Counterpoint: Standardization of Cardiac Troponin I Assays Will Not Occur in My Lifetime

Abstract

I have chosen to address the argument of why cardiac troponin I (cTnI)3 assays will likely never be standardized. It is not that I do not wish it would happen, but I think the scientific, evidence-based story I will present regarding this cardiac specific protein will speak for itself. As a means to present this argument, I would like to consider the following analogy. I am an avid trail runner. During the second week of June, I set out every morning to run a 10-km rolling trail along the Ammonoosuc River in Bretton Woods, New Hampshire. Although the path I navigated each day was the same, what I observed each morning at 7 AM was dramatically different. It was influenced by the temperature, the wind, rain with thunder and lightning, puddles in rocky crevasses, tree branches covering the path, and, twice, minor course changes due to a fox and 2 black bears that appeared both upstream and downstream on the trail I was navigating. So, what does this analogy have to do with the cTnI story? cTnI, a protein of 110 amino acid residues with a unique N-terminal sequence that differentiates it from skeletal muscle forms, is part of the myofibril 3 subcomplex (cTnI–cTnC–cTnT) in the heart with specific structural and regulatory functions (1). Shortly after cell death caused by an acute myocardial infarction (MI), cTnI is released from the damaged myocardium into the extracellular space, and 1–6 h after onset of the index event, cTnI can be detected circulating in the blood (2). Data indicate that cTnI circulates mainly as a binary complex with cTnC in the blood of patients with acute MI, but it also circulates as free cTnI and as a ternary complex, cTnICT. In addition, C- and N-terminal proteolytic degradation, oxidized and reduced forms, and phosphorylated forms of both free and complexed cTnI have been well described (3). Immunoassays that use anti-cTnI monoclonal antibodies are dependent on the epitope regions recognized by the antibodies incorporated into each assay. Protein regions within the cTnI molecule that are susceptible to the modifications noted above—as well as the variable nature of epitope-dependent heparin (4), heterophile antibody (5), and autoantibody (6) binding—are likely to exhibit variable immunoreactivity and even loss of reactivity in some cases. Such variability in immunoreactivity can lead to altered signal generation within sandwich-type cTnI immunoassays that use capture and detection antibodies directed against these modified regions. With the acute ongoing release of cTnI during the early post-MI phase, with the ongoing cTnI modifications in both myocardial tissue and blood, and with different clearance mechanisms and rates of clearance for the multiple isoforms of cTnI that accumulate over the first 24 h after an MI, no 2 patients' blood samples are likely to contain the same cTnI isoform composition (7). In addition, practical issues, such as the preanalytical effects of hemolysis and temperature, have been known to differentially affect assays. Furthermore, as high-sensitivity assays have begun to improve our understanding of the clinical implications of measurable cTnI concentrations in healthy individuals (8), we do not yet have any evidence regarding whether the distributions of cTnI isoforms are the same from person to person, or even whether they remain the same within healthy individuals. For example, some high-sensitivity assays show differences between the sexes, and others do not. Early studies by Wu et al. first showed that commercial assays generated different relative responses for a given cTnI concentration that were based on different responses to the multiple complex forms of cTnI (9). These data showed it was not possible to compare absolute concentrations obtained with assays from different manufacturers. Standardization of cTnI assays will not be as simple as determining and optimizing a primary or secondary reference material. The goals of the IFCC Working Group on Standardization of Cardiac Troponin I (the opposing point of view in this Point/Counterpoint) appear to be 2-fold (10): first, development of a serum-based secondary reference material with cTnI value assignment via a reference measurement procedure; and second, assignment to this material, based on long-term calibration traceability, of a cTnI calibration value through the reference measurement procedure. Although I applaud the efforts of the Working Group (WG) and think these studies will lead to several published reports, the following arguments suggest that the WG will likely fall short of their goals. For true cTnI immunoassay standardization to be achieved, universal adoption of antibodies, including standardization of both capture and detection/signal antibodies, that are suitable to all manufacturers will be a required first step. That goal is unlikely to be achieved. Furthermore, even when identical antibodies have been used by the same manufacturer on different platforms, there have been disconc