Overlap index. An alternative to sensitivity and specificity in comparing the Utility of a laboratory test.

The utility of a laboratory test for differential diagnosis depends on the degree that test results for different disease states overlap. I developed a new overlap index that ranges from zero, for an ideal test with no overlap, to one if the median test result is the same for the two disease states. The index does not depend on defining a normal range or estimating the percentage of observations that lie outside of the normal range. For these reasons it is superior to sensitivity and specificity for comparing laboratory tests, particularly if only limited test data are available.