A new screening program detects prostate cancer at an average age of 62 years, whereas clinically diagnosed cases in the pre-screening era presented at an average age of 67 years. Survival after diagnosis appears to improve from 4 years to 9 years, yet age at death is unchanged. The apparent survival gain is best explained by:

• A Lead time bias, because diagnosis simply occurred earlier in the natural history ✓
• B Length bias, because indolent tumors are preferentially detected
• C Selection bias, because volunteers for screening are healthier
• D Recall bias in dating symptom onset among screened men

Explanation

Lead time is the interval between detection by screening and the point at which the disease would have been diagnosed clinically. Screening shifts the date of diagnosis earlier without delaying death, artificially inflating measured survival. Because age at death is unchanged here, the entire apparent gain is lead time. Length bias refers to overrepresentation of slowly progressing tumors among screen-detected cases, selection bias concerns volunteer differences, and recall bias affects self-reported past exposures, none of which explains an unchanged death age.

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

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Written and medically reviewed by the StethoPrep medical team.