Episode Overview

This episode examines why medical screening is not simply a matter of finding disease as early as possible. Austin Baraki discusses the distinction between screening asymptomatic people and investigating symptoms, the probabilistic interpretation of test results, and the ways in which screening can produce false positives, overdiagnosis, overtreatment and diagnostic cascades.

The discussion also considers direct-to-consumer testing, disease mongering, the pressure on clinicians to act, and the role of shared decision-making. The examples and guideline positions described are those discussed in the recording. They should not be treated as current clinical recommendations without checking up-to-date guidance for the relevant population and jurisdiction.

Useful Terminology

• Screening: Testing people who do not have relevant symptoms in order to identify an unrecognised disease, a precursor state or a risk factor.
• Diagnostic evaluation: Investigation prompted by symptoms, signs or other clinical information, with the aim of explaining the presentation and guiding management.
• Pre-test probability: The estimated probability that a person has a condition before the result of a particular test is known.
• Post-test probability: The updated probability of the condition after the test result and other relevant information are considered.
• Sensitivity: The proportion of people with the condition who test positive. A highly sensitive test produces relatively few false negatives.
• Specificity: The proportion of people without the condition who test negative. A highly specific test produces relatively few false positives.
• Positive predictive value: Among people who test positive, the proportion who truly have the condition. This depends on the test and the prevalence of the condition in the tested population.
• Negative predictive value: Among people who test negative, the proportion who truly do not have the condition. This also varies with prevalence.
• False positive: A positive test result in a person who does not have the condition.
• False negative: A negative test result in a person who does have the condition.
• Overdiagnosis: Correctly identifying a condition that would never have caused symptoms or death during the person’s expected lifetime.
• Misdiagnosis: Assigning a diagnosis that the patient does not in fact have.
• Overtreatment: Treatment that is unnecessary or provides no net benefit. It can follow overdiagnosis, although the two are not identical.
• Lead-time bias: The appearance of longer survival because a condition is detected earlier, even when the time of death is unchanged.
• Length-time bias: The tendency of screening to detect more slow-growing, long-duration disease than rapidly progressive disease.
• Disease mongering: Expanding or promoting disease definitions in ways that turn ordinary experiences, low-risk states or weakly consequential findings into medical problems.
• Commission bias: A preference for action over inaction, even when observation or deliberate non-intervention may be the better choice.
• Diagnostic cascade: A sequence of further tests, consultations or treatments triggered by an initial finding, including an incidental or false-positive result.

Screening and Diagnosis Answer Different Questions

Baraki defines screening as a search for an unidentified condition in people who appear well and do not report relevant symptoms. Diagnosis begins from a different starting point: a symptom, sign or other clinical concern changes the prior probability of disease and creates a reason to investigate.

• Screening begins with an asymptomatic population.
  • It may be offered to an entire population, to a demographic group or to a more selective high-risk group.
  • The aim is not merely to find an abnormality. The intended aim is to reduce later morbidity,