Oxford American handbook of oncology. Second Edition. Oxford University Press (2015)
It is crucial that treating the disease to be screened at an early stage is more effective than treating it at a later stage.
To justify a screening program, one should not compare the outcome of screen-detected disease with that of symptomatic disease, because three biases operate in favor of screen-detected disease.
- Lead-time bias arises from the fact that, if early diagnosis advances the time of diagnosis of a disease, then the period from diagnosis to death will lengthen irrespective of whether treatment has altered the natural history of the disease. If patients die of their cancer at the same age at which this event would have occurred without screening, no benefit has been afforded by screening. Screening will only be of value if it improves the survival curve of a screened population compared with that of an unscreened population.
- Length bias operates because slow-growing tumors are more likely to be detected by screening tests than fast-growing tumors, which are more likely to present with symptoms before a screening test can be applied or between tests. Thus, screen-detected tumors will tend to be less aggressive and associated with a relatively better prognosis than interval-detected tumors.
- Selection bias results from the characteristics of individuals who accept an invitation to be screened. Such a person is more likely to be health conscious than one who refuses or ignores screening and may therefore be more likely to survive longer, irrespective of the disease process.