Fig. 3. Receiver operating characteristics (ROC) curve showing the relationship between sensitivity (true positive) and 1 − specificity (true negative) in determining the predictive value of Brain Natriuretic Peptide (BNP) for cardiogenic pulmonary edema in elderly patients (>65 yr) admitted to the emergency department for acute dyspnea. (A ) The empirical ROC curve is shown by the line containing points  that corresponds to different cutoff; the area under the empirical ROC curve was 0.87 (95% confidence interval 0.80–0.91); the ROC curve is also shown by the continuous line , which was fitted to the binomial distribution. The dotted line  is the identity line. (B ) The best cutoff was chosen as that one which minimizes the mathematical distance (d) to the ideal point (sensitivity = specificity = 1), corresponded to a concentration of BNP of 250 pg/ml with a sensitivity of 0.78 and a specificity of 0.90. But the best cutoff should be preferably chosen as that one which maximizes the distance (j) between the ROC curve and the identity line, for example, that which maximizes the Youden index (sensitivity + specificity − 1), which, in the present case provided the same cutoff value. These two options do not take into account the prevalence and the cost-benefit analysis (see text). Adapted from data from Ray et al .31