Potential Biases of Screening Tests The common biases of screening are lead time, length-biased sampling, and selection. These biases can make a screening test seem beneficial when actually it is not (or even causes net harm). Whether beneficial or not, screening can create the false impression of an epidemic by increasing the number of cancers diagnosed. It can also produce a shift in proportion of patients diagnosed at an early stage that improves survival statistics without reducing mortality (., the number of deaths from a given cancer relative to the number of those at risk for the cancer). In. | Chapter 078. Prevention and Early Detection of Cancer Part 7 Potential Biases of Screening Tests The common biases of screening are lead time length-biased sampling and selection. These biases can make a screening test seem beneficial when actually it is not or even causes net harm . Whether beneficial or not screening can create the false impression of an epidemic by increasing the number of cancers diagnosed. It can also produce a shift in proportion of patients diagnosed at an early stage that improves survival statistics without reducing mortality . the number of deaths from a given cancer relative to the number of those at risk for the cancer . In such a case the apparent duration of survival measured from date of diagnosis increases without lives being saved or life expectancy changed. Lead-time bias occurs when a test does not influence the natural history of the disease the patient is merely diagnosed at an earlier date. When lead-time bias occurs survival appears increased but life is not really prolonged. The screening test only prolongs the time the subject is aware of the disease and spends as a patient. Length-biased sampling occurs when slow-growing less aggressive cancers are detected during screening. Cancers diagnosed due to the onset of symptoms between scheduled screenings are on average more aggressive and treatment outcomes are not as favorable. An extreme form of length bias sampling is termed overdiagnosis the detection of pseudo disease. The reservoir of some undetected slow-growing tumors is large. Many of these tumors fulfill the histologic criteria of cancer but will never become clinically significant or cause death. This problem is compounded by the fact that the most common cancers appear most frequently at ages when competing causes of death are more frequent. Selection bias must be considered in assessing the results of any screening effort. The population most likely to seek screening may differ from the general population to .
