Ultrasensitive flow-based immunoassays using single-molecule counting

The dynamic range problem in the analysis of the plasma proteome

One of the greatest challenges in analyzing the plasma proteome is the wide range of concentration of different proteins. The current study examines the range of protein concentration for 18 proteins measured over a year in a clinical laboratory to provide data on pathological extremes in protein concentrations. The complete measured range, from upper limits for albumin to lowest values for thyroid-stimulating hormone (TSH), represented more than 10 logs of molar abundance. A number of plasma proteins measured in the clinical laboratory varied over a concentration range spanning more than 4 logs, and limits of detection of clinical assays were inadequate to assess full concentration ranges of several proteins. Considering reported values from studies using higher sensitivity assays suggest that plasma concentrations of prostate-specific antigen (PSA), human chorionic gonadotropin (hCG), and cardiac troponin I vary by more than 7 logs. All of the plasma proteins measured in the present study represent secretory proteins or highly expressed components of specific tissues. Thus, the dynamic range for these components is likely to greatly underestimate the total range of protein concentration in the plasma proteome.

Autoantibodies to Cardiac Troponin Associate with Higher Initial Concentrations and Longer Release of Troponin I in Acute Coronary Syndrome Patients

Background: Cardiac troponin (cTn) is an established marker of myocardial infarction. Pronounced heterogeneity and the minute amounts released into the circulation constitute significant challenges for cTn detection. Recently, autoantibody formation to cTn was shown to be common and to interfere with immunoassay performance. In this study, we investigated cTn autoantibodies and cardiac troponin I (cTnI) in acute coronary syndrome (ACS) patients over a 1-year period after the index event.

Methods: We used a second-generation cTnI assay designed to reduce the interference of cTn autoantibodies. The assay for cTn autoantibodies used 2 anti-cTnI antibodies to capture the ternary cTnI-complex, enabling unrestricted binding of the autoantibodies, which were detected with a labeled antihuman IgG antibody. We analyzed serum samples from 81 non–ST-elevation ACS patients taken at admission and after 1 week and 3 and 12 months.

Results: We found 14 cTn autoantibody–positive patients (21%) among the 67 cTnI-positive and none among the 14 cTnI-negative patients. Nine were autoantibody-positive at admission, and 5 became positive at 1 week. Autoantibody signals significantly increased in the 1-week and 3-month samples. At all time points, cTnI was significantly increased in the autoantibody-positive group relative to the negative group. Persistent cTnI elevations at 3 and 12 months were seen in the patients already autoantibody positive at admission.

Conclusions: During ACS, patients with cTn autoantibodies have higher cTnI release and therefore larger myocardial damage than patients without autoantibodies. Their cTnI release also lasts longer, at least months. The possible prognostic impact of these observations must be evaluated in larger clinical cohorts.

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).

Interpretation of high sensitivity cardiac troponin I results: reference to biological variability in patients who present to the emergency room with chest pain: case report series

BACKGROUND

The development of highly sensitive cardiac troponin (cTnI) assays has increased the number of true and false positive results for patients suspected of acute myocardial infarction (AMI). Cases are reported whereby the use of serial testing, the 99th percentile cutoff, and the application of biological variation of cTnI were used to help determine ischemic vs. non-ischemic causes of myocardial injury.

METHODS

cTnI was measured using the Siemens Ultra assay from 13 representative patients who presented to the emergency department with symptoms suggestive of acute cardiac disease. Based on a previous study, reference change values of a 46% increase and 32% decrease were used to interpret results. These differences were compared against the patient's discharge diagnosis.

RESULTS

Two patients who subsequently ruled in for AMI had a negative cTnI (<0.04 microg/l) and borderline positive cTnI (0.07 microg/l) at admission, respectively. While the 4-6 h results were also borderline, there was a significant increase from the baseline (+575% and +50%, respectively) to suggest the presence of an acute cardiac event. Two other AMI cases document the significant cTnI decline in results after peak values. In 7 other non-AMI cases (heart and renal failure, gastrointestinal bleeding, stroke and venous thrombosis), while baseline concentrations were clearly positive (0.18-2.12 microg/l), subsequent serial samples were not significantly increased or decreased from baseline. These findings were not typical for AMI. There were 2 cases with acute blunt cardiac trauma and intracranial hemorrhage, respectively, that produced cTnI results that were initially low (<0.04 and 0.05 microg/l, respectively), but significantly increased with serial testing thereby producing false positive Delta cTnI results for AMI.

CONCLUSIONS

Serial testing for troponin was useful in differentiating early AMI from non-ischemic causes of troponin increases. However, non-AMI patients with acute cardiac injury can produce troponin results that mimic AMI. Therefore serial troponin testing must be used in conjunction with clinical presentation and other laboratory findings.

Detection of acute changes in circulating troponin in the setting of transient stress test-induced myocardial ischaemia using an ultrasensitive assay: results from TIMI 35

AIMS

To determine whether an ultrasensitive assay can permit quantification of changes in circulating cardiac troponin (Tn) in the setting of stress test-induced myocardial ischaemia.

