Novel Troponin Fragmentation Assay to Discriminate Between Troponin Elevations in Acute Myocardial Infarction and End-Stage Renal Disease

Advancements and challenges in high-sensitivity cardiac troponin assays: diagnostic, pathophysiological, and clinical perspectives

Although significant progress has been made in recent years, some important questions remain regarding the analytical performance, pathophysiological interpretation and clinical use of cardiac troponin I (cTnI) and T (cTnT) measurements. Several recent studies have shown that a progressive and continuous increase in circulating levels of cTnI and cTnT below the cut-off value (i.e. the 99th percentile upper reference limit) may play a relevant role in cardiovascular risk assessment both in the general population and in patients with cardiovascular or extra-cardiac disease. International guidelines recommend the use of standardized clinical algorithms based on temporal changes in circulating cTnI and cTnT levels measured by high-sensitivity (hs) methods to detect myocardial injury progressing to acute myocardial infarction. Some recent studies have shown that some point-of-care assays for cTnI with hs performance ensure a faster diagnostic turnaround time and thus significantly reduce the length of stay of patients admitted to emergency departments with chest pain. However, several confounding factors need to be considered in this setting. A novel approach may be the combined assessment of laboratory methods (including hs-cTn assay) and other clinical data, possibly using machine learning methods. In the present document of the Italian Study Group on Cardiac Biomarkers, the authors aimed to discuss these new trends regarding the analytical, pathophysiological and clinical issues related to the measurement of cardiac troponins using hs-cTnI and hs-cTnT methods.

Cardiac Troponin I and T Ratio and Risk of Cardiovascular or Non-Cardiovascular Events in a General Population

BACKGROUND

Emerging evidence suggests that the ratio between cardiac troponin (cTn) I and T may provide information on the risk of adverse outcomes in individuals with cardiovascular disease. Whether the cTn I/T ratio provides prognostic insights in the general population is unknown.

METHODS

The cTn I/T ratio was calculated in 8855 participants (43% female, median age 56 years) from the Generation Scotland Study where both cTnI and cTnT concentrations were above the limit of blank. Multivariable cause-specific Cox proportional hazard models were used to estimate the associations between cTn I/T ratio and the primary outcome of cardiovascular or non-cardiovascular death.

RESULTS

The median cTn I/T ratio was 0.5 (25th-75th percentile, 0.3-0.8) and median follow-up was 11.4 (10.8-12.7) years. Individuals in the highest ratio tertile (≥0.64) were more likely to be male, have a higher body mass index and systolic blood pressure, and a history of cardiovascular disease. Those in the lowest ratio tertile (<0.38) were more likely to be smokers or have diabetes. After adjustment for cardiovascular risk factors, the cTn I/T ratio was positively associated with cardiovascular death (per doubling increase, adjusted hazard ratio [HR] 1.16 [95% CI, 1.05-1.28]), while an inverse association was observed for non-cardiovascular death (HR 0.89 [95% CI, 0.81-0.99]).

CONCLUSIONS

The cTn I/T ratio is positively associated with cardiovascular death in the general population, while inversely associated with non-cardiovascular death. Future research is needed to unravel underlying mechanisms and determine whether the cTn I/T ratio provides valuable information regarding risk of cardiovascular and non-cardiovascular mortality to guide further management.

Cardiac Troponin: Fragments of the Future?

Cardiac troponin is the gold standard biomarker for the diagnosis of acute myocardial infarction (AMI). Development of high-sensitivity troponin platforms has revolutionized triage of chest pain patients, but specificity for type 1 AMI remains a clinical limitation. Consequently, differentiating type 1 AMI from other forms of myocardial injury is a common conundrum, heightened by the risks associated invasive coronary angiography. The troponin complex is a dynamic structure comprising of 3 subunits which variably fragment prior to measurement in the blood. Documenting the fragmentation patterns of cardiac troponin may help identify the cause of myocardial injury. This review explores the biology underlying troponin fragmentation and summarizes multiple lines of evidence that it can improve the specificity for diagnosis of type 1 AMI.

