A health authority implements the first-ever colon cancer screening program in its territory.

When a screening program is introduced, the incidence rate appears to rise because more cases (including subclinical ones) are identified earlier. This is known as “lead-time bias” or “ascertainment bias.”

Toronto Notes 2023 – Public Health, Screening and Epidemiology:

“Screening increases the apparent incidence of disease as more early or latent cases are detected.”

MCCQE1 Objectives – Preventive Medicine > Screening:

“Candidates should understand how implementation of screening programs affects disease incidence and epidemiologic metrics.”

Case fatality rate (A) may decrease. PPV (B) depends on prevalence. Positive biopsy rate (C) may remain stable. Treatment rate (E) could increase, but incidence is the most directly and consistently affected.

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