Lead time bias

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Lead Time is the length of time between disease detection (usually based on new, experimental criteria) and usual clinical presentation and diagnosis (based on traditional criteria). This is the time between an early diagnosis with screening and the time at which the diagnosis will be made without screening. This is an important factor when evaluating the effectiveness of a particular test.

Video Lead time bias

The relationship between playback and survival

With screening, the goal is to diagnose the disease earlier than without screening. Without screening, the disease can be found later, when symptoms appear.

Early diagnosis with screening may not prolong a person's life but only determine the person's predisposition to a disease or medical condition such as with a DNA test. No additional life span has been obtained and the patient may even experience additional anxiety because the patient has to live longer with knowledge of the disease. For example, a genetic disorder of Huntington's disease is diagnosed when the symptoms appear at about 50, and the person dies at around 65. The typical patient, therefore, lives about 15 years after diagnosis. Genetic testing at birth makes it possible to diagnose this disorder beforehand. If the newborn dies at age 65, the person will "survive" 65 years after diagnosis, without actually living longer than those diagnosed without DNA detection.

Crude statistics can make screening seem to increase the survival time (called lead time). If the person dies at a time in a life that has previously been a common disease compared to when detected by an early examination, the person's life is not renewed.

Detection by advanced screening does not necessarily prolong survival.

Lead time bias may affect the interpretation of the five-year survival rate.