The paper deals with the term Medical Diagnosis (D). It is the process of identifying a disease by its signs, symptoms and results of various diagnostic (laboratory or imaging) procedures. D. is part of clinical reasoning. It includes the following phases: 1. observation (anamnesis, medical examination); 2. problem determination; 3. hypotheses generation; 4. hypothesis control (through laboratory and imaging tests); 5. working D.; 6. therapeutic/caring decisions; 7. final D., 8. follow-up. In order to choose correctly a diagnostic test (DT), it is necessary to answer the following questions: 1. Is an evidence about the accuracy of a DT valid? 2. Does the valid evidence show that this test can accurately distinguish patients who do and do not have a specific disorder? 3. How applying a valid, accurate diagnostic test to a specific patient?