Development and Preliminary Clinical Validation of a High Sensitivity Assay for Cardiac Troponin Using a Capillary Flow (Single Molecule) Fluorescence Detector

Rapid exclusion of acute myocardial infarction in patients with undetectable troponin using a high-sensitivity assay

OBJECTIVES

This paper sought to evaluate whether high sensitivity troponin (hs-cTnT) can immediately exclude acute myocardial infarction (AMI) at a novel 'rule out' cut-off.

BACKGROUND

Subgroup analysis of recent evidence suggests that undetectable hs-cTnT may exclude AMI at presentation.

METHODS

In a cohort study, we prospectively enrolled patients with chest pain, evaluating them with standard troponin T and testing for hs-cTnT (Roche Diagnostics, Basel, Switzerland) at presentation. The primary outcome was a diagnosis of AMI. We also followed up patients for adverse events within 6 months. After subsequent clinical implementation of hs-cTnT, we again evaluated whether initially undetectable hs-cTnT ruled out a subsequent rise.

RESULTS

Of 703 patients in the cohort study, 130 (18.5%) had AMI, none of whom initially had undetectable hs-cTnT (sensitivity: 100.0%, 95% confidence interval [CI]: 95.1% to 100.0%, negative predictive value: 100.0%, 95% CI: 98.1% to 100.0%). This strategy would rule out AMI in 27.7% of patients, 2 (1.0%) of whom died or had AMI within 6 months (1 periprocedural AMI, 1 noncardiac death). We evaluated this approach in an additional 915 patients in clinical practice. Only 1 patient (0.6%) with initially undetectable hs-cTnT had subsequent elevation (to 17 ng/l), giving a sensitivity of 99.8% (95% CI: 99.1% to 100.0%) and a negative predictive value of 99.4% (95% CI: 96.6% to 100.0%).

CONCLUSIONS

Undetectable hs-cTnT at presentation has very high negative predictive value, which may be considered to rule out AMI, identifying patients at low risk of adverse events. Pending further validation, this strategy may reduce the need for serial testing and empirical treatment, enabling earlier reassurance for patients and fewer unnecessary evaluations and hospital admissions.

Design and development of a field-deployable single-molecule detector (SMD) for the analysis of molecular markers

Single-molecule detection (SMD) has demonstrated some attractive benefits for many types of biomolecular analyses including enhanced processing speed by eliminating processing steps, elimination of ensemble averaging and single-molecule sensitivity. However, it's wide spread use has been hampered by the complex instrumentation required for its implementation when using fluorescence as the readout modality. We report herein a simple and compact fluorescence single-molecule instrument that is straightforward to operate and consisted of fiber optics directly coupled to a microfluidic device. The integrated fiber optics served as waveguides to deliver the laser excitation light to the sample and collecting the resulting emission, simplifying the optical requirements associated with traditional SMD instruments by eliminating the need for optical alignment and simplification of the optical train. Additionally, the use of a vertical cavity surface emitting laser and a single photon avalanche diode serving as the excitation source and photon transducer, respectively, as well as a field programmable gate array (FPGA) integrated into the processing electronics assisted in reducing the instrument footprint. This small footprint SMD platform was tested using fluorescent microspheres and single AlexaFluor 660 molecules to determine the optimal operating parameters and system performance. As a demonstration of the utility of this instrument for biomolecular analyses, molecular beacons (MBs) were designed to probe bacterial cells for the gene encoding Gram-positive species. The ability to monitor biomarkers using this simple and portable instrument will have a number of important applications, such as strain-specific detection of pathogenic bacteria or the molecular diagnosis of diseases requiring rapid turn-around-times directly at the point-of-use.

High-Sensitivity Cardiac Troponin I Measurement for Risk Stratification in a Stable High-Risk Population

BACKGROUND

Past investigations regarding the utility of high-sensitivity cardiac troponin I (cTnI) assays have been focused primarily on the acute coronary syndrome setting. We assessed whether such assays can predict future ischemic cardiovascular events in a stable high-risk population.

METHODS

We quantified serum cTnI using an investigational high-sensitivity assay (hs-cTnI IUO, Beckman Coulter) in 2572 participants from the Heart Outcomes Prevention Evaluation (HOPE) study. The derived ROC curve cutoff and the 99th percentile for the hs-cTnI assay were assessed by Kaplan–Meier and Cox analyses for the primary outcome [composite of myocardial infarction (MI), stroke, and cardiovascular death] at 4.5 years of follow-up. We also assessed individual outcomes (MI, stroke, cardiovascular death) and the combined outcome (MI/cardiovascular death) by regression analyses to determine hazard ratios (HRs) and c statistics in models that included established risk factors, C-reactive protein, and N-terminal pro-B–type natriuretic peptide (NT-proBNP).

RESULTS

Participants with hs-cTnI >6 ng/L (ROC cutoff) were at higher risk for the primary outcome (HR 1.38, 95% CI 1.09–1.76; P = 0.008, adjusted models). For the individual outcomes, participants with hs-cTnI above the 99th percentile (≥10 ng/L) had higher risk for cardiovascular death (HR 2.15, 95% CI 1.32–3.52; P = 0.002) and MI (HR 1.49, 95% CI 1.05–2.10; P = 0.025) but not stroke (HR 1.38, 95% CI 0.76–2.47; P = 0.288, adjusted models). Addition of hs-cTnI to an established risk model with NT-proBNP also yielded a higher c statistic for the combined outcome of MI/cardiovascular death.

CONCLUSIONS

The investigational Beckman Coulter hs-cTnI assay provides prognostic information for future MI and cardiovascular death in a stable high-risk population.

Increased microRNA-1 and microRNA-133a levels in serum of patients with cardiovascular disease indicate myocardial damage

BACKGROUND

Recently, elevation of circulating muscle-specific microRNA (miRNA) levels has been reported in patients with acute myocardial infarction. However, it is still unclear from which part of the myocardium or under what conditions miRNAs are released into circulating blood. The purpose of this study was to identify the source of elevated levels of circulating miRNAs and their function in cardiovascular diseases.

