We have designated the validity of a diagnostic test by a single function S taking into account the values of clinical specificity, sensitivity, and the morbidity rate. Thus, by the use of this function each possible diagnostic test can be characterized and easily differentiated from each other.When integrating function S over all x’s (morbidity rates), a parameter Is is obtained which can provide information on the total diagnostic capacity of a test because it comprises the diagnostic properties of both the positive and the negative test results (p+ and p-) in all possible samples with the morbidity rate (x) ranging from 0 to 1.p+ and p- parameters being a posteriori probabilities were used to denote the diagnostic validity,of the positive and the negative test results respectively. By these parameters diagnostic conclusions could be drawn even from unselected population samples (with x values between 0 and 1) in which sensitivity and specificity parameters have no diagnostic properties.