Ideally, diagnostic test properties will be assessed in a population in which the spectrum of disease in the target positive patients includes all those in whom clinicians might be uncertain about the diagnosis, and the target negative patients include all those with conditions easily confused with the target condition. Spectrum bias may occur when the accuracy of a diagnostic test is assessed in a population that differs from this ideal. Examples of spectrum bias would include a situation in which substantial proportion of the target positive population have advanced disease or and target-negative participants are ‘normal’ or asymptomatic. Such situations typically occur in diagnostic case-control studies (for instance, comparing those with advanced disease to normals). Such studies are liable to yield an overly sanguine estimate of the usefulness of the test.