METHODS AND RESULTS

Serum levels of miRNA (miR)-1 and miR-133a were increased significantly in patients not only with acute myocardial infarction but also with unstable angina pectoris and Takotsubo cardiomyopathy without elevation of serum creatine phosphokinase or cardiac troponin. MicroRNA microarray analysis of the heart from a mouse model of myocardial infarction indicated that the levels of miR-1, miR-133a, miR-208a, and miR-499 were significantly reduced in the infarcted myocardium. In situ hybridization of miR-133a also showed that miR-133a levels were very low in the infarcted and peri-infarcted myocardium. It has been shown that circulating miRNAs are localized inside exosomes, which are released after Ca(2+) stimulation. We stimulated H9c2 cardiomyoblasts with A23187 and measured miR-133a levels in the exosome fraction of the culture medium. A23187 induced a dose-dependent release of miR-133a, and significant elevation was observed only at concentrations where dead cells were detected. We also found that miR-133a-containing exosomes reduced the luciferase activity of 293FT cells transfected with an miR-133a sensor vector.

CONCLUSIONS

These results suggest that elevated levels of circulating miR-133a in patients with cardiovascular diseases originate mainly from the injured myocardium. Circulating miR-133a can be used as a marker for cardiomyocyte death, and it may have functions in cardiovascular diseases.

Baseline Serum Cardiac Troponin I Concentrations in Sprague-Dawley, Spontaneous Hypertensive, Wistar, Wistar-Kyoto, and Fisher Rats as Determined with an Ultrasensitive Immunoassay

Cardiac troponins have proved to be reliable blood biomarkers for identifying a variety of myocardial alterations in humans and animals. Recently, an ultrasensitive cTnI assay (Erenna IA) has been used to demonstrate increases in baseline cTnI resulting from drug-induced myocardial injury in rats, dogs, and monkeys, as well as to document baseline cTnI ranges in Sprague-Dawley (SD) rats. The present study was initiated to use the Erenna cTnI assay to further document baseline cTnI concentrations in normal control animals from multiple strains, including SD, Spontaneous Hypertensive (SHR), Wistar, Wistar-Kyoto (WKY), and Fisher strains. Baseline cTnI concentrations were quantified in all rats tested, and males had higher mean cTnI concentrations than females of the same strain. SHR males had the highest mean cTnI concentrations and the largest cTnI variability. Interestingly, cTnI concentrations increased in castrated SHR compared with unaltered male SHR, whereas cTnI concentrations decreased in ovariectomized SHR compared with unaltered female SHR. These results show significant differences in cTnI concentrations between strains, sexes, and noncardiac surgical alterations in control animals, and identify these as potential contributing factors to cTnI baseline variability that should be taken into account when using ultrasensitive cTnI as a biomarker to assess preclinical cardiotoxicity.

Biological variation and reference change value of high-sensitivity troponin T in healthy individuals during short and intermediate follow-up periods

BACKGROUND

Acute myocardial infarction is defined by a troponin concentration >99th percentile with an acute increase and/or decrease, the magnitude of which has not yet been well defined. To aid the interpretation of changes in cardiac troponin concentration, we sought to establish biological variation and reference change values (RCVs) by applying both the normal and lognormal approaches for cardiac troponin T (cTnT) sampled at hourly and weekly intervals in healthy individuals and measured on the Roche E 170 and Elecsys® 2010 automated platforms.

METHODS

High-sensitivity cTnT (hsTnT) was measured at baseline, and after 1, 2, 3, and 4 h and after 1, 2, 3, and 4 weeks in 20 and 17 healthy individuals, respectively. A healthy status was established by physical examination, MRI analysis at rest and during dobutamine stress, lung function testing, and blood sample testing.

RESULTS

Hourly total and within-individual CVs were 18% and 15%, respectively, for the E 170 assay, and 24% and 21%, respectively, for the Elecsys 2010 assay. Weekly total and within-individual CVs for these assays were 32% and 31%, respectively, for the E 170 assay, and 32% and 30%, respectively, for the Elecsys 2010 assay. The RCVs for the E 170 and Elecsys 2010 assays were ±46% and ±62% (hourly), respectively, and ±87% and ±86% (weekly), respectively. The corresponding lognormal values were +64%/-39% and +90%/-47% (hourly), and +138%/-58% and +135%/-58% (weekly).

CONCLUSIONS

RCVs appear attractive for interpreting hsTnT results. The short-term biological variation of hsTnT is low but becomes somewhat more important at intermediate sampling intervals. Knowledge of this variation is important for interpreting results from patients in whom cTnT values increase from low concentrations.

A comparison of mortality and cardiac biomarker response between three outbred stocks of Sprague Dawley rats treated with isoproterenol

The authors compared the mortality and cardiac biomarker responses in three outbred stocks of Sprague Dawley rats (CD/IGS, Sasco, Harlan) treated with isoproterenol hydrochloride. Cardiac injury was confirmed by histologic evaluation, and increases in cardiac troponin I concentration in serum were measured by two methods. CD/IGS rats had a higher incidence and earlier mortality compared with Sasco or Harlan rats. Harlan rats had lower severity scores for cardiomyocyte degeneration/necrosis compared with the other stocks. Post-isoproterenol treatment cardiac troponin I concentrations were greater in CD/IGS and Sasco rats compared with Harlan rats. Concentrations of cardiac troponin T followed a similar pattern to that of cardiac troponin I in rats treated with isoproterenol. Myosin, light chain 3 concentrations increased in all rats treated with isoproterenol, but there was no difference between the three stocks in the magnitude or pattern of the dose response. Increases in fatty acid binding protein 3 concentrations were detected in only the highest dose group at the earliest timepoint postdose for all three stocks of rats. Results of these studies illustrate the need for investigators to recognize the potential differences in response between stocks of Sprague Dawley rats treated with cardiotoxicants or novel chemical entities.

