Time-dependent degradation pattern of cardiac troponin T following myocardial infarction

Highly Sensitive Immunoassay for Long Forms of Cardiac Troponin T Using Upconversion Luminescence

BACKGROUND

Long cardiac troponin T (cTnT) has been proposed to be a promising and more specific biomarker of acute myocardial infarction (AMI). As it represents a subfraction of circulating cTnT, detection of very low concentrations is a requirement. The aim of this study was to develop a novel, highly sensitive immunoassay for long cTnT.

METHODS

A two-step sandwich-type immunoassay for long cTnT was developed, utilizing upconverting nanoparticles (UCNPs) as reporters. The limits of detection and quantitation were determined for the assay. Linearity and matrix effects were evaluated. Performance with clinical samples was assessed with samples from patients with non-ST elevation myocardial infarction (NSTEMI, n = 30) and end-stage renal disease (ESRD, n = 37) and compared to a previously developed time-resolved fluorescence (TRF)-based long cTnT assay and a commercial high-sensitivity cTnT assay.

RESULTS

The novel assay reached a 28-fold lower limit of detection (0.40 ng/L) and 14-fold lower limit of quantitation (1.79 ng/L) than the previously developed TRF long cTnT assay. Li-heparin and EDTA plasma, but not serum, were found to be suitable sample matrixes for the assay. In a receiver operating characteristics curve analysis, the troponin ratio (long/total cTnT) determined with the novel assay showed excellent discrimination between NSTEMI and ESRD with an area under the curve of 0.986 (95% CI, 0.967-1.000).

CONCLUSIONS

By utilizing upconversion luminescence technology, we developed a highly sensitive long cTnT assay. This novel assay can be a valuable tool for investigating the full potential of long cTnT as a biomarker for AMI. ClinicalTrials.gov Registration Number: NCT04465591.

Differences in Cardiac Troponin T Composition in Myocardial Infarction and End-Stage Renal Disease Patients: A Blood Tube Effect?

BACKGROUND

Cardiac troponin T (cTnT) is key in diagnosing myocardial infarction (MI) but is also elevated in end-stage renal disease (ESRD) patients. Specific larger cTnT proteoforms were identified for the acute phase of MI, while in serum of ESRD patients solely small cTnT fragments were found. However, others allocated this to a pre-analytic effect due to abundant thrombin generation in serum. Therefore, we investigated the effect of various anticoagulation methods on cTnT composition and concentration and compared the cTnT composition of MI and ESRD patients.

METHODS

The agreement of cTnT concentrations between simultaneously collected serum, lithium-heparin (LH) plasma, and ethylenediaminetetraacetic acid (EDTA) plasma was studied using the high-sensitivity (hs-)cTnT immunoassay. cTnT proteoform composition was investigated in a standardized time-dependent manner through spike experiments and in simultaneously collected blood matrixes of MI and ESRD patients.

RESULTS

Excellent hs-cTnT concentration agreements were observed across all blood matrixes (slopes > 0.98; 95% CI, 0.96-1.04). Time-dependent degradation (40 kDa intact:29 kDa fragment:15 to 18 kDa fragments) was found in LH plasma and EDTA plasma, and serum in ratios (%) of 90:10:0, 0:5:95, and 0:0:100, respectively (48 h after blood collection). Moreover, gel filtration chromatography (GFC) profiles illustrated mainly larger cTnT proteoforms in MI patients, while in ESRD patients mainly 15 to 18 kDa fragments were found for all matrices.

CONCLUSIONS

The extent of cTnT degradation in vitro is dependent on the (anti)coagulation method, without impacting hs-cTnT concentrations. Furthermore, mainly larger cTnT proteoforms were present in MI patients, while in ESRD patients mainly small 15 to 18 kDa cTnT fragments were found. These insights are essential when developing a novel hs-cTnT assay targeting larger cTnT proteoforms.

Pathophysiological mechanisms underlying increased circulating cardiac troponin in noncardiac surgery: a narrative review

Assay-specific increases in circulating cardiac troponin are observed in 20-40% of patients after noncardiac surgery, depending on patient age, type of surgery, and comorbidities. Increased cardiac troponin is consistently associated with excess morbidity and mortality after noncardiac surgery. Despite these findings, the underlying mechanisms are unclear. The majority of interventional trials have been designed on the premise that ischaemic cardiac disease drives elevated perioperative cardiac troponin concentrations. We consider data showing that elevated circulating cardiac troponin after surgery could be a nonspecific marker of cardiomyocyte stress. Elevated concentrations of circulating cardiac troponin could reflect coordinated pathological processes underpinning organ injury that are not necessarily caused by ischaemia. Laboratory studies suggest that matching of coronary artery autoregulation and myocardial perfusion-contraction coupling limit the impact of systemic haemodynamic changes in the myocardium, and that type 2 ischaemia might not be the likeliest explanation for cardiac troponin elevation in noncardiac surgery. The perioperative period triggers multiple pathological mechanisms that might cause cardiac troponin to cross the sarcolemma. A two-hit model involving two or more triggers including systemic inflammation, haemodynamic strain, adrenergic stress, and autonomic dysfunction might exacerbate or initiate acute myocardial injury directly in the absence of cell death. Consideration of these diverse mechanisms is pivotal for the design and interpretation of interventional perioperative trials.

New reference limits for cardiac troponin T and N-terminal b-type natriuretic propeptide in elders

BACKGROUND AND AIMS

Our aim was to define reference limits for cardiac troponin T (cTnT) and N-terminal pro B-type natriuretic peptide (proBNP) that would better reflect their concentrations in older people. In addition, the incidence of acute myocardial infarctions (AMIs) was studied using these reference limits in an older population with and without previous heart diseases.

MATERIALS AND METHODS

A population-based study with a ten-year follow-up. The reference population was formed by 763 individuals aged over 64 years, with no diagnoses of heart or kidney diseases.

RESULTS

There was a significant increase in cTnT and proBNP concentrations with age. The 99 % reference limits for cTnT were 25 ng/L, 28 ng/l, 38 ng/l, and 71 ng/l for men in five-year-intervals starting from 64 to 69 years to 80 years and older, and 18 ng/L, 22 ng/l, 26 ng/l, and 52 ng/L for women, respectively. The 97.5 % reference limits for proBNP were 272 ng/L, 287 ng/l, 373 ng/l and 686 ng/L for men, and 341 ng/L, 377 ng/l, 471 ng/l, and 794 ng/L for women, respectively. Elevated proBNP was statistically significantly associated with future AMIs in subjects with and without a previous heart disease.

CONCLUSIONS

Age-specific reference limits for cTnT and proBNP are needed to better evaluate cardiac symptoms.

The short-term effect of hemodialysis on the level of high-sensitive cardiac troponin T - A systematic review

INTRODUCTION

Patients with end-stage renal disease (ESRD) have an increased risk of cardiovascular disease, but interpreting cardiac troponin is difficult in this population. The effect of renal replacement therapy (RRT) is important to consider when interpreting serial cardiac troponin T (cTnT) results for patients with ESRD suspected of acute coronary syndrome (ACS). The aim of this systematic review is to answer how low-flux hemodialysis (LF-HD), high-flux hemodialysis (HF-HD), and hemodiafiltration (HDF) affect the blood concentration of high-sensitive cardiac troponin T (hs-cTnT).

