Length-time bias in screening programmes means that screen-detected cancers tend to have a better prognosis because:

• A Screening identifies disease early, before symptoms appear
• B Slow-growing tumours with longer detectable preclinical phases are disproportionately identified by periodic screening ✓
• C Screen-detected cases receive more aggressive treatment
• D Patients identified by screening are more health-conscious and comply better with therapy

Explanation

Length-time bias arises because screening is more likely to detect slow-growing tumours (which spend more time in the detectable preclinical phase) than aggressive fast-growing tumours that progress rapidly from preclinical to symptomatic stage. As a result, screen-detected cancers appear to have a better prognosis, but this is a sampling artefact, not a true screening benefit. This is separate from lead time bias (early diagnosis) and overdiagnosis (detecting cancers that would never cause symptoms).

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.