Biological variation of high sensitive Troponin T in stable heart failure patients with ischemic or dilated cardiomyopathy

INTRODUCTION

High sensitive Troponin (hsTn) assays enable detection of minimal marker elevation in heart failure patients previously deemed Troponin negative. Biovariability, reference change values (RCV), and index of individuality (II) have not been previously described for hsTnT although serial testing is important in interpreting low concentrations. For these values, a difference between ischemic heart disease (IHD) and dilated cardiomyopathy (dCMP) appears conceivable.

METHODS

Change in hsTnT was determined alongside with clinical variables in 41 patients with stable chronic systolic dysfunction at 2-week-, 1-month-, 2-month-, and 3-month-intervals (IHD n = 17; dCMP n = 24).

RESULTS

HsTnT was detectable in all patients. Individual hsTnT-variations at 2-week, 1-month, 2-month, and 3-month follow-up were 7.2, 22.6, 28.9, and 15.7%, respectively, corresponding to RCVs of 20.1, 62.5, 80.0, and 43.3%, respectively, for crude values. For log-normalised values, individual variations were 3.2, 2.8, 2.7, and 3.5%, respectively, corresponding to RCVs of 8.8, 7.9, 7.6, and 9.7%, respectively. The II was 0.03 to 0.33 according to interval. Aetiology of heart failure was not a consistent determinant of variation (p = 0.28; p = 0.07; p = 0.98; p = 0.03 for 2-week, 1-month, 2-month, and 3-month follow-up, respectively).

CONCLUSION

While short-term biological variation of hsTnT is low, it becomes relatively more important for intermediate follow-up. It is not related to aetiology of heart failure. The corresponding indices of individuality indicate high individuality of values.

Novel biomarkers in cardiovascular disease: update 2010

The rapid evaluation of patients presenting with symptoms suggestive of an acute coronary syndrome is of great clinical relevance. Biomarkers have become increasingly important in this setting to supplement electrocardiographic findings and patient history because one or both can be misleading. Today, cardiac troponin is still the only marker used routinely in this setting due to its myocardial tissue specificity and sensitivity, as well as its established usefulness for therapeutic decision making. However, even current generation troponin assays have certain limitations such as insufficient sensitivity for diagnosing unstable angina. Novel high-sensitivity assays for cardiac troponin have the potential to overcome these limitations. Further studies are needed to answer some critical questions regarding the best cutoffs for diagnosis and risk assessment and the optimal work-up for rule-out of acute myocardial infarction. Other nonmyocardial tissue-specific markers might help in this setting. Myeloperoxidase, copeptin, and growth differentiation factor 15 reflect different aspects of the development of atherosclerosis or acute ischemia. Each has demonstrated impact in risk stratification of acute coronary syndromes. Limited data also show that copeptin may, when used together with cardiac troponin, improve the sensitivity for diagnosing acute myocardial infarction, and growth differentiation factor 15 may help in selection of patients that benefit from invasive therapy. Further evaluation is needed before these markers can be adopted routinely in clinical practice.

New features of troponin testing in different clinical settings

Cardiac troponin levels are routinely measured for diagnosing acute myocardial infarction. Cardiac troponin measurements also provide information concerning prognosis and the effect of early intervention in patients with acute coronary syndromes. The recent development of highly sensitive cardiac troponin assays permits detection of very low circulating levels. Use of sensitive troponin assays improves overall diagnostic accuracy in patients with suspected acute coronary syndromes, and these assays provide strong prognostic information in stable coronary artery disease and chronic heart failure. However, increased sensitivity comes with a cost of decreased specificity, and serial testing, as well as clinical context and judgment, is likely to become increasingly important in the interpretation of troponin assay results.

Recommendations for the use of cardiac troponin measurement in acute cardiac care

The release of cardiomyocyte components, i.e. biomarkers, into the bloodstream in higher than usual quantities indicates an ongoing pathological process. Thus, detection of elevated concentrations of cardiac biomarkers in blood is a sign of cardiac injury which could be due to supply-demand imbalance, toxic effects, or haemodynamic stress. It is up to the clinician to determine the most probable aetiology, the proper therapeutic measures, and the subsequent risk implied by the process. For this reason, the measurement of biomarkers always must be applied in relation to the clinical context and never in isolation. There are a large number of cardiac biomarkers, but they can be subdivided into four broad categories, those related to necrosis, inflammation, haemodynamic stress, and/or thrombosis. Their usefulness is dependent on the accuracy and reproducibility of the measurements, the discriminatory limits separating pathology from physiology, and their sensitivity and specificity for specific organ damage and/or disease processes. In recent years, cardiac biomarkers have become important adjuncts to the delivery of acute cardiac care. Therefore, the Working Group on Acute Cardiac Care of the European Society of Cardiology established a committee to deal with ongoing and newly developing issues related to cardiac biomarkers. The intention of the group is to outline the principles for the application of various biomarkers by clinicians in the setting of acute cardiac care in a series of expert consensus documents. The first of these will focus on cardiac troponin, a pivotal marker of cardiac injury/necrosis.

Identification of myocardial injury in the emergency setting

Within the past decade, the use of biomarkers to detect myocardial injury in the emergency department (ED) has been given increasing prominence as evident by the numerous studies and guidelines documenting their use. This review details the scope of the clinical problem, the history of changes in the definition of myocardial infarction (MI) and the new approaches, as well as suggestions for using laboratory biomarkers in the early detection of MI in the ED.

