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The pre-test probability of a target condition is the prevalence of the target condition in the population to which the person being tested, by a screening or diagnostic test, belongs.
Signs, symptoms, other characteristics and the results of other diagnostic tests can be used to define different populations with different pre-test probabilities.
For example, the pre-test probability of breast cancer in a group of women who have been referred for breast biopsy is estimated to be 20%. This means that a woman who has a biopsy because of a suspicious breast lump has a 20% chance that the breast lump will be cancerous before the test is performed.
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