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Prognostic value of exercise testing and catheterization in post-MI patients.

To the Editor: De Feyter and associates (Circulation 66: 527, 1982) have reported a very valuable study comparing the prognostic utility of exercise testing and cardiac catheterization in post-myocardial infarction patients. Certain aspects of their data analysis require further discussion. Cardiac death and recurrent infarction data are initially presented in reference to follow-up intervals; however, final classification into highand low-risk groups utilized a numerator (events) derived from a 36month follow-up, but a denominator (population) from only a 12-month follow-up. The prevalence of cardiac events in the 12-month period in which the entire population is followed is 7.3% (13 of 179), a substantially lower event rate than the 12.8% (23 of 179) presented, which is not adjusted for the variable-interval follow-up period. This shortcoming calls into question all subsequent predictive analyses of sensitivity and specificity of exercise testing and catheterization, since there are insufficient raw data provided to calculate those variables over a uniform follow-up period. When a test such as a catheterization or exercise test does not have a binary outcome, but a continuum of variables, any of which may be selected as the boundary between normal and abnormal, the sensitivity and specificity vary with the value selected as the cutoff point. This concept may be expressed graphically by constructing a receiver operator characteristic (ROC) curve,1 a plot of sensitivity vs (I specificity) for varying cutoff rates. Variation in interpretive criteria will modify a particular test's position on the curve. The less stringent the interpretive criteria, the more sensitive the test and the higher the false-positive rate. Conversely, the more stringent the interpretive criteria, the more specific the test and the higher the false-negative rate. Plotting a hypothetical ROC curve for De Feyter's data exemplifies this concept (fig. 1). Since the tests are plotted on the same curve, they may be considered of equal quality. The preference for one test over the other must then be based upon two considerations: the risks, effectiveness and costs of the intervention suggested by the test result and the risk to the patient of misclassification. In general, false-positive responders are subjected to unnecessary additional diagnostic testing or therapy. Conversely, false-negative responders are denied potentially life-saving interventions. The importance of these considerations must be viewed in the context of the test