Quantifying the value of diagnostic tests

This paper considers the different statistics that can be derived from studies on the value of diagnostic tests. Well-accepted quantities, such as sensitivity and specificity, are reviewed and the value of the likelihood ratio in communicating particular test results is highlighted. A new quantity, the 'diagnostic value index', is proposed and various examples of its application in nuclear medicine studies are presented. This new index is based on a weighted average of the likelihood ratios for all possible outcomes of the test, the average being weighted according to how frequently a particular outcome occurs. This index provides an objective and quantitative means of comparing the diagnostic value of different tests, or of comparing the value of a given test in different patient groups. It is hoped that this will be a useful aid to clinical decision-making concerning referrals for diagnostic tests.