ABSTRACT
An alternative method to the sensitivity and the specificity of expressing the accuracy of a test is by calculating the positive and negative predictive values of an abnormal and a normal test result, respectively. The positive predictive value of a test indicates the proportion of patients with an abnormal test result that actually have the disease out of the total number of patients with an abnormal test result. In the example of the D-dimer test for venous thrombosis in the chapter on sensitivity and specificity this would be 34 of 50 patients, which gives a positive predictive value of 68%. In other words, of all patients with an abnormal D-dimer result, 68% will have thrombosis and 32% will not (this is not very helpful). The negative predictive value is determined by calculating the proportion of patients with a normal test result and no thrombosis out of the total number of patients with a normal test result. In the example the negative predictive value would be 49/50=98%, in other words a normal D-dimer virtually excludes the presence of thrombosis (which may be helpful in clinical practice).
