A new screening test for prostate cancer is introduced. Five years after implementation, the average survival time after diagnosis has doubled, but the age-specific mortality rate from prostate cancer is unchanged. The most likely explanation is:

• A Length bias, because slow-growing tumours are preferentially detected
• B Selection bias, because only motivated patients underwent screening
• C Lead time bias, because diagnosis occurs earlier without delaying death ✓
• D Overdiagnosis bias, because non-progressive lesions were treated

Explanation

Lead time bias occurs when screening advances the time of diagnosis without changing the time of death, so survival measured from diagnosis increases even though nothing has changed biologically. The unchanged mortality rate is the decisive clue, since a genuinely effective intervention must lower cause-specific mortality. Length bias refers to over-representation of indolent cases, and overdiagnosis refers to detecting lesions that would never become clinically apparent; neither explains prolonged post-diagnosis survival with stable mortality.

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.