METHOD

Several databases were searched and identified records were evaluated independently by two of the authors. Pre- and postdialysis hs-cTnT concentrations together with other relevant data were extracted from the included studies. The quality (potential bias and applicability issues) were assessed for each of the included studies.

RESULTS

The literature search identified 2,540 records and 15 studies were included. The relative pre- to postdialysis change of hs-cTnT varied from -41 to 29%. LF-HD increased the hs-cTnT concentration with relative changes between 2 and 17%. HDF decreased the concentration with relative changes from -41% to -9%. Both increases and decreases were seen for HF-HD (-16% to 12%).

DISCUSSION/CONCLUSION

In this systematic review, we found LF-HD to increase the hs-cTnT concentration and HDF to decrease the concentration. Results for HF-HD and unspecified HD are more heterogeneous. Because of the differences between the included studies, a meta-analysis was not meaningful. This systematic review can help with the assessment of patients with ESRD suspected of ACS in relation to hemodialysis/HDF treatment.

Fragmentation of human cardiac troponin T after acute myocardial infarction

BACKGROUND

Blood measurement of cardiac troponin T (cTnT) is one of the most widespread methods of acute myocardial infarction (MI) diagnosis. cTnT degradation may have a significant influence on the precision of cTnT immunodetection; however, there are no consistent data describing the level and sites of cTnT proteolysis in the blood of MI patients. In this study, we bordered major cTnT fragments and quantified their relative abundance in the blood at different times after MI.

METHODS

Serial heparin plasma samples were collected from 37 MI patients 2-37 h following the onset of MI. cTnT and its fragments were studied by western blotting and immunofluorescence analysis using monoclonal antibodies specific to various cTnT epitopes.

RESULTS

cTnT was present in the blood of MI patients as 23 proteolytic fragments with an apparent molecular mass of ∼ 8-37 kDa. Two major sites of cTnT degradation were identified: between amino acid residues (aar) 68 and 69 and between aar 189 and 223. Analysis of the abundance of cTnT fragments showed an increase in the fraction of free central fragments in the first few hours after MI, while the fraction of the C-terminal fragments of cTnT remained almost unchanged.

CONCLUSION

cTnT progressively degrades after MI and appears in the blood as a mixture of 23 proteolytic fragments. The cTnT region approximately bordered by aar 69-158 is a promising target for antibodies used for measurement of total cTnT.

Cardiac troponins: Mechanisms of release and role in healthy and diseased subjects

The cardiac troponins (cTns), cardiac troponin C (cTnC), cTnT, and cTnI are key elements of myocardial apparatus, fixed as protein complex on the thin filament of sarcomere and are involved in the regulation of excitation-contraction coupling of cardiomyocytes in the presence of Ca²⁺ . Circulating cTnT and cTnI (cTns) increase following cardiac tissue necrosis, and they are consolidated biomarkers of acute myocardial infarction (AMI). However, the use of high sensitivity (hs)-immunoassay tests for cTnT and cTnI has made it possible to identify a multitude of other clinical conditions associated with increased circulating levels of cTns. cTns can be measured also in the peripheral circulation of healthy subjects or athletes, suggesting that different mechanisms are involved in the release of cTns in the blood independently of cardiac cell necrosis. In this review, the molecular/cellular mechanisms involved in cTns release in blood and the exploitation of cTnI and cTnT as biomarkers of cardiac adverse events, in addition to cardiac necrosis, are discussed.

Cardiac Troponins as Biomarkers of Cardiac Myocytes Damage in Case of Arterial Hypertension: From Pathological Mechanisms to Predictive Significance

Background. Many pathological conditions of both cardiovascular and non-cardiac origin in the course of their development cause damage to contractile cardiac muscle cells—cardiac myocytes (CMCs). One of the most sensitive and specific criteria for detecting CMCs are cardiac troponins (CTs), which are regulatory protein molecules that are released into the blood serum from CMCs upon their death or damage. New (high-sensitive) methods for detecting CTs allow the detection of minor CMCs damages at the earliest stages of cardiovascular diseases and can therefore change the understanding of disease development mechanisms and open up new diagnostic possibilities. One of the most common and dangerous early diseases of the cardiovascular system is arterial hypertension. The purpose of this paper is to summarize the pathophysiological mechanisms underlying CMCs damage and CTs release into the bloodstream in the case of arterial hypertension and to state the clinical significance of increased CTs levels in patients with arterial hypertension. Materials and methods. This is a descriptive review, which was prepared using the following databases: Embase, Pubmed/Medline and Web of Science. The following key words were used in the literature search: “myocardial injury” and “arterial hypertension” in combination with the terms “cardiac troponins” and “mechanisms of increase”. Conclusions. According to a literature analysis, CMCs damage and CTs release in the case of arterial hypertension occur according to the following pathophysiological mechanisms: myocardial hypertrophy, CMCs apoptosis, damage to the CMC cell membrane and increase in its permeability for CTs molecules, as well as changes in the glomerular filtration rate. Most often, increased CTs serum levels in case of arterial hypertension indicate an unfavorable prognosis. Data on the CTs predictive significance in case of arterial hypertension open the prospects for the use of these biomarkers in the choice of patient management plans.

High-sensitivity cardiac troponins in pediatric population

Apparently healthy children often complain of chest pain, especially after physical exercise. Cardiac biomarker levels are often measured, but the clinical relevance of these assays in children is still debated, even when a cardiac disease is present. Coronary artery disease is exceedingly rare in children, but elevated circulating levels of cardiac troponin I (cTnI) and T (cTnT) in an acute setting may help detect heart failure due to an unknown cardiac disorder, or worsening heart failure, particularly in combination with other biomarkers such as B-type natriuretic peptides. However, the interpretation of biomarkers is often challenging, especially when institutions transition from conventional cTn assays to high-sensitivity (hs-cTn) methods, as well demonstrated in the emergency setting for adult patients. From a clinical perspective, the lack of established reference values in the pediatric age is the main problem limiting the use of hs-cTn methods for the diagnosis and managements of cardiac diseases in infants, children and adolescents. This review aims to discuss the possibility to use hs-cTnI and hs-cTnT to detect cardiac disease and to explore age-related differences in biomarker levels in the pediatric age. We start from some analytical and pathophysiological considerations related to hs-cTn assays. Then, after a systematic literature search, we discuss the current evidence and possible limitations of hs-cTn assay as indicators of cardiac disease in the most frequently cardiac disease in pediatric setting.