Clinical applications of highly sensitive troponin assays

Cardiac troponin is the biomarker of choice for the diagnosis of acute myocardial infarction. Recent consensus recommendations have adopted a concentration of troponin above the 99th percentile of a healthy population to diagnose myocardial infarction. Until recently, there was no assay capable of achieving recommended precision; however, with the development of "highly sensitive" troponin assays, it is now possible to accurately measure troponin concentrations at and below the current 99th percentile of a healthy population. These assays have enormous potential in not only identifying more patients with acute myocardial infarction, and providing superior risk prediction in those so afflicted, in addition highly sensitive troponins assays may be useful for long-term risk assessment of the patient with coronary disease. In this article, we will review the clinical applications, novel concepts, challenges, and limitations of using highly sensitive troponins assays.

Cardiac troponin I is released following high-intensity short-duration exercise in healthy humans

It has been previously shown that cardiac troponin (cTn) is released in response to prolonged endurance exercise. The influence of short-duration high-intensity exercise upon the release of cTn is not known. We examined cardiac troponin I (cTnI) release pre-, during and post-30 min of high-intensity running exercise in eight recreationally active males (age 29±3 years; VO2peak 53±11 ml kg min(-1)). Following exercise, cTnI increased in six of the eight participants. Four participants showed a minimal response (<0.05 µg/l) post-exercise. In contrast, two participants showed a progressive increase in cTnI (>0.1 µg/l) following exercise which peaked 3-4 h post-exercise. cTnI returned below the detection limit of the assay in all bar one of the participants 24 h post-exercise. These data are the first to show that cTnI can be released following short-duration high-intensity exercise. Clinicians should be aware that exercise-induced release of cTnI is not limited to prolonged endurance activity.

Cardiac troponin may be released by ischemia alone, without necrosis

Whilst it is formally stated that cardiac troponin is only released when cardiac myocytes undergo necrosis, there are a number of clinical situations where troponin is present in the circulation, without any apparent cardiac injury. In these cases, troponin half-life in the circulation is usually substantially shorter than that seen when troponin is released following myocardial infarction with frank necrosis. A mechanism has been described in liver, where large cytoplasmic molecules can pass from the intra- to extra-cellular space without cellular necrosis occurring. This occurs by the formation of membranous blebs which bud off from the plasma membrane of the cell. Blebs develop during cellular ischemia. If the ischemia is limited and re-oxygenation occurs, the blebs may be released into the circulation without rupture of the plasma membrane, resulting in a one-off release of cytoplasmic contents including macromolecules. Evidence from cardiac studies is presented supporting the presence of membranous blebs in cardiac myocytes, enabling troponin to be released from cardiac cells due to ischemia alone, without necrosis.

Analytical validation of a highly sensitive microparticle-based immunoassay for the quantitation of IL-13 in human serum using the Erenna immunoassay system

IL-13 is a Th2 cytokine that has been shown to be an important mediator of airway inflammation contributing to asthma lesions. Given its proposed role in asthma, measurements of this cytokine in serum may provide insights into disease mechanisms, progression and pharmacodynamic effects of IL-13 targeted therapeutics. However, current commercially available ELISA immunoassays are frequently unable to detect baseline concentrations of IL-13 in serum from healthy individuals, which are below the limit of detection. Here we describe the use of the novel microparticle-based Erenna IL-13 human immunoassay (Singulex, Inc.), which utilizes proprietary antibodies and single molecule counting technology, to quantify IL-13 from 100 microL of serum from apparently healthy subjects and clinically defined symptomatic and asymptomatic asthma subjects. The lower limit of quantification of the Erenna assay was validated at 0.07 pg/mL and the assay detected baseline concentrations of IL-13 in 98% of serum samples tested. The calibration curve showed good precision over the entire linear range of 0.07-50 pg/mL, with inter-assay imprecision <10% CV except at the lowest concentration tested (<15%). The intra- and inter-assay imprecision of spiked serum samples containing three different IL-13 concentrations (2, 8, and 25 pg/mL) ranged from 2.2-2.4% and 6.1-6.8%, respectively. Using the Erenna IL-13 assay, we observe that serum IL-13 concentrations range from <0.07-1.02 pg/mL in apparently healthy subjects (N=60) with similar ranges in asymptomatic (0.07-0.66 pg/mL, N=26) and symptomatic (<0.07-1.26 pg/mL, N=96) asthma subjects. The Erenna immunoassay improved sensitivity by over two full logs compared to previous ELISA methods, while using smaller sample volumes. In addition, the Erenna assay reliably measured IL-13 in endogenous and spiked human serum samples that were not quantifiable using other methods. Taken together, these results show that this novel assay offers a significant improvement over previous methods for high-sensitive quantitative measurement of IL-13 in human serum samples obtained from both apparently healthy and asthmatic subjects, and can be used in future clinical studies to accurately measure concentrations of this cytokine prior to and following drug therapy in human serum.

Longitudinal Studies of Cardiac Troponin-I Concentrations in Serum from Male Sprague Dawley Rats: Baseline Reference Ranges and Effects of Handling and Placebo Dosing on Biological Variability

Serum cardiac troponin-I has been validated as a biomarker for cardiotoxicity in numerous animal models; however, baseline reference ranges for cTnI concentration in a healthy population of laboratory rats, as well as an investigation of biological cTnI variability in rats with respect to time, handling, and placebo dosing methods, have not been reported. In this study, we used an ultrasensitive cTnI immunoassay to quantify hourly concentrations of cTnI in live rats handled under standard laboratory conditions using 15 μL of serum per determination. The baseline reference range (mean 4.94 pg/mL, range 1–15 pg/mL, 99% confidence interval [CI]) of cTnI concentration in rats was consistent with previously reported reference ranges for cTnI in humans (1–12 pg/mL) and with preliminary studies in dogs (1–4 pg/mL) and monkeys (4–5 pg/mL) using the same cTnI assay method. In addition, cTnI concentrations in individual rat serum samples show minimal biological variability over a twenty-four-hour interval when compared to a meaningful reference change value of 193% to 206%. Furthermore, measurements of cTnI concentration were consistent within the reference limits in individual rats over long periods and under three different standard laboratory handling conditions. Thus, using this new method, rats can be followed longitudinally at hourly intervals, and a doubling of cTnI concentration would be significant above biological variability. This is a new paradigm for preclinical testing, which allows transient changes in cTnI concentration to be accurately quantified. This understanding of baseline and biological variability in rats will be fundamental for designing and analyzing future studies that assess potential cardiotoxicity in drug development.

