LETTERS TO THE EDITOR Comparison of Troponin T and Creatine Kinase-MB Fraction in Evaluating Cardiac Patients Postoperatively

Dr. Januzzi and colleagues in a recent study (1) published in the Journal suggest replacement of creatine kinase-MB fraction (CKMB) with serum troponin testing for postoperative evaluation of the cardiac surgical patient. An important point to remember is the release pattern of these markers. Although the rise (CK-MB 4 to 8 h; CK-MB isoform 2 to 6 h; cardiac troponin I [cTnI] 4 to 6 h; cardiac troponin T [cTnT] 4 to 8 h) and peak (CK-MB 12 to 24 h; CK-MB isoform 18 h; cTnI 12 h; cTnT 12 to 28 h) are similar, the markers differ greatly in their return to normal (CK-MB 72 to 96 h; CK-MB isoform 24 h; cTnI 3 to 10 days; cTnT 7 to 10 days) (2). Because of their prolonged elevation in the blood (up to 10 days), cardiac troponins may reflect a summation of preoperative, perioperative, and postoperative events, thereby limiting the ability to detect perioperative injury exclusively. In contrast, CK-MB and its isoforms have a more rapid clearance and return to normal more quickly, thus facilitating better timing of myocardial injury (3). The long circulating half-lives of the cardiac troponins make it difficult to distinguish new episodes of myocardial necrosis from earlier episodes (4,5). Hence, reinfarctions may be difficult to diagnose with the sole use of cardiac troponin assays, if the initial myocardial infarction (MI) occurred within a week of cardiac surgery. Subsequently, both CK-MB and cardiac troponins are concurrently useful and not mutually exclusive in the diagnosis of postoperative MI and reinfarction after coronary artery bypass graft surgery.