Use of high-sensitivity cardiac troponins in the emergency department for the early rule-in and rule-out of acute myocardial infarction without persistent ST-segment elevation (NSTEMI) in Italy

Serial measurements of cardiac troponin are recommended by international guidelines to diagnose myocardial infarction (MI) since 2000. However, some relevant differences exist between the three different international guidelines published between 2020 and 2021 for the management of patients with chest pain and no ST-segment elevation. In particular, there is no agreement on the cut-offs or absolute change values to diagnose non-ST-segment elevation MI (NSTEMI). Other controversial issues concern the diagnostic accuracy and cost-effectiveness of cut-off values for the most rapid algorithms (0 h/1 h or 0 h/2 h) to rule-in and rule-out NSTEMI. Finally, another important point is the possible differences between demographic and clinical characteristics of patients enrolled in multicenter trials compared to those routinely admitted to the Emergency Department in Italy. The Study Group of Cardiac Biomarkers, supported by the Italian Scientific Societies Società Italiana di Biochimica Clinica, Italian Society of the European Ligand Assay Society, and Società Italiana di Patolgia Clinica e Medicina di Laboratorio decided to revise the document previously published in 2013 about the management of patients with suspected NSTEMI, and to provide some suggestions for the use of these biomarkers in clinical practice, with a particular focus on the Italian setting.

Exercise-Induced Cardiac Troponin Elevations: From Underlying Mechanisms to Clinical Relevance

Supplemental Digital Content is available in the text.

Serological assessment of cardiac troponins (cTn) is the gold standard to assess myocardial injury in clinical practice. A greater magnitude of acutely or chronically elevated cTn concentrations is associated with lower event-free survival in patients and the general population. Exercise training is known to improve cardiovascular function and promote longevity, but exercise can produce an acute rise in cTn concentrations, which may exceed the upper reference limit in a substantial number of individuals. Whether exercise-induced cTn elevations are attributable to a physiological or pathological response and if they are clinically relevant has been debated for decades. Thus far, exercise-induced cTn elevations have been viewed as the only benign form of cTn elevations. However, recent studies report intriguing findings that shed new light on the underlying mechanisms and clinical relevance of exercise-induced cTn elevations. We will review the biochemical characteristics of cTn assays, key factors determining the magnitude of postexercise cTn concentrations, the release kinetics, underlying mechanisms causing and contributing to exercise-induced cTn release, and the clinical relevance of exercise-induced cTn elevations. We will also explain the association with cardiac function, correlates with (subclinical) cardiovascular diseases and exercise-induced cTn elevations predictive value for future cardiovascular events. Last, we will provide recommendations for interpretation of these findings and provide direction for future research in this field.

Cardiac Troponin Composition Characterization after Non ST-Elevation Myocardial Infarction: Relation with Culprit Artery, Ischemic Time Window, and Severity of Injury

BACKGROUND

Troponin composition characterization has been implicated as a next step to differentiate among non-ST elevation myocardial infarction (NSTEMI) patients and improve distinction from other conditions with troponin release. We therefore studied coronary and peripheral troponin compositions in relation to clinical variables of NSTEMI patients.

METHODS

Samples were obtained from the great cardiac vein (GCV), coronary sinus (CS), and peripheral circulation of 45 patients with NSTEMI. We measured total cTnI concentrations, and assessed both complex cTnI (binary cTnIC + all ternary cTnTIC forms), and large-size cTnTIC (full-size and partially truncated cTnTIC). Troponin compositions were studied in relation to culprit vessel localization (left anterior descending artery [LAD] or non-LAD), ischemic time window, and peak CK-MB value.

RESULTS

Sampling occurred at a median of 25 hours after symptom onset. Of total peripheral cTnI, a median of 87[78-100]% consisted of complex cTnI; and 9[6-15]% was large-size cTnTIC. All concentrations (total, complex cTnI, and large-size cTnTIC) were significantly higher in the CS than in peripheral samples (P < 0.001). For LAD culprit patients, GCV concentrations were all significantly higher; in non-LAD culprit patients, CS concentrations were higher. Proportionally, more large-size cTnTIC was present in the earliest sampled patients and in those with the highest CK-MB peaks.

CONCLUSIONS

In coronary veins draining the infarct area, concentrations of both full-size and degraded troponin were higher than in the peripheral circulation. This finding, and the observed associations of troponin composition with the ischemic time window and the extent of sustained injury may contribute to future characterization of different disease states among NSTEMI patients.

Diagnostic algorithms for non-ST-segment elevation myocardial infarction: open issues

The use of serial measurement of cardiac troponin (cTn) is recommended by international guidelines for the diagnosis of myocardial infarction (MI) since 2000. This article focuses on factors influencing temporal changes in high-sensitive cTn (hs)-cTn and the impact of these factors on the diagnosis of non-ST-segment elevation MI (NSTEMI). The recommendations proposed by three different international guidelines published in 2020-2021 for the management of acute coronary syndromes (ACS) in patients presenting without persistent ST-segment elevation (NSTE) show some discrepancies. In particular, there is no agreement among these guidelines about cut-off or absolute change values to be used for the rule-in, especially regarding the use of sex-specific cut-off values. Furthermore, there are no sufficient evidences on the diagnostic accuracy and cost effectiveness related to cut-off values suggested for algorithms to be used by some hs-cTnI methods.

Cardiac Troponin T: The Impact of Posttranslational Modifications on Analytical Immunoreactivity in Blood up to the Excretion in Urine

Cardiac troponin T (cTnT) is a sensitive and specific biomarker for detecting cardiac muscle injury. Its concentration in blood can be significantly elevated outside the normal reference range under several pathophysiological conditions. The classical analytical method in routine clinical analysis to detect cTnT in serum or plasma is a single commercial immunoassay, which is designed to quantify the intact cTnT molecule. The targeted epitopes are located in the central region of the cTnT molecule. However, in blood cTnT exists in different biomolecular complexes and proteoforms: bound (to cardiac troponin subunits or to immunoglobulins) or unbound (as intact protein or as proteolytic proteoforms). While proteolysis is a principal posttranslational modification (PTM), other confirmed PTMs of the proteoforms include N-terminal initiator methionine removal, N-acetylation, O-phosphorylation, O-(N-acetyl)-glucosaminylation, N(ɛ)-(carboxymethyl)lysine modification and citrullination. The immunoassay probably detects several of those cTnT biomolecular complexes and proteoforms, as long as they have the centrally targeted epitopes in common. While analytical cTnT immunoreactivity has been studied predominantly in blood, it can also be detected in urine, although it is unclear in which proteoform cTnT immunoreactivity is present in urine. This review presents an overview of the current knowledge on the pathophysiological lifecycle of cTnT. It provides insight into the impact of PTMs, not only on the analytical immunoreactivity, but also on the excretion of cTnT in urine as one of the waste routes in that lifecycle. Accordingly, and after isolating the proteoforms from urine of patients suffering from proteinuria and acute myocardial infarction, the structures of some possible cTnT proteoforms are reconstructed using mass spectrometry and presented.