Short- and long-term risk stratification using a next-generation, high-sensitivity research cardiac troponin I (hs-cTnI) assay in an emergency department chest pain population

BACKGROUND

The next-generation, high-sensitivity cardiac troponin assays can measure quantifiable concentrations of cTn in a majority of individuals, but there are few studies assessing these assays for risk stratification. The present study was undertaken to determine if a research hs-cTnI assay can be useful for predicting death/myocardial infarction (MI), both short- and long-term, in an emergency department acute coronary syndrome (ACS) population.

METHODS

In a cohort of 383 subjects, originally recruited in 1996, presenting to the emergency department with symptoms suggestive of ACS, the heparin plasma obtained at initial presentation was thawed and measured in 2007 with a research hs-cTnI assay. AccuTnI (Beckman Coulter) measurements were made on these same samples in 2003. The population was divided into 4 groups by hs-cTnI: <5.00, 5.00-9.99, 10.00-40.00, and >40.00 ng/L. Kaplan-Meier, Cox proportional hazards, ROC curves, and logistic regression analyses were used to identify which hs-cTnI concentrations were predictive of death/MI within 10 years after presentation.

RESULTS

There were significant differences between the hs-cTnI groups for the probability of death/MI up to 10 years after presentation (P < 0.05). At 6 months, patients with hs-cTnI > or =10.00 ng/L were at higher risk for death/MI (hazard ratio >3.7; P < 0.05) compared with those having hs-cTnI <5.00 ng/L. ROC curve analysis for death/MI at 30 days with the hs-cTnI assay had an area under the curve of 0.74 (95% CI 0.65-0.82), with logistic models yielding an optimal assay threshold of 12.68 ng/L.

CONCLUSIONS

This research hs-cTnI assay appears useful for risk stratification for death/MI in an ACS population.

Analytic and Clinical Utility of a Next-Generation, Highly Sensitive Cardiac Troponin I Assay for Early Detection of Myocardial Injury

Background: Improvements in cardiac troponin (cTn) assays have increased the rapidity with which clinicians can identify patients with changing cTn concentrations (rise or fall) indicative of acute myocardial injury. The aim of the present study was to characterize a new, high-sensitivity cTnI (hs-cTnI) assay and examine whether increased sensitivity can result in still earlier detection of evolving injury.

Methods: We determined the limit of detection, precision profiles, and preliminary estimates of the 99th percentile for the Beckman Coulter hs-cTnI assay in 125 healthy individuals (age &lt;55 years, 54% male). We compared AccuTnI® and hs-cTnI to assess whether change criteria for early concentration changes (i.e., ≥3SD for low concentrations and 20% difference for concentrations &gt;0.10 μg/L) were exceeded in the first 2 specimens (median time between specimens, 1 h; 25th–75th percentile, 1–3 h) from subjects with symptoms suggestive of cardiac ischemia (n = 290).

Results: The limit of detection for the hs-cTnI assay was 2.06 ng/L, and the 20% CV and 10% CV concentrations were 2.95 and 8.66 ng/L, respectively. The preliminary 99th percentile estimates in lithium heparin, serum, and EDTA plasma were 9.20, 8.00, and 8.60 ng/L, respectively. In 108 patients with myocardial injury based on the peak AccuTnI concentration, applying the change criteria on the 2 earliest specimens identified 81% (95% CI 73%–88%) of patients using the hs-cTnI assay compared to 62% (53%–71%) using the AccuTnI assay (P &lt; 0.001).

Conclusions: Although more extensive validation studies are required, this Beckman Coulter hs-cTnI assay appears to detect patients with evolving myocardial injury earlier.

Value of Cardiac Troponin I Cutoff Concentrations below the 99th Percentile for Clinical Decision-Making

Background: The aim of this study was to evaluate factors influencing the 99th percentile for cardiac troponin I (cTnI) when this cutoff value is established on a highly sensitive assay, and to compare the value of this cutoff to that of lower cutoffs in the prognostic assessment of patients with coronary artery disease.

Methods: We used the recently refined Access AccuTnI assay (Beckman-Coulter) to assess the distribution of cTnI results in a community population of elderly individuals [PIVUS (Prospective Study of the Vasculature in Uppsala Seniors) study; n = 1005]. The utility of predefined cTnI cutoffs for risk stratification was then evaluated in 952 patients from the FRISC II (FRagmin and Fast Revascularization during InStability in Coronary artery disease) study at 6 months after these patients had suffered acute coronary syndrome.

Results: Selection of assay results from a subcohort of PIVUS participants without cardiovascular disease resulted in a decrease of the 99th percentile from 0.044 μg/L to 0.028 μg/L. Men had higher rates of cTnI elevation with respect to the tested thresholds. Whereas the 99th percentile cutoff was not found to be a useful prognostic indicator for 5-year mortality, both the 90th percentile (hazard ratio 3.1; 95% CI 1.9–5.1) and the 75th percentile (hazard ratio 2.8; 95% CI 1.7–4.7) provided useful prognostic information. Sex-specific cutoffs did not improve risk prediction.

Conclusions: The 99th percentile of cTnI depends highly on the characteristics of the reference population from which it is determined. This dependence on the reference population may affect the appropriateness of clinical conclusions based on this threshold. However, cTnI cutoffs below the 99th percentile seem to provide better prognostic discrimination in stabilized acute coronary syndrome patients and therefore may be preferable for risk stratification.

Short- and Long-Term Biological Variation in Cardiac Troponin I Measured with a High-Sensitivity Assay: Implications for Clinical Practice

Background: The improved detection limit and precision in new-generation commercial assays for cardiac troponin I (cTnI) have lowered the 99th-percentile cutoff value, yielding higher frequencies of positive test results. Because serial testing is important in interpreting low concentrations, we evaluated the biological variation of cTnI in both the short (hours) and long (weeks) terms and determined reference change values (RCVs) and the index of individuality (II) for cTnI.

