[The step toward a specific diagnosis. Evaluation of the value of diagnostic methods in the individual case with special attention to probability].

The preliminary diagnosis based on the patient's history and the results of a physical examination, is considered to be the most probable one. In general, however, differential diagnoses with a smaller degree of probability must also be taken into account. This article shows how--making use of simple probability theory--an assessment can be made as to whether an available diagnostic instrument is capable of increasing the probability of a post-test diagnosis vis-a-vis the pre-test probability level. The terms sensitivity and specificity are explained with the aid of clinical examples, and the possibility of increasing the probability of the diagnosis by iteration is discussed.