Challenges of serial troponin testing: A symphony in need for harmony

Abstract

In an earlier issue of this journal [1], we reported on early rule-out and rule-in of non-STEMI using 3 h and 6 h protocols of high sensitivity cardiac troponin T in an emergency department population. With interest, we now read a letter to the editor from Dr. Lippi [2] referring to a recent studybyCardinaels et al. [3]who found several cTnT degradation products in sera of patients admitted for STEMI and underwent primary PCI. Lippi now raised the possibility that cTnT degradation products might confound serial troponin testing that is endorsed by current ESC Guidelines on patients with non-ST segment elevationacute coronary syndromes [4]. The observationof Cardinaels et al. [3] is interesting but requires further validation of Western blot findings, as well as clinical evaluation of its clinical relevance. Lippi and Cervellin [2] suspect that antibodies of the current hsTnT assaymay fail to detect variousmolecular isoformsdue toheterogeneous immunoreactivity. For support, they refer to Cardinaels et al. [3] who demonstrated a consistentdegradationpatternusingantibodies directed against the C-terminal end of cTnTas compared to the pattern seenwith standard cTnT antibodies, but no fragments when the antibody was directed against the N-terminal end. Notably, the epitope of the cTnT assay is located centrally and thereby protected against epitope loss by protease cleavage [5,6]. In fact, this issue might be more relevant for some cTnI assays due to a higher susceptibility to posttranslational modifications, and heterogeneous epitope locations [5]. In addition to the central location of the epitope, cTnT is not prone to oxidation– reduction, phosphorylation or major modifications [5]. Nevertheless, it would be scientifically interesting to investigate the effect of degradation on measurable cTnT and cTnI concentrations over time. In clinical practice, the strength of hsTnT is earlier andmore accurate detection of NSTEMI. For STEMI, monitoring of reperfusion success is nowadays less important than in the era of fibrinolytic therapy, and estimation of infarct size using values beyond the first 24 h is not within the label. Therefore, the hsTnT assay was optimized at the low concentration range and truncated at the upper range as compared to the 4th generation cTnT assay [7]. Due to the excellent negative predictive value of hsTn assays in large clinical trials [8,9], current ESC guidelines on NSTE-ACS [4] endorse an early rule-out protocol with cTnTmeasurements on admission and after 3 h (optionally after 6 h). The finding that intact cTnT disappears 4 h after admission to the ED, and cTnT fragments start to become themajor fraction of cTnT does however not support the notion that cTnT fragments might disturb the diagnostic performance of hsTnT for early rule-out of NSTEMI. In addition, the mechanisms behind the formation of cTn degradation products remain unclear. Further work is needed to investigate the occurrenceof degradationproducts after small myocyte necrosis as compared to large STEMI with the subsequent release of large quantities of proteases, or the role of ischemia/ reperfusion injury after primary PCI. Although, the concerns of Dr. Lippi cannot be ruled out fully, his hypothesis is not supported by clinical experience where clinicians are rather confronted with issues of overdiagnosis of NSTEMI due to reduced clinical specificity rather than underdiagnosis. At least, at the moment, no data support an underestimation of MI due to epitope loss or modification, and the study by Cardinaels et al. [3] studied cTnT degradation products in a patient population that is not in the scope of troponin testing. In addition, as this is thefirst report,findings require confirmationusingmass spectroscopy followed by peptide sequencing to validate the identity of peptides. Furthermore, testing for fragments should be executed in heparin plasma, as well. Another issue raised by Dr. Lippi [2] is the analytical imprecision of hsTnT values below the LoQ which depends on the analytical platform used. Accordingly, Lippi [2] questioned the interpretation of small absolute concentration values for the diagnosis of NSTEMI. In our study [1], a diagnosis of NSTEMI was not only based on the presence of respective hsTnTchanges but required that the first or second hsTnT value exceeded the 99th percentile value of 14 ng/L. So far, the symphony is ready to be played but musicians still need to practice for harmony.