A new mammography programme reports that women whose tumours were detected through screening survive a median of 8 years after diagnosis, compared with 4 years among women diagnosed outside screening. Mortality rates from breast cancer in the screened population are unchanged. The inflated apparent survival benefit is best explained by:

• A Length bias
• B Healthy worker effect
• C Hawthorne effect
• D Lead time bias ✓

Explanation

Lead time is the interval between detection by screening and the point at which the diagnosis would have been made clinically. Screening advances the date of diagnosis without delaying the date of death, so post-diagnosis survival rises even though mortality is unchanged. Length bias refers to over-representation of indolent disease in screened populations, which is a related but distinct screening artefact.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.