Limites et Biais de l’Evaluation Retrospective d’une Technique de Diagnostic
In order to assess retrospectively the diagnostic value of radionuclide brain scans, 399 cases which fulfilled the required diagnosis criteria were reviewed. These 399 cases constituted only 22% of the scans performed during one year in the hospital. In this sample, the test overall accuracy was equal to 79.5. False positive errors (4%) were less frequent than false-negative ones (16.5 %). By comparing scintigraphic diagnoses with preliminar clinical diagnoses, the main result appeared to be the following. When these two diagnoses were identical, the final diagnosis agreed with the preliminar clinical hypothesis in 97 % of the cases; but when they were different, final diagnosis and preliminar clinical hypothesis agreed in only 15% of the cases.
But limits and biases of such a retrospective study make suspectáble this apparently positive conclusion. Indeed, they lead to i) overestimate the proportion of abnormal scans; ii) under estimate the proportion of false-negative errors; iii) over estimate, on the overall, the diagnosis value of this test. But it seems, unfortunately, that neither a prospective study nor a change in the efficacy criteria which are taken into account, can permit to eliminate important errors in assessment of diagnostic tests.
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