New approach to standardisation of human cardiac Troponin I (cTnI)

Imprecision of cardiac marker analyses among laboratories on the basis of external quality assurance results: Finnish experience

The purpose of this report was to evaluate the reproducibility and harmonization of cardiac marker tests and to describe the current situation concerning quality of assays for cardiac markers on the basis of the results of the external quality control schemes (EQAS) of Labquality Ltd., Helsinki, Finland in the period 2002 to 2005. Finnish EQAS surveys obtained for proficiency samples at low marker concentration indicated that the overall coefficient of variation (CV) between laboratories for CK-MBmass and troponin I exceeded 10 %, while for cardiac troponin T the CV was 8.6 %. Intra-laboratory reproducibility was investigated in a single laboratory using concomitant testing in the same EDTA plasma samples to establish cut-off limits for one CK-MBmass and three troponin assays. The 10 % imprecision limit obtained from the concomitant testing in the same samples for CK-MBmass was (by Elecsys) 8.5 microg/L, for cardiac troponin T (by Elecsys) 0.023 microg/L and for cardiac troponin I (by AxSYM) and by Immulite 2000) 0.85 microg/L and 0.63 microg/L. At present, it is recommended that laboratories determine the concentration at which the 10 % imprecision for a specific cardiac marker assay is reached, because the assays generally do not reach that imprecision at the level of the 99th percentile value, usually taken as decisional level. However, common efforts of scientific societies and professional diagnostic industry associations internationally are needed if consensus is to be reached on standardization of immunoassays for cardiac markers and uniform results obtained among laboratories.

Present and Future Biochemical Markers for Detection of Acute Coronary Syndrome

The use of biochemical markers in the diagnosis and management of patients with acute coronary syndrome has increased continually in recent decades. The development of highly sensitive and cardiac-specific troponin assays has changed the view on diagnosis of myocardial infarction and also extended the role of biochemical markers of necrosis into risk stratification and guidance for treatment. The consensus definition of myocardial infarction places increased emphasis on cardiac marker testing, with cardiac troponin replacing creatine kinase MB as the "gold standard" for diagnosis of myocardial infarction. Along with advances in the use of more cardiac-specific markers of myocardial necrosis, biochemical markers that are involved in the progression of atherosclerotic plaques to the vulnerable state or that signal the presence of vulnerable plaques have recently been identified. These markers have variable abilities to predict the risk of an individual for acute coronary syndrome. The aim of this review is to provide an overview of the well-established markers of myocardial necrosis, with a special focus on cardiac troponin I, together with a summary of some of the potential future markers of inflammation, plaque instability, and ischemia.

The importance of analytical quality specifications for biomarker assays currently used in acute cardiac care

It is very important that cardiac biomarkers on which clinically relevant decisions will rest are measured with highly reliable assays. Adequate studies are needed before new methods can be implemented in the laboratory routine, and only well-documented assays should be considered for clinical use. Therefore, it is critical that, as new biomarkers are proposed, quality specifications are developed. Only after appropriate analytical quality specifications are addressed, the issues pertaining to methodological differences that result in non-harmonized concentration values, and clinical interpretation of biomarker concentrations will be reconciled. Today, the technology to address many analytic problems is at hand, but commitment on the part of manufacturers and their customers in the laboratory and clinical communities is essential. The design control loop is not closed until the finished in vitro diagnostic system is adequately validated to meet the customer needs, including analytical quality specifications. It is essential to determine the attributes and performance characteristics of relevant competitive systems and their degree of acceptance by clinical laboratories in order to demonstrate that user needs are definitely met. The responsibility of defining and implementing these issues must be a shared responsibility among laboratorians, clinicians, industry, and regulatory agencies on an international front. To date, two sets of quality specifications have been published, one for cardiac troponin assays and one for B-type natriuretic peptide assays. Both address analytical factors, such as calibrator characterization, antibody specificity, assay sensitivity and imprecision, and interferents, as well as preanalytical factors, such as sample type and stability. It would be ideal if regulatory agencies, such as FDA in the United States, accept these criteria for premarket approval clearance applications.

Beckman Access versus the Bayer ACS:180 and the Abbott AxSYM cardiac Troponin-I real-time immunoassays: an observational prospective study

Background

Reliability of cardiac troponin-I assays under real-time conditions has not been previously well studied. Most large published cTnI trials have utilized protocols which required the freezing of serum (or plasma) for delayed batch cTnI analysis. We sought to correlate the presence of the acute ischemic coronary syndrome (AICS) to troponin-I values obtained in real-time by three random-mode analyzer immunoassay systems: the Beckman ACCESS (BA), the Bayer ACS:180 (CC) and the Abbott AxSYM (AX).

Methods

This was an observational prospective study at a university tertiary referral center. Serum from a convenience sampling of telemetry patients was analyzed in real-time for troponin-I by either the BA-CC (Arm-1) or BA-AX (Arm-2) assay pairs. Presence of the AICS was determined retrospectively and then correlated with troponin-I results.

