A screening programme for a disease detects cases earlier than they would present symptomatically. However, 5-year survival is better in screen-detected cases simply because the time of diagnosis is moved earlier, not because of any real benefit. This is called:

• A Lead time bias ✓
• B Volunteer bias
• C Length-time bias
• D Hawthorne effect

Explanation

Lead time bias occurs when earlier detection through screening appears to prolong survival because the clock starts earlier (at screen detection) rather than at symptom onset, without any actual extension of life. Length-time bias occurs because screening preferentially detects slow-growing (longer preclinical phase) tumours that have better prognosis. Both biases can inflate the apparent benefit of screening programmes.

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.