The programme

♦ Adequate facilities for diagnosis in those with a positive test

♦ High quality of treatment for screen-detected disease

♦ Screening repeated at intervals if the disease is of insidious onset

♦ Benefit must outweigh physical and psychological harm

♦ Benefit must justify financial cost

It is crucial that treating the disease to be screened at an early stage is more effective than treating at a later stage. To justify a screening programme one cannot compare the outcome of screen-detected disease with that of symptomatic disease, because three biases operate in favour 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 or not treatment has altered the natural history of the disease. Screening will only be of value if it shifts the survival curve upwards.

♦ Length bias operates as slow-growing tumours are more likely to be detected by screening tests when compared to fast-growing tumours which are more likely to present with symptoms before a screening test can be applied or between tests. Thus, screen-detected tumours will tend to be less aggressive and associated with a relatively good prognosis.

♦ Selection bias results from the type of person who accepts an invitation to be screened. Such a person is more likely to be health conscious than one who refuses or ignores screening and is therefore likely to survive longer, irrespective of the disease process.

These three biases make patients with screen-detected tumours appear to have a better prognosis than when tumours present with symptoms. For the true effect of screening to be revealed, screening research must take these biases into account. The only way to do this is to carry out randomized, controlled trials in which mortality from a specific disease is compared between a population offered screening (including those who present with symptoms before screening can take place or between screens and those who refuse to be screened) and a population not offered screening. Such trials have been done in breast and colorectal cancer.

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