Association between short-term exposure to ambient fine particulate matter and myocardial injury in the CATHGEN cohort

Exposure to fine particulate matter (PM_2.5) has been associated with a higher risk for coronary events. Elevated circulating cardiac troponins (cTn) are suggestive of myocardial injury in both ischemic and non-ischemic conditions. However, little is known about the association between PM_2.5 and cTn. In this study, we investigated short-term PM_2.5 effects on cardiac troponin T (cTnT), as well as N-terminal-pro brain natriuretic peptide (NT-pro BNP) and inflammatory biomarkers among cardiac catheterized participants. We analyzed 7444 plasma cTnT measurements in 2732 participants who presented to Duke University Hospital with myocardial infarction symptoms between 2001 and 2012, partly along with measurements of NT-pro BNP and inflammatory biomarkers. Daily PM_2.5 concentrations were predicted by a neural network-based hybrid model and were assigned to participants' residential addresses. We applied generalized estimating equations to assess associations of PM_2.5 with biomarker levels and the risk of a positive cTnT test (cTnT > 0.1 ng/mL). The median plasma cTnT concentration at presentation was 0.05 ng/mL and the prevalence of a positive cTnT test was 35.4%. For an interquartile range (7.6 μg/m³) increase in PM_2.5 on the previous day, cTnT concentrations increased by 7.7% (95% CI: 3.4-12.3) and the odds ratio of a positive cTnT test was 1.08 (1.01-1.16). Participants under 60 years (effect estimate: 15.2%; 95% CI: 7.4-23.5) or living in rural areas (12.3%; 95% CI: 4.8-20.3) were more susceptible. There was evidence for increases in fibrinogen and NT-pro BNP associated with elevated PM_2.5 on the concurrent and previous two days. Our study suggests that acute PM_2.5 exposure may elevate indicators of myocardial tissue damage. This finding substantiates the association of air pollution exposure with adverse cardiovascular events.

Evaluation and Comparison with Other High-Sensitivity Methods of Analytical Performance and Measured Values of a New Laboratory Test for Cardiac Troponin I Assay

Bakground

The aim of this study was to evaluate both analytical characteristics and clinical results of a new chemiluminescent method for the measurement of cardiac troponin I (cTnI), named VITROS ® High Sensitivity Troponin I Assay, using the VITROS® 3600 automated platform. The results found with this new method were compared to those observed with hs-cTnI ARCHITECT and ECLIA hs-cTnT ELECSYS methods.

Methods

For evaluation of analytical performance and comparison of clinical results, plasma samples (lithium-heparin), were collected from apparently healthy subjects and patients with cardiovascular diseases.

Results

The hs-cTnI VITROS method showed values for limit of blank (LoB 0.33 ng/L), limit of detection (LoD, 0.91 ng/L), limit of quantifications at 20% (LoQ 20% CV, 1.82 ng/L), and 10% (LoQ 10% CV, 4,74 ng/L), which are comparable to those previously reported for other hs-cTnI methods. Moreover, the clinical results of the hs-cTnI VITROS method were found to be closely correlated to those of hs-cTnI ARCHITECT (R = 0,9883, N = 198) and ECLIA hs-cTnT Elecsys (R = 0,9704, N = 293) methods.

Conclusions

The hs-cTnI VITROS method shows analytical performance comparable to other cTnI and cTnT assay. The results of this study confirm that there are significant systematic differences among hs-cTnI methods. Further multicenter studies using larger reference populations are needed in order to obtain a better estimation, especially of the 99° percentile URL values categorized for sex and age of hs-cTnI and hs-cTnT methods.

Cardiac troponins: are there any differences between T and I?

The most recent international guidelines recommend the measurement of cardiac troponin I (cTnI) and cardiac troponin T (cTnT) using high-sensitivity methods (hs-cTn) for the detection of myocardial injury and the differential diagnosis of acute coronary syndromes. Myocardial injury is a prerequisite for the diagnosis of acute myocardial infarction, but also a distinct entity. The 2018 Fourth Universal Definition of Myocardial Infarction states that myocardial injury is detected when at least one value above the 99th percentile upper reference limit is measured in a patient with high-sensitivity methods for cTnI or cTnT. Not infrequently, increased hs-cTnT levels are reported in patients with congenital or chronic neuromuscular diseases, while the hs-cTnI values are often in the normal range. Furthermore, some discrepancies between the results of laboratory tests for the two troponins are occasionally found in individuals apparently free of cardiac diseases, and also in patients with cardiac diseases. In this review article, authors discuss the biochemical, pathophysiological and analytical mechanisms which may cause discrepancies between hs-cTnI and hs-cTnT test results.

Development of a Novel Sensor System Based on Magnetic Microspheres to Detect Cardiac Troponin T

Acute myocardial infarction (AMI) causes irreversible injury to cardiomyocytes in a short time and may result in various complications, severely threatening patient safety. Therefore, it is necessary to predict the possibility of AMI in the prophase. Prognostic detection of biomarkers that specifically reflect myocardial damage in a patient’s blood has become an essential mediating measure to prevent the serious occurrence of AMI. The present study is aimed at exploring a novel sensing system with high specificity and precision based on magnetic microspheres developed to detect cardiac troponin T (cTnT), which is the most specific diagnostic marker for AMI in cardiovascular diseases. Naive human cTnT protein in serum samples and antigens on functional magnetic microspheres will competitively bind with limited specific antibodies. After rapid removal of heterogeneous elements in the sera using a magnetic separator, fluorescein isothiocyanate-labeled immunoglobulin G is added to react with specific antibodies on the magnetic microspheres. Then, a flow cytometer is used to collect signals of different fluorescence intensities. The results show that the method is characterized by economy, high accuracy, and novelty. It can be used for the detection of cTnT in blood at 1.7–106.1 ng/mL, with a detection limit of 0.5 ng/mL. Thus, the proposed sensor improves the accuracy and efficiency of diagnosis before clinical deterioration of AMI.

Discrepancy between Cardiac Troponin Assays Due to Endogenous Antibodies

BACKGROUND

Despite well-described analytical effects of autoantibodies against cardiac troponin (cTn) I on experimental assays, no study has systematically examined their impact on cTn assays in clinical use. We determined the effects of endogenous antibodies on 5 different cTnI assays and a cTnT assay.

METHODS

cTn was measured by 6 methods: Siemens hs-cTnI Centaur, Siemens hs-cTnI Vista, Abbott hs-cTnI Architect, Beckman hs-cTnI Access, Beckman cTnI Access, and Roche hs-cTnT Elecsys. Measurements were repeated on 5 assays (all except Siemens hs-cTnI Vista) following immunoglobulin depletion by incubation with protein A. Low recovery of cTnI (<40%) following immunoglobulin depletion was considered positive for macro-cTnI. Protein A findings were validated by gel filtration chromatography and polyethylene glycol precipitation.

RESULTS

In a sample of 223 specimens selected from a community laboratory that uses the Siemens hs-cTnI Centaur assay and from which cTn was requested, 76% of samples demonstrated increased cTnI (median, 88 ng/L; interquartile range, 62-204 ng/L). Macro-cTnI was observed in 123 (55%) of the 223 specimens. Comparisons of cTnI assays markedly improved once patients with macro-cTnI were removed. Passing-Bablok regression analysis between hs-cTnI assays demonstrated different slopes for patients with and without macro-cTnI. In patients with macro-cTnI, 89 (72%) showed no effect on the recovery of cTnT, whereas 34 (28%) had reduced recovery of cTnT. The proportion of results above the manufacturers' 99th percentile varied with the cTn assay and macro-cTnI status.

