Fig. 1

Probability distribution of a diagnostic marker (x) with continuous values (arbitrary unit) in disease-free (gray curve) and diseased (red curve) populations. Vertical solid lines outline the reference range of the marker values in the disease-free population. Any value equal to or greater than the cut-off value (the vertical dashed line) is considered test-positive; otherwise, it is test-negative. The pink area is equivalent to the sensitivity (Se); the blue area, specificity (Sp) [4]. Note that the curves are density functions (area under each curve is equal to 1)