A diagnostic test is a measurement used to indicate the presence or absence of a specific disease as condition in a patient from a specific patient population. The parameters that could be calculated are Sensitivity and Specificity. More exactly, in medicine we use clinical sensitivity and specificity. In order to calculate those parameters for a new diagnostic test it is necessary to develop a comparative procedure also called „golden standard”. When this „golden standard” is not available, the construction of a new one becomes appropriate. Furthermore, if that construction is not possible we can calculate a level of agreement. Evaluating a diagnostic or even screening test could be a difficult task, if the methodological issues are not followed.