The diagnostic process always involves two sequential steps: the first assesses the patient's clinical situation through data obtained from the history and physical examination, and the second requests tests to confirm the diagnostic hypothesis. The first step belongs to cognitive psychology and has its own biases and learning methods. The second is based on scientific data that defines the discriminatory power of tests from studies comparing a new test (the so-called index test) and the gold standard for diagnosing the disease. In this article we present a classification of the diagnostic characteristics of tests using a 2x2 contingency table. This information can be used to support the rational use of diagnostic tests.