American Board of Surgery Qualifying Exam (ABS QE) Practice Test

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How is lead time bias defined?

Bias introduced by early disease detection through screening
A distortion caused by differences in outcome assessment
Loss of follow-up subjects affecting outcome certainty
The odds of an outcome for individuals with versus without exposure

The correct answer is: Bias introduced by early disease detection through screening

Lead time bias is defined as the bias introduced by early disease detection through screening. This occurs when a disease is diagnosed earlier due to screening efforts, but the time from diagnosis to death does not actually change – patients may live longer after diagnosis (since it is detected earlier), but their overall survival may not actually improve. Essentially, lead time bias often gives the illusion that a screening program is effective in extending life because the diagnosis happens sooner, while the actual course of the disease remains unchanged. The other options address different types of biases or issues in research and study design, such as confounding factors or variations in how outcomes are measured, but they do not accurately capture the essence of lead time bias, which specifically revolves around the early detection of a disease affecting perceived survival rates.