Extensive Troponin I and T Modification Detected in Serum From Patients With Acute Myocardial Infarction

A recent article in Circulation 1 characterizes the stability of human recombinant cardiac troponin I spiked into normal human serum.The authors incorrectly concluded on the basis of these studies that the accuracy of the Bayer Immuno 1 troponin I method would decrease with time after the onset of an ischemic event, increasing the risk of a false-negative diagnosis in patients with minor myocardial damage and late admittance to the emergency department.More than 90% of cardiac troponin I in serum is complexed to troponin C, and little or none is found in a free, uncomplexed form.2,3 Moreover, the apparent stability of cardiac troponin I in serum is greatly enhanced by complexation with troponin C, which is due in part to proteolytic shielding.4 Thus, the stability characteristics of free recombinant cardiac troponin I do not mimic those of the troponin I complex as it exists in patient sera and, therefore, they have little clinical relevance.In a recent study, troponin I measurements by the Bayer Immuno 1 method were performed with actual patient samples incubated at room temperature and at 4°C for up to 48 hours.5 Cardiac troponin I recoveries ranged between 96% and 105%, indicating no decline in recovery.The troponin I epitopes recognized by the antibodies used in the Bayer Immuno 1 method are contained on a proteolytic-resistant region between amino acid residues 30 and 110. 4 This accounts for the troponin I stability in samples from patients with acute myocardial infarction when measured by the Bayer Immuno I method.Given that the serial patient profiles of troponin I values closely parallel those of creatine kinase-MB (which is totally independent of the in vitro study), it is highly unlikely that the reduction of troponin I concentrations is due to "some modification of troponin I . . .(that) alters its detectability by Immuno 1." 1 An additional mechanism contributing to the time-dependent quantitative loss of immunoreactive troponin I observed by the authors when spiking recombinant troponin I in vitro may be adsorption of the free troponin I to the container surfaces.Free troponin I has a strong net positive charge, and it has been established that, without appropriate precautions, significant amounts may be lost to the surface of glass tubes.3 Troponin I-troponin C complexes are less prone to this phenomena.For this reason, manufacturers recommend that glass not be used for the long-term storage of troponin I samples.