Am Fam Physician. 1998;57(2):226-229
to the editor: The article by Dr. Chesebro1 provides useful and timely information regarding the often difficult diagnosis of myocardial infarction. Two points should be made regarding the use of cardiac troponin I (c-TnI) in this setting. First, rapid assays are available for c-TnI; they are currently in use at our institution. The turn-around time for c-TnI is equivalent to that of MB isoenzyme of creatine kinase (CK-MB) assays. Second, elevated c-TnI levels do not seem to be found in patients with renal failure (as Dr. Chesebro correctly points out), unlike CK-MB and cardiac troponin T, though the data to support this conclusion is lacking. We have found that c-TnI is a more accurate predictor of myocardial injury than CK-MB assays in patients with renal failure [Martin GS. February 1997. Unpublished data.], and that non-specific elevations of c-TnI do not seem to occur. Moreover, our data for patients with renal failure support Antman's conclusion2 about the population in general—that elevated c-TnI levels are associated with increased short-term mortality. With rapid c-TnI assays available, immediate decisions can be made regarding diagnosis of chest pain as well as stratifying patient risk. Cardiac troponin I and CK-MB complement each other because of their respective kinetics (rising in tandem, with CK-MB falling within 36 hours and c-TnI remaining positive for four to 10 days). As c-TnI assays become more widely available, evaluation of patients with suspected myocardial injury should become both simpler and more precise.
in reply: I appreciate Dr. Martin's letter, as well as correspondence from Spectral Diagnostics, Inc., regarding their product, the Cardiac STATus Troponin I test kit, which uses seven to eight drops of whole blood, serum or plasma and gives results within 15 minutes. The improved predictive values, both positive and negative, of using c-TnI can improve our diagnostic capabilities when symptoms suggest acute myocardial infarction, and this improved capability can save lives through early intervention. Unfortunately, we are not readily discarding the older, less specific tests as the newer, more specific types become available. We may be creating more confusion by keeping the older tests (such as creatine phosphokinase [CPK] and CPK-MB), and combining them with myoglobin, troponin T and troponin I than if we relied on one or two with better positive and negative predictive values.