Introduction

You may wonder why we have a separate chapter on screening tests. After all, now that you have learned about sensitivity, specificity, likelihood ratios, Receiver Operating Characteristic curves and so forth, it seems like you should be well equipped to evaluate screening tests. However, whereas diagnostic tests are done on sick people to determine the cause of their symptoms, screening tests are generally done on healthy people with a low prior probability of disease. The problem of false positives and possible harms of unnecessary treatment looms larger. The questions of whether the patient benefits from being diagnosed and whether this benefit justifies the possible harms and costs of the test are more salient for screening. Finally, because decisions about screening are often made at the population level, political and other factors may be more influential. Thus, in this chapter, we focus explicitly on the question of whether doing the test improves health, not just whether it gives the right answer, and we pay particular attention to biases and nonmedical factors that can lead to excessive screening.

Definition and types of screening

Our favorite definition of screening is that suggested by Eddy (1991): “the application of a test to detect a potential disease or condition in people with no known signs or symptoms of that disease or condition.” The “test” being applied may be a laboratory test or x-ray, or it may be nothing more than a standard series of questions, as long as the goal is to detect a disease or condition of which the patient has no known symptoms.