CONCLUSION

We suggest that the observed discrepancy between hs-cTnI assays may be attributed in part to the presence of macro-cTnI.

High-sensitivity methods for cardiac troponins: The mission is not over yet

The measurement of cardiac troponin (cTn) is recommended by all guidelines as the gold standard for the differential diagnosis of Acute Coronary Syndromes. The aim of this review is to discuss in details some key issues regarding both analytical and clinical characteristics of the high-sensitivity methods for cTn (hs-cTn), which are still considered controversial or unresolved. In particular, the major clinical concern regarding hs-cTn methods is the difficulty to differentiate the pathophysiological mechanism responsible for biomarker release from cardiomyocytes after reversible or irreversible injury, respectively. Indeed, recent experimental and clinical studies have demonstrated that different circulating forms of cTnI and cTnT can be respectively measured in plasma samples of patients with reversible or irreversible myocardial injury. Accordingly, a new generation of hs-Tn methods should be set up, based on immunometric immunoassays or chromatographic techniques, specific for circulating peptide forms more characteristics for reversible or irreversible myocardial injury. It is conceivable that this new generation of hs-cTn methods will complete the mission regarding the laboratory tests for specific cardiac biomarkers, started more than 20 years ago, which has already revolutionized the diagnosis, prognosis and management of patients with cardiac diseases.

Urinary troponin concentration as a marker of cardiac damage in pregnancies complicated with preeclampsia

Pregnant women with preeclampsia experience significant hemodynamic changes which lead to an increased myocardial workload. In response to increased demands in pregnancy, the heart muscle responds with ventricular remodeling process which involves cardiac muscle hypertrophy. Opposed to occurrence of eccentric ventricular hypertrophy in normal pregnancy, myocardial remodeling in a form of concentric hypertrophy will occur in pregnant patients with preeclampsia. Increased myocardial workload is manifested by an increased troponin release. As process of troponin degradation continue, filtration of degradation fragment through glomerular membrane occur, raising the possibility of it's detection in urine. Degradation fragments of troponin molecules are estimated to be 20 kDa with preserved immunoreactivity to high-sensitivity assays. Some of the authors suggest that serum levels of cardiac troponin I might be elevated in patients with hypertension, as well as in preeclamptic pregnant women. It is to be expected that evaluation of severity of the myocardial damage in pregnant woman with preeclampsia may be performed by measuring levels of troponin in the urine using high-sensitivity assays. Designing of urine dipstick will help to detect an early phase of myocardial involvement in preeclamptic pregnancies.

Change in troponin concentrations in patients with macrotroponin: An in vitro mixing study

INTRODUCTION

Macrotroponin is a complex formed between endogenous cardiac troponin autoantibodies (cTnAABs) and circulating cardiac troponin (cTn). The potential effect of macrotroponin on current high sensitivity cTn assays has not been fully explored but has recently been identified as a major cause of discrepancy in cTn results between assays. In this study we investigated the effects of mixing troponin (cTn) standards to specimens with and without macrotroponin.

METHOD

Macrotroponin was identified in specimens by a recovery of cTnI < 40% following protein A immunoglobulin depletion. Troponin standards containing cTn-IC and cTn-TIC complexes were mixed with serum samples, with (n = 20) and without (n = 10) the presence of macrotroponin. Specimens were tested for cTn before and after mixing by three commercially available high sensitivity cTn assays. Gel filtration chromatography was carried out on five specimens with macrotroponin and each fraction was analzyed by multiple cTn assays.

FINDINGS

Following mixing with cTn-TIC standard, all specimens with macrotroponin had a markedly reduced absolute increase in cTnI, indicating negative analytical interference due to macrotroponin. Following mixing with the cTn-IC standard, specimens with macrotroponin demonstrated highly variable changes in cTnI, suggesting significant heterogeneity in macrotroponin complex reactivity between individuals. When the ratio of change, calculated by dividing the absolute change between two cTn assays, was compared between specimens with and without macrotroponin, significant differences were observed (p < 0.001). These findings were supported by variable migration of peak cTn activity on gel filtration chromatography.

CONCLUSION

Macrotroponin leads to assay dependent analytical interference affecting current high sensitivity troponin I assays. Furthermore, endogenously occurring cTnAABs are conformationally specific and the analytical effects vary between assays and individuals.

Early evaluation of myocardial injury by means of high-sensitivity methods for cardiac troponins after strenuous and prolonged exercise

All the latest international guidelines recommend that cardiac troponin (cTn) I and T should be considered the preferred biomarkers for diagnosis of acute myocardial infarction. However, only in the last 5 years, a progressive improvement in analytical performance of immunometric assays has allowed the measurement of circulating levels of cTnI and cTnT in the large part of apparently healthy adult subjects. The routine use of these high-sensitivity methods for cardiac troponin (hs-cTn) assay has in a short time demonstrated that cardiac troponin concentrations frequently increase after strenuous prolonged exercise in healthy athletes. This acute response of hs-cTn assay following exercise was at first considered to be physiological and without long-term adverse consequences. More recent studies have suggested that exercise-induced increases in hs-cTn values may not be always a physiological response to exercise, but, conversely, it should sometimes be considered as an early cardiovascular risk marker. The aim of this review is to provide an overview of acute and chronic effects of strenuous physical exercise on hs-cTn circulating levels and also to discuss the potential pathophysiological and clinical implications of biomarker responses.

A Possible Mechanism behind Faster Clearance and Higher Peak Concentrations of Cardiac Troponin I Compared with Troponin T in Acute Myocardial Infarction

BACKGROUND

Although cardiac troponin I (cTnI) and troponin T (cTnT) form a complex in the human myocardium and bind to thin filaments in the sarcomere, cTnI often reaches higher concentrations and returns to normal concentrations faster than cTnT in patients with acute myocardial infarction (MI).

METHODS

We compared the overall clearance of cTnT and cTnI in rats and in patients with heart failure and examined the release of cTnT and cTnI from damaged human cardiac tissue in vitro.

RESULTS

Ground rat heart tissue was injected into the quadriceps muscle in rats to simulate myocardial damage with a defined onset. cTnT and cTnI peaked at the same time after injection. cTnI returned to baseline concentrations after 54 h, compared with 168 h for cTnT. There was no difference in the rate of clearance of solubilized cTnT or cTnI after intravenous or intramuscular injection. Renal clearance of cTnT and cTnI was similar in 7 heart failure patients. cTnI was degraded and released faster and reached higher concentrations than cTnT when human cardiac tissue was incubated in 37°C plasma.

CONCLUSION

Once cTnI and cTnT are released to the circulation, there seems to be no difference in clearance. However, cTnI is degraded and released faster than cTnT from necrotic cardiac tissue. Faster degradation and release may be the main reason why cTnI reaches higher peak concentrations and returns to normal concentrations faster in patients with MI.

