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Lead-time Bias — SCE Medical Oncology MCQ

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HardClinical Trials & StatisticsLead-time BiasSCE Medical Oncology

A screening study reports that median survival from diagnosis rose from 4 to 7 years, exactly matching a three-year advance in diagnosis. Age at cancer death and population disease-specific mortality were unchanged. Which interpretation is most defensible?

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Correct answer: A — This is lead-time bias; require mortality or morbidity reduction to infer screening benefit

Explanation lettering: B = shown as A · E = shown as B · A = shown as C · C = shown as D · D = shown as E

B is correct. Moving diagnosis three years earlier automatically adds three years to measured post-diagnosis survival even when death occurs at the same age. That is lead-time bias, which is why UK screening evaluation emphasises cause-specific mortality and meaningful morbidity rather than survival among diagnosed cases. Length-time bias concerns preferential detection of slower-growing disease and is not established merely by a stage shift. Overdiagnosis may coexist but cannot be proven for every case from these aggregate data. Post-diagnosis survival is therefore not interchangeable with a population mortality endpoint.

Reference: UK National Screening Committee screening standards: https://www.gov.uk/government/publications/population-screening-our-approach-to-screening-standards--2/nhs-population-screening-standards