Methods: To assess short- and long-term variation, we collected blood from 12 healthy volunteers hourly for 4 h and from 17 healthy individuals once every other week for 8 weeks, measured cTnI with a high-sensitivity assay (detection limit, 0.2 ng/L), and computed analytical, intraindividual, interindividual, and total CVs (CVA, CVI, CVG, and CVT, respectively; CVT = CVA + CVI + CVG) as well as the II. Because of the slight right-skewness of the data, RCVs were calculated with a lognormal approach.

Results: Within-day CVA, CVI, and CVG values were 8.3%, 9.7%, and 57%, respectively; the corresponding between-day values were 15%, 14%, and 63%. Within- and between-day IIs were 0.21 and 0.39, respectively. Lognormal within-day RCVs were 46% and −32%, respectively; the corresponding between-day values were 81% and −45%.

Conclusions: The low II indicates that population-based reference intervals are less useful for interpreting cTnI values than following serial changes in values in individual patients. This criterion is particularly important for interpreting results from patients who show cTnI increases at low concentrations measured with very high-sensitivity assays, from patients presenting with chest pain (short term), and for evaluating drugs for cardiotoxicity (long term).

Single molecule studies of multiple-fluorophore labeled antibodies. Effect of homo-FRET on the number of photons available before photobleaching

Advancements in single molecule detection (SMD) continue to unfold powerful ways to study the behavior of individual and complex molecular systems in real time. SMD enables the characterization of complex molecular interactions and reveals basic physical phenomena underlying chemical and biological processes. We present here a systematic study of the quenching efficiency of Förster-type energy-transfer (FRET) for multiple fluorophores immobilized on a single antibody. We simultaneously monitor the fluorescence intensity, fluorescence lifetime, and the number of available photons before photobleaching as a function of the number of identical emitters bound to a single IgG antibody. The detailed studies of FRET between individual fluorophores reveal complex through-space interactions. In general, even for two or three fluorophores immobilized on a single protein, homo-FRET interactions lead to an overall non-linear intensity increase and shortening of fluorescence lifetime. Over-labeling of protein in solution (ensemble) results in the loss of fluorescence signal due to the self-quenching of fluorophores making it useless for assays applications. However, in the single molecule regime, over-labeling may bring significant benefits in regards to the number of available photons and the overall survival time. Our investigation reveals possibilities to significantly increase the observation time for a single macromolecule allowing studies of macromolecular interactions that are not obscured by ensemble averaging. Extending the observation time will be crucial for developing immunoassays based on single-antibody.

Reference population and marathon runner sera assessed by highly sensitive cardiac troponin T and commercial cardiac troponin T and I assays

BACKGROUND

Endurance exercise can increase cardiac troponin (cTn) concentrations as high as those seen in cases of minor myocardial infarction. The inability of most cTn assays to reliably quantify cTn at very low concentrations complicates a thorough data analysis, and the clinical implications of such increases remain unclear. The application of recently developed highly sensitive cTn immunoassays may help resolve these problems.

METHODS

We evaluated the precommercial highly sensitive cardiac troponin T (hs-cTnT) assay from Roche Diagnostics and the Architect cardiac troponin I (cTnI-Architect) assay from Abbott Diagnostics by testing samples from a reference population of 546 individuals and a cohort of 85 marathon runners. We also measured the samples with the current commercial cTnT assay for comparison.

RESULTS

Although the hs-cTnT and cTnI-Architect assays were capable of measuring cTn concentrations at low concentrations (<0.01 microg/L), only the hs-cTnT assay demonstrated a CV of <10% at the 99th percentile of the reference population and a near-gaussian distribution of the measurements. After a marathon, 86% of the runners had cTnT concentrations greater than the 99th percentile with the hs-cTnT assay, whereas only 45% of the runners showed increased concentrations with the current cTnT assay. cTn concentrations remained significantly increased the day after the marathon. A multiple regression analysis demonstrated marathon experience and age to be significant predictors of postmarathon cTn concentrations (P < 0.05).

CONCLUSIONS

The hs-cTnT assay was the only assay tested with a performance capability sufficient to detect cTn concentrations in healthy individuals. The number of runners with increased cTn concentrations after a marathon depends highly on an assay's limit of detection (LOD). The assay with the lowest LOD, the hs-cTnT assay, showed that almost all runners had increased cTn concentrations. The clinical implications of these findings require further investigation.

Ultrasensitive Cross-species Measurement of Cardiac Troponin-I Using the Erenna Immunoassay System

Serum cardiac troponin-I (cTnI) has been validated as a biomarker for cardiotoxicity in numerous animal models; however, owing to sensitivity issues cTnI concentrations in healthy, resting animals used in toxicology studies have not been established. Serum from healthy and isoproterenol hydrochloride (iso)-treated rats, dogs, and monkeys were assayed using the Erenna system. The Erenna cTnI assay provided sensitivity < 1 ng/L across human, rat, dog, and monkey cTnI. Linear responses ( R2= 0.99) were observed for all species. Precision studies yielded interassay CVs of curve fit quantification from 2% to 4% between 1.6 and 5000 ng/L, and 23% at 0.78 ng/L. Strong correlation ( R2= 0.99) was obtained between Erenna and Beckman Access cTnI. Concentrations of cTnI in healthy animals ranged from 1 to 9 ng/L. In longitudinal studies of iso-treated animals, the concentrations of cTnI in the control vehicle-treated groups were 10–20 ng/L for rats (N = 10) and predose values of 2–3 ng/L for dogs (N = 3). Measured with the Erenna assay system, cTnI was quantifiable at all time intervals tested in all animals treated with iso. The Erenna system provides sensitive measurement of cTnI in rats, dogs, and monkeys, makes it possible to determine small changes from normal concentrations, and provides cTnI values from small volumes of serum.