Analytical performance of cardiac troponin assays - Current status and future needs

Concurrent with the introduction of cardiac troponin measurements into the diagnostic definition of myocardial infarction (MI), clinicians and laboratory professionals signaled a clear clinical need for improved analytical quality. This was an important precipitant for developing high-sensitivity cardiac troponin (hs-cTn) assays, currently used in rapid algorithms guiding investigations of patients presenting to the emergency department with possible MI. The hs-cTn assays were also important for the detection and monitoring of low-grade chronic myocardial injury, a condition that has been linked to increased long-term risk of cardiovascular morbidity and mortality. This review summarizes the general recommendations for defining analytical performance specifications while providing relevant clinical situations related to analytical performance. Importantly, outcome studies suggest analytical quality performance for hs-cTn is sufficient for early discharge of patients investigated for possible MI. However, bias due to change in calibrators or reagents may significantly affect the percentage of patients discharged. Biological variation data is suitable for defining performance specifications when hs-cTn measurements are used for diagnosing and monitoring chronic myocardial injury. Further improvement in analytical performance for hs-cTn testing may result in even faster decision making in the emergency setting; while also identifying those with chronic injury at risk for an adverse cardiac event.

Skeletal myopathies as a non-cardiac cause of elevations of cardiac troponin concentrations

Skeletal myopathies have been suggested as a non-cardiac cause of elevations of cardiac troponin (cTn), particularly cardiac troponin T (cTnT). This is of major clinical relevance and concern as cTn plays a major role in the early diagnosis of myocardial infarction (MI). While both the incidence as well as the true pathophysiology (cardiac versus non-cardiac) underlying elevations in cTn in skeletal myopathies remain largely unknown, re-expression of cTnT in regenerating adult skeletal muscle has been suggested as a possible contributor. However, unequivocal protein characterization in skeletal muscle and quantification of the relative amounts of this possible signal versus the cTn signal derived from true cardiomyocyte injury remains elusive. Alternatively, minor cross-reactivity of the cTnT (and possibly at times also cTnI) detection and capture antibodies used in current monoclonal immunoassays with the skeletal troponin T or I isoform may be considered. Both would represent "false positive" elevations from a clinical perspective and would need to be reliably differentiated from "true positive elevations" from subclinical cardiomyocyte injury not detectable by currently available imaging techniques such as echocardiography and contrast enhanced magnetic resonance imaging (MRI), which have at least a 5 times lower sensitivity for cardiomyocyte injury. This review aims to explore the currently available data, its methodological limitations and provide guidance to clinicians to avoid misinterpretation of cTn concentrations.

Prognostic Value of Troponins in Patients With or Without Coronary Heart Disease: Is it Dependent on Structure and Biology?

Convincing evidence has emerged that cardiac troponins (cTns) T and I are the biochemical gold standard for diagnosing cardiac injury, and may also be used as efficient screening and risk stratification tools, especially when measured with the new high-sensitivity (hs-) immunoassays. In this narrative review, we aim to explore and critically discuss the results of recent epidemiological studies that have attempted to characterise the prognostic value of cTns in patients with or without cardiovascular disease, and then interpret this information according to cTn biology. Overall, all recent studies agree that higher blood levels of cTns reflect the larger risk of cardiovascular events and/or death, both in the general population and in patients with cardiovascular disease. Additional evidence has shown that the clinical information provided by assessment of both cTns molecules is greater compared to that of either protein alone, and this is mostly due to differential metabolism and clearance of cTnI and cTnT after release in the bloodstream. Although it seems likely that the prognostic value of these biomarkers may be higher than that of other conventional cardiovascular risk factors such as cholesterol or C reactive protein, large and reliable cost-effectiveness investigations are needed to define whether cTns-based population screening may be biologically plausible, clinically effective and economically sustainable.

Multi-Site Coronary Vein Sampling Study on Cardiac Troponin T Degradation in Non-ST-Segment-Elevation Myocardial Infarction: Toward a More Specific Cardiac Troponin T Assay

Background Cardiac troponin T ( cTnT ) is seen in many other conditions besides myocardial infarction, and recent studies demonstrated distinct forms of cTnT . At present, the in vivo formation of these different cTnT forms is incompletely understood. We therefore performed a study on the composition of cTnT during the course of myocardial infarction, including coronary venous system sampling, close to its site of release. Methods and Results Baseline samples were obtained from multiple coronary venous system locations, and a peripheral artery and vein in 71 non- ST -segment-elevation myocardial infarction patients. Additionally, peripheral blood was drawn at 6- and 12-hours postcatheterization. cTnT concentrations were measured using the high-sensitivity- cTnT immunoassay. The cTnT composition was determined via gel filtration chromatography and Western blotting in an early and late presenting patient. High-sensitivity - cTnT concentrations were 28% higher in the coronary venous system than peripherally (n=71, P<0.001). Coronary venous system samples demonstrated cT n T-I-C complex, free intact cTnT , and 29 kD a and 15 to 18 kD a cTnT fragments, all in higher concentrations than in simultaneously obtained peripheral samples. While cT n T-I-C complex proportionally decreased, and disappeared over time, 15 to 18 kD a cTnT fragments increased. Moreover, cT n T-I-C complex was more prominent in the early than in the late presenting patient. Conclusions This explorative study in non- ST -segment-elevation myocardial infarction shows that cTnT is released from cardiomyocytes as a combination of cT n T-I-C complex, free intact cTnT , and multiple cTnT fragments indicating intracellular cTnT degradation. Over time, the cT n T-I-C complex disappeared because of in vivo degradation. These insights might serve as a stepping stone toward a high-sensitivity- cTnT immunoassay more specific for myocardial infarction.

Full-Size and Partially Truncated Cardiac Troponin Complexes in the Blood of Patients with Acute Myocardial Infarction

BACKGROUND

The measurement of cardiac isoforms of troponin I (cTnI) and troponin T (cTnT) is widely used for the diagnosis of acute myocardial infarction (AMI). However, there are conflicting data regarding what forms of cTnI and cTnT are present in the blood of AMI patients. We investigated cTnI and cTnT as components of troponin complexes in the blood of AMI patients.

METHODS

Gel filtration techniques, sandwich fluoroimmunoassays, and Western blotting were used.

RESULTS

Plasma samples from patients with AMI contained the following troponin complexes: (a) a cTnI-cTnT-TnC complex (ITC) composed of full-size cTnT of 37 kDa or its 29-kDa fragment and full-size cTnI of 29 kDa or its 27-kDa fragments; (b) ITC with lower molecular weight (LMW-ITC) in which cTnT was truncated to the 14-kDa C-terminal fragments; and (c) a binary cTnI-cTnC complex composed of truncated cTnI of approximately 14 kDa. During the progression of the disease, the amount of ITC in AMI samples decreased, whereas the amounts of LMW-ITC and short 16- to 20-kDa cTnT central fragments increased. Almost all full-size cTnT and a 29-kDa cTnT fragment in AMI plasma samples were the components of ITC. No free full-size cTnT was found in AMI plasma samples. Only 16- to 27-kDa central fragments of cTnT were present in a free form in patient blood.

CONCLUSIONS

A ternary troponin complex exists in 2 forms in the blood of patients with AMI: full-size ITC and LMW-ITC. The binary cTnI-cTnC complex and free cTnT fragments are also present in patient blood.

