Lead time bias
Lead time is the length of time between the detection of a disease (usually based on new, experimental criteria) and its usual clinical presentation and diagnosis (based on traditional criteria).
Lead time bias is that factor when comparing, in detection of disease, a traditional test with a new or experimental test but the outcome of the disease is unchanged; the new or experimental test merely identifies the disease earlier than the previous and thus gives the impression that survival is prolonged. It is an important factor when evaluating the effectiveness of a specific test.
Relationship between screening and survival
By screening, the intention is to diagnose a disease earlier than it would be without screening. Without screening, the disease may be discovered later when symptoms appear.
Early diagnosis through screening may not prolong the life of someone, just determine the propensity of the person to a disease or medical condition such as through DNA testing. No additional life span has been gained; the patient may be subject to added anxiety as the patient must live for longer with knowledge of the disease. For example, the genetic disorder Huntington's disease is diagnosed when symptoms appear around age 50, and the person dies around age 65. The typical patient therefore lives about 15 years after diagnosis. A genetic test at birth makes it possible to diagnose this disorder earlier. If this newborn baby dies around age 65, he or she will have "survived" 65 years after diagnosis, without having actually lived any longer than those diagnosed without DNA detection.
Raw statistics can make screening appear to increase survival time (this gain is called lead time). If the person dies at a time in life that previously has been the usual course of the disease, than when detected by early screening, then the person's life has not been prolonged. Detection through advanced screening does not always mean prolong survival.