Cardiac troponin complexes and fragments: potential targets for improved clinical performance

High-sensitivity assays for cardiac troponin (cTn) have improved rule-out algorithms for acute myocardial infarction (AMI). However, reduced specificity specifically to AMI posed new clinical challenges. Studies involving the composition of troponin released into the circulation after injury may provide insights to improve specificity. In MI patients, cTnI primarily exists of cTnIC and truncated cTnTIC complexes. Larger-sized cTnT forms, as part of the cTnTIC complex, predominate with shorter ischemic time windows. Over time, mildly and heavily truncated cTnT forms increase, whereas for cTnI this is less certain. Targeting the central part of cTnT, the current high-sensitivity assay also identifies heavily truncated forms as seen in end-stage renal disease and after exercise. This review on composition of circulating troponin covers different populations and outlines first initiatives toward more specific assays by targeting larger-sized troponin forms.

Design and Analytical Evaluation of Novel Cardiac Troponin Assays Targeting Multiple Forms of the Cardiac Troponin I-Cardiac Troponin T-Troponin C Complex and Fragmentation Forms

BACKGROUND

Current studies suggest that cardiac troponin (cTn) forms in the circulation may vary in different clinical scenarios. Our aim was to design a combination of cTn assays specific to the main cTn forms and to evaluate their analytical performance.

METHODS

We developed immunoassays specific for measuring (1) long-cTnT cTnI-cTnT-TnC (ITC) ternary complex, with cTnT in long form without cleavage at the C-terminal amino acids residue 189-223, designated "long-cTnT ITC complex assay;" (2) both the long-cTnT ITC complex plus short-cTnT ITC complex, designated "hs-total ITC complex assay;" (3) the central part of cTnT of both the long-cTnT ITC complex and free cTnT, designated "hs-cTnT assay." Sex-specific 99th percentile upper reference limits (URLs) were determined. High-sensitivity performance was assessed by examining the imprecision and detectable results above limit of detection (LoD) in the healthy population.

RESULTS

Both complex immunoassays exhibited excellent analytical sensitivity, precision, and specificity. The 99th percentile URLs were as follows: long-cTnT ITC complex: male 0.90 ng/L, female 0.87 ng/L; hs-total ITC complex: male 16.15 ng/L, female 10.08 ng/L; hs-cTnT: male 15.57 ng/L, female 14.28 ng/L. The total imprecision at or below the sex-specific 99th percentile URLs was <5% for all assays. The hs-total ITC complex and the hs-cTnT assays showed >50% of measurable concentrations above the LoD. However, <20% were measurable for the long-cTnT ITC complex assay.

CONCLUSIONS

The cTn assays detected concentrations of major cTn forms in the circulation with high sensitivity, precision, and specificity, supporting their use for monitoring cTn complex and fragmentation forms during myocardial injuries.

Characterization of Cardiac Troponin Fragment Composition Reveals Potential for Differentiating Etiologies of Myocardial Injury

BACKGROUND

Increased cardiac troponin (cTn) concentrations occur in acute myocardial injury and chronic diseases. Characterization of cTn composition in the circulation may assist in differentiating etiologies of myocardial injury. Our goal was to study cTn composition and kinetics in patients following type 1 myocardial infraction (T1MI), cardiac procedures, and chronic heart diseases to establish the relationship between cTn composition and clinical diagnosis.

METHODS

Plasma samples were collected from 201 patients with T1MI, 78 undergoing cardiac surgeries, and 218 with chronic cardiomyopathy or chronic heart failure. Major cTn forms in the circulation and their ratios were analyzed using cTn composition immunoassays, targeting (a) the long-cTnT cTnI-cTnT-TnC (ITC) ternary complex, short-cTnT ITC complex cleaved at amino acids residues 189-223 of cTnT, and the binary cTnI-TnC (IC) complex, and designated the "high-sensitivity (hs)-cTnI assay;" (b) the long-cTnT ITC complex, and designated the "long-cTnT ITC complex assay;" (c) the long-cTnT ITC complex and short-cTnT ITC complex, and designated the "hs-total ITC complex assay;" and (d) the central part of cTnT of both the long-cTnT ITC complex and free cTnT, and designated the "hs-cTnT assay."

RESULTS

Early-stage T1MI patients showed a high ratio of long-cTnT ITC complex to cTnI (long-cTnT ITC complex/cTnI, R1). Similarly, patients after acute cardiac surgery exhibited increased cTn concentrations with high R1, which decreased rapidly. In chronic disease, cTn composition exhibited stable and low R1 and high ratios of cTnT to cTnI (cTnT/cTnI, R3).

