Lead time bias

Lead time bias is a form of systematic error that can arise in diagnostic studies. It refers to the apparent increase in survival time observed among individuals who are diagnosed earlier, even if the treatment for their condition is not effective. This bias occurs because survival is typically measured from the point of diagnosis. Early diagnosis simply adds time to the patient’s measured survival without necessarily extending their actual life span from the onset of the disease. If the delay period (between early diagnosis and late diagnosis, usually when symptoms are apparent) is known, it is relatively easy to make adjustments to survival estimates. However, in most cases, the delay period is not properly known. To solve this, diagnostic studies might be designed as randomised controlled trials, with patients either randomised to early testing or to a non-testing arm. However, such designs face ethical challenges and would likely take many years to provide mature data.