Assessment of diagnostic tests

An increasing number of clinicians is to day aware of the need to assess their diagnostic tools. Despite the fact that monetary value is only one of the components of the cost of a diagnostic test, economical problems are decisive stimuli for such an effort. Any diagnostic test involves a cost which is borne by the patient. So, test request is admissible only if its result may provide an useful information to the clinician. Behind this apparently simple formulation, one can foresee difficulties arising from the fact that criteria of judgement are essentially subjective, both for the cost and the usefulness. Several indices such as sensitivity, specificity, predictive values and accuracy, are commonly used for the assessment of the diagnostic value of a test. But these indices are not always calculated from valid data. And biases and lack of precisions are sometimes an obstacle to the transferability of the conclusions. Running from the most simple to the most complex, the aim of the assessment may be: 1. To judge the diagnostic value of a single test by reference to the final diagnosis or to the clinician's initial hypothesis. 2. To compare two diagnostic tests A and B and to discuss the possible interest of their combination. 3. To define the exact place of a test in a complex diagnostic strategy. In each of these situations, an ideal protocol can be envisaged. It will be often difficult to apply. But it will help to an informative description of both data and results, making easier utilisation of the conclusions of the study by the whole medical community.