Defining a role for novel biomarkers in acute coronary syndromes

BACKGROUND

Biomarkers play a pivotal role in the diagnosis and treatment of patients with cardiovascular disease. Active investigation has brought forward an increasingly large number of novel candidate markers; however, few of these markers have yet to be incorporated into routine clinical use.

CONTENT

This review discusses biomarkers currently used in the setting of acute coronary syndromes. In this context, we assess the contemporary unmet needs for novel biomarkers in acute ischemic heart disease and the related challenges faced in developing new biomarkers to the point of integration into clinical practice. In particular, we address the impact of the availability of increasingly sensitive biomarkers of myocardial necrosis on the potential roles for novel biomarkers of inflammation, thrombosis, and ischemia.

SUMMARY

Although active investigation has produced a growing list of candidate novel biomarkers for the care of patients with cardiovascular disease, it has become increasingly challenging to find appreciable incremental clinical benefit for their addition to existing markers, in particular newer, more analytically sensitive cardiac troponin assays. A major challenge for researchers and clinicians will be to demonstrate whether candidate novel markers are useful in improving diagnosis and guiding clinical treatment.

Highly Sensitive Cardiac Troponin T Values Remain Constant after Brief Exercise- or Pharmacologic-Induced Reversible Myocardial Ischemia

Background: Using a new precommercial high-sensitivity cardiac troponin T (hsTnT) assay, we evaluated whether hsTnT increases after reversible myocardial ischemia.

Methods: In 195 patients undergoing nuclear stress testing (ST) using single-photon emission computed tomography (SPECT) for suspected ischemic heart disease, we measured hsTnT before and 18 min, 4 h, and 24 h after the stress test. Thirty patients were excluded before ST because of cardiac troponin T (cTnT) &gt;30 ng/L (0.03 μg/L) as measured by the fourth-generation commercial test. Another 65 patients were excluded because of a combination of fixed and reversible perfusion defects (PDs) after SPECT.

Results: We studied 18 patients with reversible PDs, 41 patients with fixed PDs, and 41 patients without any PDs. Of these 100 patients, 61 received dynamic ST and 39 pharmacological ST. Median baseline hsTnT concentrations (25th, 75th percentile) were comparable in patients with reversible, fixed, and no PDs [5.57 (2.47, 12.60), 8.01 (4.55, 12.44), and 6.90 (4.63, 10.59) ng/L, respectively]. After ST, median hsTnT concentrations did not change in the reversible, fixed, or no PD groups from baseline to 18 min [−0.41 (−0.81, 0.01), 0.01 (−0.75, 0.79), and 0.36 (−0.42, 1.01) ng/L] or from baseline to 4 h [−0.56 (−1.82, 0.74), 0.24 (−0.60, 1.45), and 0.23 (−0.99, 1.15) ng/L]. Median baseline hsTnT concentrations tended to be higher in patients undergoing pharmacological vs dynamic ST; however, there were no significant increases in hsTnT concentrations after either type of ST.

Conclusions: Elevation of cTnT is rather a consequence of irreversible myocyte death than reversible myocardial ischemia after exercise or pharmacologic myocardial ischemia.

The determination of the 99th centile level for troponin assays in an Australian reference population

Background

Current guidelines for the diagnosis and risk assessment of patients presenting with myocardial infarction recommend a single decision cut-off point for cardiac troponin (cTn) based on the 99th centile of a reference population. The 99th centile level for eight troponin assays was determined in an apparently cardio-healthy Australian reference population.

Methods

Nine laboratories measured troponin in serum and plasma collected from 111 reference individuals. An imprecision profile was determined using up to 10 serum samples analysed on 10 separate days. Method comparison using 100 routinely tested plasma samples was performed to estimate method concordance.

Results

Generally 99th centile values determined in this study were lower than, or the same as manufacturers' levels, except for cTnI by Architect (0.020 vs. 0.012 μg/L), and imprecision at the 99th centile was 20% coefficient of variation (CV) or higher. Troponin concentrations at 10% CV were greater than those quoted in the manufacturer's package insert except by AxSYM, 0.06 vs. 0.16 μg/L cTnI, and by E-170, 0.02 vs. 0.03 μg/L cTnT. In the method comparison 74, 70, 65, 75, 58, 66, 58 and 77 samples measured by Access, Architect, AxSYM, Centaur, Dimension RxL, E-170, i-STAT and Vitros ECi assays, respectively, had troponin concentrations above the study 99th centile.

Conclusions

Depending on the selected reference population for troponin, there is likely to be variability in the 99th centile as shown in this study. Some differences in sample concordance at the 99th centile cut-off were observed between cTn methods and may result in different clinical classification.

The clinical need for high-sensitivity cardiac troponin assays for acute coronary syndromes and the role for serial testing

BACKGROUND

Cardiac troponin is the biomarker of choice for the serologic diagnosis of acute coronary syndromes. International cardiology and laboratory medicine guidelines have suggested that the cutoff concentration be set at the 99th percentile of a healthy population, with an assay imprecision of 10% or less. Unfortunately, most commercial troponin assays do not have the sensitivity and precision to reliably detect troponin in sera of healthy subjects. Therefore, there is a need to develop troponin assays with higher sensitivity, which cannot be achieved while also improving the assay's precision.

METHODS AND RESULTS

Novel prototype analytical testing devices have been developed that are 5- to 10-fold more sensitive than existing commercial troponin assays. These tests should enable an earlier detection of myocardial infarction relative to the time of presentation and detect a higher percentage of emergency department chest pain patients who are at risk for short-term major adverse cardiac events. However, use of a high-sensitivity troponin assay will also result in detection of more patients who have cardiac necrosis due to a nonischemic etiology.

