Lead time bias

However, the lead time itself biases survival statistics: people with diseases detected by screening appear to have a longer survival (the time the person has lived after diagnosis) only because screening starts the clock sooner.

[citation needed] Lead time bias affects the interpretation of the five-year survival rate, effectively making it appear that people survive longer with cancer even in cases where the course of cancer is the same as in those who were diagnosed later.

[3] Another example is when early diagnosis by screening may not prolong the life of someone but just determine the propensity of the person to a disease or medical condition, such as by DNA testing.

For example, the genetic disorder Huntington's disease is diagnosed when symptoms appear at around 50, and the person dies at around 65.

If this newborn baby dies at around 65, the person will have "survived" 65 years after diagnosis, without having actually lived any longer than those diagnosed without DNA detection.