Evaluation of analytical performance of immunoassay methods for cTnI and cTnT: From theory to practice

Current guidelines worldwide recommend cardiac troponins I (cTnI) and T (cTnT) as the biomarkers of choice for the differential diagnosis of acute coronary syndrome (ACS), and the measurement of the 99th upper reference population limit (URL) value for cardiac troponins, with an imprecision of ≤10 CV%. Measuring the 99th URL of cTnI and cTnT is a challenging analytical task due to low biomarker concentrations present in healthy subjects. Therefore, since the year 2006, several manufacturers have established new generation cTnI and cTnT immunoassays with an improved analytical sensitivity in accordance with the quality specifications described in international guidelines, the more recent of which state that only immunoassays that meet the required quality specifications should be considered "high-sensitivity" methods. For the early diagnosis of ACS, and for the stratification of cardiovascular risk in cardiac patients and the general population, high-sensitivity methods should be employed. It is therefore important for laboratory professionals and clinicians to gain a thorough understanding of the analytical performances of immunoassay methods for cTnI and cTnT, especially at low to normal concentration ranges. The aim of the present study was to analyze critical aspects related to definition, analytical performance, pathophysiological interpretations, and the clinical relevance of high-sensitivity cardiac troponin assays.

Can copeptin and troponin T ratio predict final infarct size and myocardial salvage index in patients with ST-elevation myocardial infarction: A sub-study of the DANAMI-3 trial

BACKGROUND

Primary percutaneous coronary intervention (pPCI) is recommended in patients presenting with ST-elevation myocardial infarction (STEMI) within <12 h of symptom onset. However, patients-reported symptom duration is not always reliable. Cardiac specific troponin T (cTnT) and the endogenous stress marker copeptin have different temporal release patterns for myocardial infarction MI. We hypothesized that copeptin/troponin-ratio is associated to the duration of coronary occlusion and therefore inversely proportional to myocardial salvage.

METHOD

Patients older than 18 years with first time STEMI referred to pPCI were eligible. cTnT and copeptin values were measured at admission. A cardiac magnetic resonance scanning (CMR) was done during the index admission for assessment of area at risk (AAR), and later 3 months to assess final infarct size (FIS). Myocardial salvage index (MSI) was calculated based on these measurements.

RESULTS

A total of 468 patients were included. The median time from patient-reported onset of symptoms to pPCI was 192 min (IQR 150 min - 290 min). At presentation 416 (89%) patients had hs-cTnT values above the 99th percentile, median hs-cTnT was 53 ng/l (IQR 24 ng/l-146 ng/l) and 318 (68%) patients had copeptin values above the 99th percentile (18.9 pmol/l), median copeptin was 50 pmol/l (IQR 14 pmol/l-131 pmol/l). Symptom duration showed a weak but significant association with AAR (R² = 0.02, p = .04), FIS (R² = 0.03, p < .01) and MSI (R² = 0.04, p < .01). Copeptin/troponin-ratio was significantly associated with symptom duration (R² = 0.19, p < .01), but not AAR (R² = 0.02, p = .19), FIS (R² = 0.02, p = .12), or MSI (R² = 0.01, p = .25).

CONCLUSION

Copeptin/troponin-ratio is associated with patient-reported symptom duration, but there was no association with area at risk, final infarct size or myocardial salvage index.

Mass Spectrometric Identification of Cardiac Troponin T in Urine of Patients Suffering from Acute Myocardial Infarction

BACKGROUND

Because of its high cardiospecificity, cardiac troponin T (cTnT) is one of the first-choice biomarkers to diagnose acute myocardial infarction (AMI). cTnT is extensively fragmented in serum of patients suffering from AMI. However, it is currently unknown whether all cTnT is completely degraded in the body or whether some cTnT fragments can leave the body via urine. The aim of the present study is to develop a method for the detection of cTnT in urine and to examine whether cTnT is detectable in patient urine.

METHODS

Proteins in urine samples of 20 patients were precipitated using a cTnT-specific immunoprecipitation technique and a nonspecific acetonitrile protein precipitation. After in-solution digestion of the precipitated proteins, the resulting peptides were separated and analyzed using HPLC and mass spectrometry with a targeted selected ion monitoring assay with data-dependent tandem mass spectrometry (t-SIM/dd-MS2).

RESULTS

The t-SIM/dd-MS2 assay was validated using a synthetic peptide standard containing 10 specific cTnT peptides of interest and with purified human intact cTnT spiked in urine from healthy individuals. Using this assay, 6 different cTnT-specific peptides were identified in urine samples from 3 different patients, all suffering from AMI.

CONCLUSIONS

We show here for the first time that cTnT can be present in the urine of AMI patients using a targeted LC-MS/MS assay. Whether the presence of cTnT in urine reflects a physiological or pathophysiological process still needs to be elucidated.

Sex-Related Aspects of Biomarkers in Cardiac Disease

Biomarkers play an important role in the clinical management of cardiac care. In particular, cardiac troponins (cTn) and natriuretic peptides are the cornerstones for the diagnosis of acute myocardial infarction (AMI) and for the diagnosis of heart failure (HF), respectively. Current guidelines do not make a distinction between women and men. However, the commonly used "one size fits all" algorithms are topic of debate to improve assessment of prognosis, particularly in women. Due to the high-sensitivity assays (hs-cTn), lower cTn levels (and 99th percentile upper reference limits) were observed in women as compared with men. Sex-specific diagnostic thresholds may improve the diagnosis of AMI in women, though clinical relevance remains controversial and more trials are needed. Also other diagnostic aspects are under investigation, like combined biomarkers approach and rapid measurement strategies. For the natriuretic peptides, previous studies observed higher concentrations in women than in men, especially in premenopausal women who might benefit from the cardioprotective actions. Contrary to hs-cTn, natriuretic peptides are particularly incorporated in the ruling-out algorithms for the diagnosis of HF and not ruling-in. Clinical relevance of sex differences here seems marginal, as clinical research has shown that negative predictive values for ruling-out HF were hardly effected when applying a universal diagnostic threshold that is independent from sex or other risk factors. Apart from the diagnostic issues of AMI in women, we believe that in the future most sex-specific benefits of cardiac biomarkers can be obtained in patient follow-up (guiding therapy) and prognostic applications, fitting modern ideas on preventive and personalized medicine.

24-Hour Kinetics of Cardiac Troponin-T Using a "High-Sensitivity" Assay in Thoroughbred Chuckwagon Racing Geldings after Race and Associated Clinical Sampling Guidelines

Background

A “high‐sensitivity” cardiac troponin‐T (hscTnT) assay recently has been validated for use in horses and is a specific biomarker of myocardial damage. Postexercise release kinetics of cTnT utilizing the hscTnT assay have yet to be established in horses.

Objectives

To determine: (1) cTnT release kinetics in racing Thoroughbreds after a high‐intensity 5/8th mile Chuckwagon race; (2) the effects of age on pre‐ and postrace cTnT concentrations; and (3) sampling guidelines for clinicians evaluating horses presenting after exercise.

Animals

Samples were obtained from 38 Thoroughbred geldings aged 5–16 years before racing and immediately, 2, 3, 4, 6, 12, and 24 hour postrace.

