What is lead-time bias in screening studies?

• A. Early detection through screening inflates survival time without changing mortality
• B. Screening reduces disease incidence
• C. Screening creates random misclassification of disease status
• D. Screening delays diagnosis leading to worse outcomes

Answer: (A)

Lead-time bias happens when screening detects disease earlier than it would have been found because of symptoms, so survival after diagnosis appears longer even though the actual course of the disease and time of death haven’t changed.

For example, if a cancer would be diagnosed at age 65 due to symptoms and the person dies at 70, starting treatment or surveillance at 62 via screening makes it seem like they survived longer after diagnosis (8 years) than if diagnosed at 65 (5 years), even though death occurs at the same age. The apparent benefit is just earlier detection, not a real extension of life.

Because of this, the true measure of a screening program’s effectiveness is whether it reduces disease-specific mortality, not just prolongs time from diagnosis.