Correspondence

Troponin I in the Diagnosis of Postoperative Myocardial Infarction

N Engl J Med 1994; 331:277July 28, 1994

Article

To the Editor:

It was a pleasure to read the article by Adams et al. on the diagnosis of perioperative myocardial infarction by the measurement of troponin I (March 10 issue).1 Clearly, creatine kinase levels can be misleading in the perioperative period, and a better marker for perioperative myocardial injury is needed. However, we have reservations about the authors' conclusion that troponin I is more specific than creatine kinase in the diagnosis of perioperative myocardial infarction. Fractionation of the creatine kinase enzyme allows calculation of the relative index, which is defined as MB creatine kinase divided by total creatine kinase. With use of the data supplied in Figure 4 and the relative-index reference limit of 2.5, noted in the legend, the calculated specificity of the creatine kinase relative index for myocardial infarction is 98 percent, which is not significantly different from that of the troponin I assay (99 percent). The high values for the two assays are probably due to the stringent criterion by which a diagnosis of myocardial infarction was made (i.e., a new akinetic or dyskinetic regional wall motion seen on echocardiography). This criterion would be expected to screen out many nontransmural myocardial infarcts, which frequently present the greatest diagnostic difficulty in the perioperative period.

Although we agree that troponin I appears to be more specific than total MB creatine kinase activity for the diagnosis of perioperative myocardial injury, we do not believe that the authors have demonstrated that troponin I is superior to the creatine kinase relative index. Given the wide variability in the time course of troponin I activity, shown in Figure 3, we wonder how often and for what period these values should be monitored. Information regarding the expense of the test is not reported, but it is unlikely to be cheaper than the test for creatine kinase isoenzymes, and with a relative-index specificity of 99 percent, as compared with 98 percent for MB creatine kinase, one wonders whether the troponin I test will be worth the presumably increased cost.

Michael R. Rich, M.D.
Joseph G. Murphy, M.D.
Mayo Clinic, Rochester, MN 55905

1 References

1. 1

   Adams JE III, Sicard GA, Allen BT, et al. Diagnosis of perioperative myocardial infarction with measurement of cardiac troponin I. N Engl J Med 1994;330:670-674
   Full Text | Web of Science | Medline

Author/Editor Response

The authors reply:

To the Editor: Many laboratories calculate the percentage of MB creatine kinase with respect to total creatine kinase and use a cutoff value such as the one proposed by Drs. Rich and Murphy, with the hope of improving the specificity of MB creatine kinase for the diagnosis of myocardial injury. This approach has serious problems. The percentage of MB creatine kinase in skeletal muscle, particularly in regenerating muscle, is quite variable. Thus, high percentages of MB creatine kinase in plasma and false positive diagnoses of myocardial injury occur when percentage criteria are used in patients with acute or chronic skeletal-muscle injury1. A more common clinical problem arises when severe skeletal-muscle damage causes the release of large amounts of creatine kinase into plasma. This reduces the percentage of MB creatine kinase with respect to total creatine kinase so that its sensitivity for the diagnosis of myocardial injury is lost. This problem has been reported in patients with cardiac contusion2 and was also the case in the perioperative patients we described. The sensitivity would have been only 62.5 percent if the relative index proposed had been used. It is for these reasons that there is enthusiasm for the development of sensitive and highly specific markers such as cardiac troponin I.

It is unlikely that we failed to diagnose substantial numbers of non-Q-wave infarctions in our cohort of patients, because there is 13 times more cardiac troponin I than MB creatine kinase in myocardium, and clinically, the sensitivity of cardiac troponin I is comparable to that of MB creatine kinase for the diagnosis of acute myocardial infarction3. Furthermore, elevations persist for four to seven days after infarction, providing a prolonged and thus a more convenient opportunity for a diagnosis based on analysis of daily samples.

Jesse E. Adams, III, M.D.
Jack H. Ladenson, Ph.D.
Allan S. Jaffe, M.D.
Washington University School of Medicine, St. Louis, MO 63110-1093

3 References

1. 1

   Adams JE III, Bodor GS, Davila-Roman VG, et al. Cardiac troponin I: a marker with high specificity for cardiac injury. Circulation 1993;88:101-106
   Web of Science | Medline

2. 2

   Potkin RT, Werner JA, Trobaugh GB, et al. Evaluation of noninvasive tests of cardiac damage in suspected cardiac contusion. Circulation 1982;66:627-631
   CrossRef | Web of Science | Medline

3. 3

   Adams JE III, Schechtman K, Landt V, Ladenson JH, Jaffe AS. Equivalent sensitivities of MB creatine kinase (MBCK) and cardiac troponin I for detection of acute myocardial infarction. Clin Chem (in press).
   

Citing Articles (1)

Citing Articles

1. 1

   A Edouard, C Cosson. (2003) Le dosage de la troponine I : intérêt et limites en pratique médicale courante. La Revue de Médecine Interne 24:9, 623-626
   CrossRef