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Lead time bias occurs if testing increases the perceived survival time without affecting the course of the disease. Lead time bias happens when survival time appears longer because diagnosis was done earlier (for instance, by screening), irrespective of whether the patient lived longer.
Lead time bias refers to a distortion overestimating the apparent time surviving with a disease caused by bringing forward the time of its diagnosis.
Lead-time bias is inherent in any comparison of survival. It makes survival time after screen detection—and, by extension, earlier cancer diagnosis—an inherently inaccurate measure of whether screening saves lives.
Lead time (information bias)- the systematic error of apparent increased survival from detecting disease in an early stage. Length (information bias) - the systematic error from detecting disease with a long latency or pre-clinical period.
Lead time bias occurs when cases who were detected by screening seem to have survived longer than diagnosed cases just because the disease was detected earlier, not because death was delayed. For example: Consider the following 2 scenarios of a patient who suffers from dementia since the age of 65:
Failure to account for the effects of lead-time, length-time, and overdiagnosis biases can lead to overestimation of screening benefits.
'Lead time bias' occurs when a disease is detected during a health screening procedure, before the usual diagnosis when the disease is symptomatic. The earlier a disease is diagnosed, the longer a patient's survival time appears to be.
Determination of survival time among persons with screen-detected cancer is subject to lead time and length biases. The authors propose a simple correction for lead time, assuming an exponential distribution of the preclinical screen-detectable period.
Slide 7: How can studies mitigate length and lead time bias? Include all outcomes regardless of method of detection (symptoms vs. screening). Verify findings from observation studies with RCTs showing disease specific mortality benefit for early detection and treatment.
Length time bias occurs when cases who were detected earlier by SCREENING seem to have survived longer than cases DIAGNOSED after symptoms appear just because screening tests tend to identify less aggressive cases of the disease more often than aggressive ones.