II. Process Studies (Diagnostic Test)

Abstract

The purpose of a diagnostic test is to establish the presence of health or disease, it can even graduate the degree of illness. Diagnostic tests are usually assessed mathematically. Thus, sensitivity and specificity are estimated once the existence or not of disease is known; in clinical practice, the course of action is often reverse: from positivity or negativity of a test for the presence or not of the disease and, therefore, positive and negative predictive values are used. Mathematical strategies allow for an observation to be quantified, but clinical judgement is required in order to establish the quality of that observation; in consequence, some characteristics have to be considered: a) selection under the same criteria for cases and witnesses; b) inclusion of the entire spectrum of severity of the disease (trying that all the strata include an important number of subjects); c) the interpretation of the gold standard and the test under study must be blinded and done by experts; d) the interpretation of the results must show the applicability of the test in everyday practice; e) reproducibility of the test must be proven. It is important not to forget that, usually, only a patient is seen at a time; therefore, full knowledge of the diagnostic test performance is essential, as well as considering the clinical aspects for its correct application.