studies that have only one 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.


Level-3 studies are studies that have at least two or three of the features listed above.

From: 1, Introduction

Cover of Diabetes in Pregnancy
Diabetes in Pregnancy: Management of Diabetes and Its Complications from Preconception to the Postnatal Period.
NICE Clinical Guidelines, No. 63.
National Collaborating Centre for Women's and Children's Health (UK).
London: RCOG Press; 2008 Mar.
Copyright © 2008, National Collaborating Centre for Women’s and Children’s Health.

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