Lead-time bias

Lead-time bias affects comparisons of survival between screen-detected and clinically detected disease. Screening finds disease earlier, which starts the survival clock sooner. Even if treatment does nothing to postpone death, the time from diagnosis to death is longer simply because diagnosis happened earlier, creating a false impression that screening extends life. Consider two people who would die at the same age from the same cancer; if one is diagnosed by screening five years earlier, that person appears to survive five years longer, though the date of death is identical. When reading a screening study, be cautious about claims based on five-year survival from diagnosis, and look instead for evidence on disease-specific mortality in a randomized comparison, which is not fooled by when the clock starts. Watch also for its cousin, length-time bias. The question it lets you ask: does the reported survival benefit reflect a real delay in death, or just an earlier diagnosis date?