CONCLUSIONS

Kinetic differences in multiple cTn forms contribute to the differentiation between acute injury and chronic disease, with a high proportion of long-cTnT ITC complex implying occurrence of acute injury.

Type 2 myocardial infarction: challenges in diagnosis and treatment

The Fourth Universal Definition of Myocardial Infarction recommends a classification based on aetiology, in recognition that the underlying pathophysiology of myocardial infarction influences the approach to investigation and treatment. Type 1 myocardial infarction occurs due to atherosclerotic plaque rupture with thrombosis, whereas type 2 myocardial infarction occurs due to an imbalance in myocardial oxygen supply or unmet need in myocardial oxygen demand, without atherothrombosis, usually in the context of another acute illness. In this state-of-the-art review, the diagnosis, investigation, and treatment of patients with type 2 myocardial infarction are considered, with general advice for clinical practice and a consideration of future research directions.

High-sensitivity cardiac troponin T and N-terminal b-type natriuretic propeptide are associated with cardiac and all-cause mortality in older adults - A population-based ten-year follow-up study

BACKGROUND

Cardiac troponin T (cTnT) and N-terminal B-type natriuretic propeptide (proBNP) are mainly used as biomarkers to diagnose specific conditions of the heart, but they also have predictive ability. Our aim was to study their associations with cardiovascular and all-cause mortality in an older population in non-acute conditions.

METHODS

A population-based study with a ten-year follow-up. The data comes from a community-based representative sample of an older population with 1260 participants (participation rate 82 %). Associations were analyzed using Cox proportional hazard models.

RESULTS

Altogether, 467 (37%) subjects died during the 10-year follow-up period, and 149 of those of a cardiovascular disease. Both elevated cTnT and proBNP concentrations were statistically significantly associated with cardiovascular and all-cause mortality in older adults.

CONCLUSIONS

Our study shows that older population with higher cTnT and proBNP concentrations have an increased risk of cardiovascular and all-cause mortality. Acknowledging the elevated risk may aid in targeting follow-up, prevention, and treatment adequately and more individually.

Novel troponin fragmentation assay to discriminate between Takotsubo syndrome and acute myocardial infarction

AIMS

Cardiac troponin levels are elevated in Takotsubo syndrome (TTS) with significant overlap to acute myocardial infarction (MI). Long and intact cardiac troponin T (cTnT) forms are typical for MI. This study sought to assess whether the fragmentation composition of cTnT release in TTS differs from MI.

METHODS AND RESULTS

The concentration of long molecular forms of cTnT (long cTnT) was measured with a novel upconversion luminescence immunoassay and total cTnT with a commercial high-sensitivity cTnT assay in 24 TTS patients and in 84 Type 1 MI patients. The ratio of long to total cTnT (troponin ratio) was determined as a measure of cTnT fragmentation. Troponin ratio was lower in TTS patients [0.13 (0.10-0.20) vs. 0.62 (0.29-0.96), P < 0.001]. In the receiver operating characteristic curve analyses, troponin ratio showed a better predictive power than total cTnT in discriminating TTS and MI patients {area under the curve [AUC] 0.869 [95% confidence interval (CI) 0.789-0.948] vs. 0.766 [95% CI 0.677-0.855], P = 0.047}. When restricting the analysis to patients with total cTnT below 1200 ng/L (maximal value in TTS patients), the respective AUC values for total cTnT and troponin ratio were 0.599 (95% CI 0.465-0.732) and 0.816 (95% CI 0.712-0.921) (P = 0.003). At a cut-off point of 0.12, troponin ratio correctly identified 95% of MI patients and 50% of TTS patients.

CONCLUSION

In contrast to Type 1 MI, only a small fraction of circulating cTnT in TTS exists in intact or long molecular forms. This clear difference in troponin composition could be of diagnostic value when evaluating patients with cTnT elevations and suspicion of TTS.

CLINICAL TRIAL REGISTRATION

URL: https://www.clinicaltrials.gov; Unique identifier: NCT04465591.

Composition of cardiac troponin release differs after marathon running and myocardial infarction

OBJECTIVES

Elevations of cardiac troponin T (cTnT) levels are common after strenuous exercise. We assessed whether the composition of cTnT release after marathon race differs from that of acute myocardial infarction (MI).