METHODS AND RESULTS

Blood samples were obtained before, immediately after, and 2 and 4 h after stress testing with nuclear perfusion imaging in 120 patients. Troponin was measured using commercial assays as well as with a novel, ultrasensitive cardiac TnI assay with a limit of detection of 0.2 pg/mL. Using the ultrasensitive assay, TnI was detectable in all patients before stress testing (median 4.4 pg/mL, interquartile range 3.1-8.6 pg/mL). By 4 h, troponin levels were unchanged in patients without ischaemia, whereas circulating levels had increased by a median of 1.4 pg/mL (24% increase) in patients with mild ischaemia (P = 0.002) and by 2.1 pg/mL (40% increase) in patients with moderate-to-severe ischaemia (P = 0.0006). In contrast, changes in troponin levels across patients in different ischaemic categories were indistinguishable using commercial troponin assays. When added to clinical factors, a >1.3 pg/mL increase in TnI using the ultrasensitive assay was an independent predictor of ischaemia (odds ratio 3.54, P = 0.007).

CONCLUSION

Transient stress test-induced myocardial ischaemia is associated with a quantifiable increase in circulating troponin that is detectable with a novel, ultrasensitive TnI assay.

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.

Concentrations of cardiac Troponin I before and after ovariohysterectomy in 46 female dogs with pyometra

BACKGROUND

Canine pyometra is a common disease in countries where routine spaying of young dogs is not common practice. This disease is known to lead to systemic inflammation potentially affecting multiple organs in the body, including the heart. Cardiac-specific Troponin I (cTnI) is a sensitive marker of myocardial cell damage, which can result from ischemia, trauma, toxins or inflammation. Dogs with pyometra are also exposed to anaesthesia which can potentially result in myocardial cell damage. The aims of the study were 1) to evaluate the occurrence of myocardial cell damage as indicated by increased serum concentrations of cTnI in dogs with pyometra and relate these to presence of systemic inflammation and 2) to evaluate the change in cTnI-concentrations after anaesthesia and surgery.

METHODS

Serum cTnI concentration was measured preoperatively and one day after surgery in 46 female dogs with pyometra and 15 female dogs that underwent surgery for other reasons (ovariohysterectomy and mammary tumours).

RESULTS

Forty-six female dogs of different breeds diagnosed with pyometra were included. The dogs had a median age of 8.5 years (IQR 7.5-10) and a median weight of 29 kg (IQR 9-32). Of the 46 dogs, 37 (80%) fulfilled the chosen criteria for systemic inflammatory response syndrome (SIRS) at inclusion. Thirteen (28%) of the dogs had increased cTnI concentrations (> 0.2 microg/l) before surgery and 18 (39%) had increased cTnI-concentrations the day after surgery. The cTnI concentrations in the 13 dogs with increased preoperative cTnI concentrations decreased in 8 dogs, increased in 4 dogs, and was unchanged in one dog. Seven dogs with nondetectable preoperative cTnI concentrations had increased postoperative concentrations. The only significant association between the studied laboratory or clinical variables (including SIRS) and cTnI concentration was preoperative percentage band neutrophils (PBN) and postoperative cTnI concentration (P = 0.016). In total, 20 dogs (43%) had increased pre- or postoperative cTnI concentrations. Seven dogs (15%) had pre-or postoperative concentrations of cTnI of 1.0 microg/l or higher.

CONCLUSION

Mild to moderate increases in cTnI appears to be common in dogs with pyometra before and after surgery, but the clinical importance of this finding is uncertain. None of the studied clinical variables were found to reliably predict increased preoperative cTnI concentrations. Because of the pre- and postoperative variation in cTnI concentrations, it was not possible to identify a negative effect of anaesthesia and surgery on myocardial cell integrity.

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.

Cardiac troponin is the most effective translational safety biomarker for myocardial injury in cardiotoxicity

There is an overwhelming weight of evidence that certifies cardiac troponin (cTn) as the preferred, defacto, translational, safety biomarker for myocardial injury in cardiotoxicity. As well as being the gold standard for cardiac injury in man, it has been widely used for clinical assessment and monitoring of cardiac toxicity in humans being treated for cancer. Furthermore, several dozen preclinical published studies have directly confirmed its effectiveness in laboratory animals for assessment of cardiotoxicity. It is gradually being reverse translated from human into animal use as a safety biomarker. Its use is especially merited whenever there is any safety signal indicating potential cardiotoxicity and its required inclusion as a routine biomarker in preclinical safety studies seems on the horizon. There are some considerations that are unique to use of cTn assays in animals. Lack of awareness of these has, historically, significantly inhibited the introduction of cTn as a safety biomarker in preclinical toxicology. Firstly, cross-species reactivity is usually but not always high. Secondly, there is a background of cardiac injury that needs to be controlled for, including spontaneous cardiomyopathy in Sprague Dawley rats, and inappropriate blood collection methods. Also, there are faster kinetics of clearance in rats than for humans. Also, coincident muscle injury is frequent with cardiotoxicity and requires a skeletal muscle biomarker. Because cTn assays were developed for detection of gross cardiac necrosis, such as occurs with myocardial infarct, the more sensitive assays should be used for preclinical studies. However, analytic sensitivity is higher for standard preclinical studies than for clinical diagnostic testing because of use of concurrent controls and use of batch analysis that eliminates interassay variability. No other biomarker of myocardial injury comes close to cTn in effectiveness, including CK-MB, LDH-1 and 2, myoglobin, and FABP3. In addition to the use of cTn for monitoring active myocardial degeneration, there is growing evidence that measurements of brain natriuretic peptide (BNP) may be effective for monitoring drug-induced left ventricular dysfunction.