CONCLUSIONS

Serial troponin testing will be necessary to determine the clinical significance of low levels of troponin release with use of high-sensitivity assays. Guidelines will need to be established to determine a change in troponin results that is statistically and clinically significant, and new considerations for the time interval needed between blood collections. This will enable the use of future high-sensitivity troponin assays to be more valuable.

An integrated optics microfluidic device for detecting single DNA molecules

A fluorescence-based integrated optics microfluidic device is presented, capable of detecting single DNA molecules in a high throughput and reproducible manner. The device integrates microfluidics for DNA stretching with two optical elements for single molecule detection (SMD): a plano-aspheric refractive lens for fluorescence excitation (illuminator) and a solid parabolic reflective mirror for fluorescence collection (collector). Although miniaturized in size, both optical components were produced and assembled onto the microfluidic device by readily manufacturable fabrication techniques. The optical resolution of the device is determined by the small and relatively low numerical aperture (NA) illuminator lens (0.10 effective NA, 4.0 mm diameter) that delivers excitation light to a diffraction limited 2.0 microm diameter spot at full width half maximum within the microfluidic channel. The collector (0.82 annular NA, 15 mm diameter) reflects the fluorescence over a large collection angle, representing 71% of a hemisphere, toward a single photon counting module in an infinity-corrected scheme. As a proof-of-principle experiment for this simple integrated device, individual intercalated lambda-phage DNA molecules (48.5 kb) were stretched in a mixed elongational-shear microflow, detected, and sized with a fluorescence signal to noise ratio of 9.9 +/-1.0. We have demonstrated that SMD does not require traditional high numerical aperture objective lenses and sub-micron positioning systems conventionally used in many applications. Rather, standard manufacturing processes can be combined in a novel way that promises greater accessibility and affordability for microfluidic-based single molecule applications.

Ultrasensitive flow-based immunoassays using single-molecule counting

BACKGROUND

Immunoassay (IA) technology has expanded the clinical utility of protein biomarkers, but demands for increased sensitivity, dynamic reporting ranges, and small sample volumes have limited the potential clinical usefulness of many biomarkers. We assessed the performance, including limits of detection (LODs) and the dynamic reporting range, of an IA-based technology, Erenna Immunoassay System, for a series of biomarkers, including cardiac troponin I (cTnI).

METHODS

Erenna IAs were used with 10 different and clinically important biomarkers to ascertain the LOD with various sample sizes (10 microL to 200 microL).

RESULTS

The Erenna Immunoassay System generated LODs of 10-100 pg/L using 100 microL of sample. For cTnI, the LOD was 0.2 ng/L and a 10% CV was seen between 0.78 and 1.6 ng/L.

CONCLUSIONS

The Erenna IA-based technology reproducibly measures protein biomarkers with detection limits of 10-100 pg/L, with a dynamic range of >4.5 logs in sample volumes of 50-150 microL.

Usefulness of detectable levels of troponin, below the 99th percentile of the normal range, as a clue to the presence of underlying coronary artery disease

The aim of the study was to evaluate whether markers of myocardial injury and ischemia are helpful in detecting coronary artery disease (CAD) in patients with stable angina. Venous blood was obtained before and after a bicycle exercise test in 47 outpatients with suspected CAD for measurement of cardiac troponin I (cTnI), heart-type fatty acid binding protein, and glycogen phosphorylase BB. Patients with a coronary artery stenosis >/=70% in diameter (n = 33) were compared with patients with coronary narrowing <50% (controls, n = 14). None of the markers increased after bicycle exercise testing. cTnI measured before and after exercise was higher in the CAD group than in controls (p <0.001). The area under the curve for diagnosis was greater when the cTnI value was detectable than with stress testing alone. In conclusion, baseline cTnI was of value in detecting CAD and also during follow-up in predicting the need for further revascularization.

Quantification of urinary 8-iso-PGF2alpha using liquid chromatography-tandem mass spectrometry and association with elevated troponin levels

OBJECTIVES

Increased lipid peroxidation (i.e. "oxidative stress") has been identified as a central mechanism in the development of atherosclerosis and inflammatory vascular damage. Measurement of 8-iso-PGF(2alpha) has demonstrated to be a reliable indicator of in vivo oxidative stress levels. The purpose of this study was to develop a rapid, sensitive, and specific LC-MS/MS method for detection of urinary 8-iso-PGF(2alpha), establish reference intervals, and correlate isoprostane levels with cardiac troponin I.

DESIGN AND METHODS

Urinary 8-iso-PGF(2alpha) was detected after direct injection onto a C18 silica column and monitored in the MRM mode using m/z transitions of 353.2>193.25 (8-iso-PGF(2alpha)) and 357.2>197.25 (8-iso-PGF(2alpha)-d(4)). The LC-MS/MS method was also compared to an ELISA kit. Reference interval studies were evaluated against a separate population of patients presenting with chest pain that had positive cTnI values.

RESULTS

Elution of 8-iso-PGF(2alpha) was achieved after 7 min, with a total run time of 10 min. Inter-assay CVs were 13.8-20.0% and intra-assay CVs were 10.9-17.0%. Linearity ranged from 100 pg/mL to 100 ng/mL. Deming regression of ELISA and LC-MS/MS methods for 8-iso-PGF(2alpha) levels yielded poor correlation, with a slope of 0.0265, y-intercept of 0.255 ng/mL, and R(2) value of 0.0434. Urine 8-iso-PGF(2alpha) concentrations in samples obtained from healthy individuals (n=34) ranged from 57 to 390 ng/g creatinine with a mean of 221 ng/g creatinine. 8-iso-PGF(2alpha) levels were statistically significant in troponin-positive (n=35) versus troponin-negative (n=36) patients (p<0.0049).

CONCLUSIONS

This LC-MS/MS method provides a rapid, accurate, sensitive, and cost-effective alternative to other methods for detection of 8-iso-PGF(2alpha) in urine. 8-iso-PGF(2alpha) has potential to be a great prognostic risk indicator in individuals with a high probability for future coronary events.