METHODS

Troponin composition was analysed in plasma samples taken from 45 runners after marathon race and from 84 patients with type 1 MI. The concentration of long cTnT (intact and mildly fragmented cTnT) was measured with a novel upconversion luminescence immunoassay, total cTnT with a commercial high-sensitivity cTnT assay, and the ratio of long to total cTnT (troponin ratio) was determined as a measure of troponin fragmentation.

RESULTS

Total cTnT exceeded the upper reference limit (>14 ng/L) in 37 (82%) runners. Troponin ratio was lower in runners ((IQR) 0.17 (0.11-0.24) vs 0.62 (0.29-0.96), p<0.001). With increasing troponin release the troponin ratio decreased (r=-0.497, p<0.001) in marathon runners and the concentration of long cTnT remained in all runners below 8.4 ng/L. In contrast to marathon runners, troponin ratio increased (r=0.565, p<0.001) with the increase of cTnT release in patients with MI. The median total and long cTnT concentrations were lower in marathon runners than in patients with MI (25 ng/L vs 835 ng/L and 4.1 vs 385 ng/L, p<0.001 for both).

CONCLUSION

In contrast to type 1 MI, only a small fraction of circulating cTnT exists as intact cTnT or long molecular forms after strenuous exercise and the difference in troponin composition is more pronounced in runners with higher troponin release.

TRIAL REGISTRATION NUMBER

NCT06000930.

Half-Life and Clearance of Cardiac Troponin I and Troponin T in Humans

BACKGROUND

Cardiac troponin (cTn) is key in diagnosing myocardial infarction (MI). After MI, the clinically observed half-life of cTn has been reported to be 7 to 20 hours, but this estimate reflects the combined elimination and simultaneous release of cTn from cardiomyocytes. More precise timing of myocardial injuries necessitates separation of these 2 components. We used a novel method for determination of isolated cTn elimination kinetics in humans.

METHODS

Patients with MI were included within 24 hours after revascularization and underwent plasmapheresis to obtain plasma with a high cTn concentration. After at least 3 weeks, patients returned for an autologous plasma retransfusion followed by blood sampling for 8 hours. cTn was measured with 5 different high-sensitivity cTn assays.

RESULTS

Of 25 included patients, 20 participants (mean age, 64.5 years; SD, 8.2 years; 4 women [20%]) received a retransfusion after a median of 5.8 weeks (interquartile range, 5.0-6.9 weeks) after MI. After retransfusion of a median of 620 mL (range, 180-679 mL) autologous plasma, the concentration of cTn in participants' blood increased 4 to 445 times above the upper reference level of the 5 high-sensitivity cTn assays. The median elimination half-life ranged from 134.1 minutes (95% CI, 117.8-168.0) for the Elecsys high-sensitivity cTnT assay to 239.7 minutes (95% CI, 153.7-295.1) for the Vitros high-sensitivity cTnI assay. The median clearance of cTnI ranged from 40.3 mL/min (95% CI, 32.0-44.9) to 52.7 mL/min (95% CI, 42.2-57.8). The clearance of cTnT was 77.0 mL/min (95% CI, 45.2-95.0).

CONCLUSIONS

This novel method showed that the elimination half-life of cTnI and cTnT was 5 to 16 hours shorter than previously reported. This indicates a considerably longer duration of cardiomyocyte cTn release after MI than previously thought. Improved knowledge of timing of myocardial injury may call for changes in the management of MI and other disorders with myocardial injury.

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.

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 Cardio-Kidney Patient: Epidemiology, Clinical Characteristics and Therapy

Patients with chronic kidney disease (CKD) are at high risk to develop cardiovascular disease with its manifestations coronary artery disease, heart failure, arrhythmias, and sudden cardiac death. In addition, the presence of CKD has a major impact on the prognosis of patients with cardiovascular disease, leading to an increased morbidity and mortality if both comorbidities are present. Therapeutic options including medical therapy and interventional treatment are often limited in patients with advanced CKD, and in most cardiovascular outcome trials, patients with advanced CKD have been excluded. Thus, in many patients, treatment strategies for cardiovascular disease need to be extrapolated from trials conducted in patients without CKD. The current article summarizes the epidemiology, clinical presentation, and treatment options for the most prevalent manifestations of cardiovascular disease in CKD and discusses the currently available treatment options to reduce morbidity and mortality in this high-risk population.

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.