Methods

Prospective, observational study with convenience sampling. A fifth‐generation hscTnT assay was used for plasma sample analysis, and concentrations were compared at all time‐points. Correlations were determined between cTnT concentrations and age. Biochemistry analysis was performed to assess rhabdomyolysis, renal failure, and exercise‐induced dehydration.

Results

All horses with measureable cTnT concentrations had significant postexercise increases in cTnT with a median peak (8.0 ng/L) at 3‐hour postrace. All horses had peak postexercise cTnT concentrations 2‐ to 6‐hour postrace ≤ the 99th percentile upper reference limit of 23.2 ng/L, after which all cTnT concentrations decreased until returning to baseline by 12–24 hours. There was no correlation over time between cTnT concentrations and age.

Conclusions and Clinical Importance

In racing Thoroughbreds completing short‐duration, high‐intensity Chuckwagon races, cTnT concentrations are expected to be increased 2‐ to 6‐hour postrace and to decrease by 12–24 hours while remaining ≤23.2 ng/L throughout. This study contributes to establishing guidelines for clinical use of the hscTnT assay in exercising horses.

Thrombin-Mediated Degradation of Human Cardiac Troponin T

BACKGROUND

Cardiac troponin T (cTnT) is an acknowledged biomarker of acute myocardial infarction (AMI) that is known to be prone to proteolytic degradation in serum. Such degradation is usually explained by the action of μ-calpain, although there could be other candidates for that role. In the current study, we explored the hypothesis that thrombin-mediated cTnT cleavage occurs as a result of the serum sample preparation.

METHODS

cTnT degradation was studied by using immunoblotting and mass spectrometry (MS) analysis.

RESULTS

The comparison of cTnT isolated from AMI heparin plasma and serum samples showed that cTnT in the plasma samples was mainly present as the full-sized molecule (approximately 35 kDa), while in serum samples it was present as a 29-kDa fragment. The incubation of recombinant cTnT, or native ternary cardiac troponin complex with thrombin or in normal human serum (NHS), resulted in the formation of a 29-kDa product that was similar to that detected in AMI serum samples. No cTnT degradation was observed when thrombin or NHS was pretreated with hirudin, a specific inhibitor of thrombin, or during incubation of troponin in normal heparin plasma. When the products of thrombin-mediated cTnT proteolysis were analyzed by MS, 2 fragments consisting of amino acid residues (aar) 2-68 and 69-288 were identified, which suggests that thrombin cleaves cTnT between R68 and S69.

CONCLUSIONS

The results of this study suggest that the 29-kDa fragment of cTnT in AMI serum samples mainly appears due to the cleavage by thrombin during serum sample preparation.

Quantifying the Release of Biomarkers of Myocardial Necrosis from Cardiac Myocytes and Intact Myocardium

BACKGROUND

Myocardial infarction is diagnosed when biomarkers of cardiac necrosis exceed the 99th centile, although guidelines advocate even lower concentrations for early rule-out. We examined how many myocytes and how much myocardium these concentrations represent. We also examined if dietary troponin can confound the rule-out algorithm.

METHODS

Individual rat cardiac myocytes, rat myocardium, ovine myocardium, or human myocardium were spiked into 400-μL aliquots of human serum. Blood was drawn from a volunteer after ingestion of ovine myocardium. High-sensitivity assays were used to measure cardiac troponin T (cTnT; Roche, Elecsys), cTnI (Abbott, Architect), and cardiac myosin-binding protein C (cMyC; EMD Millipore, Erenna^®).

RESULTS

The cMyC assay could only detect the human protein. For each rat cardiac myocyte added to 400 μL of human serum, cTnT and cTnI increased by 19.0 ng/L (95% CI, 16.8-21.2) and 18.9 ng/L (95% CI, 14.7-23.1), respectively. Under identical conditions cTnT, cTnI, and cMyC increased by 3.9 ng/L (95% CI, 3.6-4.3), 4.3 ng/L (95% CI, 3.8-4.7), and 41.0 ng/L (95% CI, 38.0-44.0) per μg of human myocardium. There was no detectable change in cTnI or cTnT concentration after ingestion of sufficient ovine myocardium to increase cTnT and cTnI to approximately 1 × 10⁸ times their lower limits of quantification.

CONCLUSIONS

Based on pragmatic assumptions regarding cTn and cMyC release efficiency, circulating species, and volume of distribution, 99th centile concentrations may be exceeded by necrosis of 40 mg of myocardium. This volume is much too small to detect by noninvasive imaging.

Redesigning the diagnostic pathway for chest pain patients in emergency departments

Patients presenting with chest pain at an emergency department in the United Kingdom receive troponin tests to assess the likelihood of an acute myocardial infarction (AMI). Until recently, serial testing with two blood samples separated by at least six hours was necessary in order to analyse the change in troponin levels over time. New high-sensitivity troponin tests, however, allow the inter-test time to be shortened from six to three hours. Recent evidence also suggests that the new generation of troponin tests can be used to rule out AMI on the basis of a single test if patients at low risk of AMI present with very low cardiac troponin levels more than three hours after onset of worst pain. This paper presents a discrete event simulation model to assess the likely impact on the number of hospital admissions if emergency departments adopt strategies for serial and single testing based on the use of high-sensitivity troponin. Data sets from acute trusts in the South West of England are used to quantify the resulting benefits.

Cardiac Troponin T: Smaller Molecules in Patients with End-Stage Renal Disease than after Onset of Acute Myocardial Infarction

BACKGROUND

We have found previously that in acute myocardial infarction (AMI), cardiac troponin T (cTnT) is degraded in a time-dependent pattern. We investigated whether cTnT forms differed in patients with chronic cTnT increases, as seen with renal dysfunction, from those in the acute phase of myocardial infarction.

METHODS

We separated cTnT forms by gel filtration chromatography (GFC) in end-stage renal disease (ESRD) patients: prehemodialysis (pre-HD) and post-HD (n = 10) and 2 months follow-up (n = 6). Purified (cTnT) standards, quality control materials of the clinical cTnT immunoassay (Roche), and AMI patients' sera also were analyzed. Immunoprecipitation and Western blotting were performed with the original cTnT antibodies from the clinical assay and antibodies against the N- and C-terminal end of cTnT.

RESULTS

GFC analysis revealed the retention of purified cTnT at 27.5 mL, identical to that for cTnT in quality controls. For all ESRD patients, one cTnT peak was found at 45 mL, pre- and post-HD, and stable over time. Western blotting illustrated that this peak corresponded to cTnT fragments &lt;18 kDa missing the N- and C-terminal ends. AMI patients' sera revealed cTnT peaks at 27.5 and 45 mL, respectively, corresponding to N-terminal truncated cTnT of 29 kDa and N- and C-terminal truncated fragments of &lt;18 kDa, respectively.

CONCLUSIONS

We found that cTnT forms in ESRD patients are small (&lt;18 kDa) and different from forms seen in AMI patients. These insights may prove useful for development of a more specific cTnT immunoassay, especially for the acute and diagnostic phase of myocardial infarction.