[Considerations on the information content of diagnostic tests (author's transl)]

The value of a diagnostic test lies in its ability to support the doctor's efforts to detect a certain disease. The aim is to achieve the highest possible probability that the disease is correctly diagnosed by means of the test. Doctors are not always aware of the extent to which a test can contribute to the final diagnosis. This contribution, the information content of the test, can be calculated by various algebraic manipulations. An example (carcinoembryonic antigen) is given, which demonstrates the significance of certain terms such as true and false positive results or likelihood and, moreover, the application of Baye's theorem.