Results

100 patients were enrolled in Arm-1 (38 with AICS) and 94 in Arm-2 (48 with AICS). The BA system produced 51% false positives in Arm-1, 44% in Arm-2, with negative predictive values of 92% and 100% respectively. In Arm-1, the BA and the CC assays had sensitivities of 97% and 63% and specificities of 18% and 87%. In Arm-2, the BA and the AX assays had sensitivities of 100% and 83% and specificities of 11% and 78%.

Conclusions

In real-time analysis, the performance of the AxSYM and ACS:180 assay systems produced more accurate troponin-I results than the ACCESS system.

Comparison of cardiac troponin T and I in healthy men and in aortic valve replacement

Troponins are of outstanding importance for the diagnosis of myocardial infarction. Cardiac troponin T (cTnT) and the various cardiac troponin I (cTnI) assays differ with respect to method comparison, diagnostic sensitivity and diagnostic specificity. To understand the differences in the diagnostic behavior of troponin assays, AccuTnI and Elecsys Troponin STAT were used in a group of healthy men and in the follow-up of patients with aortic valve replacement (AVR). Within the healthy subjects AccuTnI was able to differentiate two subgroups from each other, whereas the cTnT concentrations of all subjects were below the detection limit. In AVR patients, cTnT and cTnI correlated sufficiently, if the postoperative periods were taken into consideration. There was a rapid increase in cTnI within 24 h. In contrast, a broad peak was evident for cTnT between 48 and 120 h. The results emphasize more the differences in the release of cTnI and cTnT from the cytoplasm and the thin filaments of the cardiomyocytes than the modifications of the troponins circulating in the blood.

The measurement of cardiac markers: where should we focus?

Cardiac markers are presently a hot topic, with active debate on their use. They now have a major role for cost-effective management of acute chest pain and suspected acute coronary syndrome. The laboratory has a pivotal role in proper selection and interpretation of available markers, depending on the creation of evidence-based knowledge about test utilization and sources of variation. This article reviews this knowledge in the field of biomarkers determination and summarizes the major analytic and clinical issues, with reference to various recent recommendations of laboratory medicine and cardiology expert groups.

Measurement of cardiac troponins

The cardiac troponins form part of the regulatory mechanism for muscle contraction. Specific cardiac isoforms of cardiac troponin T and cardiac troponin I exist and commercially available immunoassay systems have been developed for their measurement. A large number of clinical and analytical studies have been performed and the measurement of cardiac troponins is now considered the 'gold standard' biochemical test for diagnosis of myocardial damage. There have been advances in understanding the development and structure of troponins and their degradation following myocardial cell necrosis. This has contributed to the understanding of the problems with current assays. Greater clinical use has also highlighted areas of analytical and clinical confusion. The assays are reviewed based on manufacturers' information, current published material as well as the authors' in-house experience.

Standardization of Cardiac Troponin I Assays: Round Robin of Ten Candidate Reference Materials

Background: Cardiac troponin I (cTnI) results vary 100-fold among assays. As a step toward standardization, we examined the performance of 10 candidate reference materials (cRMs) in dilution studies with 13 cTnI measurement systems.

Methods: Solutions of 10 cTnI cRMs, each characterized by NIST, were shipped to the manufacturers of 13 cTnI measurement systems. Manufacturers used their respective diluents to prepare each cRM in cTnI concentrations of 1, 10, 25, and 50 μg/L. For the purpose of ranking the cRMs, the deviation of each cTnI measurement from the expected response was assessed after normalization with the 10 μg/L cTnI solution. Normalized deviations were examined in five formats. Parameters from linear regression analysis of the measured cTnI vs expected values were also used to rank performance of the cRMs.

Results: The three cRMs demonstrating the best overall rankings were complexes of troponins C, I, and T. The matrices for these three cRMs values differed; one was reconstituted directly from the lyophilized form submitted by the supplier; one was submitted in liquid form, lyophilized at NIST, and subsequently reconstituted; and the third was evaluated in the liquid form received from the supplier. The cRM demonstrating the fourth best performance was a binary complex of troponins C and I supplied in lyophilized form and reconstituted before distribution.

Conclusions: The cRMs demonstrating the best performance characteristics in 13 cTnI analytical systems will be included in subsequent activities of the cTnI Standardization Committee of the AACC.

Time-resolved fluorometry (TRF)-based immunoassay concept for rapid and quantitative determination of biochemical myocardial infarction markers from whole blood, serum and plasma

We report the development of a time-resolved fluorometry-based immunoassay concept for the rapid measurement of three cardiac markers from whole blood, serum or plasma. Using a universal all-in-one (AIO) dry reagent concept, all the analyte specific reagents are built into a single microtire well, to which an identical assay protocol is applied. Addition of 5-20 microL sample (whole blood, serum or plasma) together with a universal buffer initiates the reaction, which is brought close to equilibrium in 15 min. After the wash step the Eu chelate-derived signal is measured directly from the dried surface. Application of this concept to the three cardiac markers illustrates its ability to provide rapid, highly sensitive and fully quantitative results over a large dynamic range with good reproducibility. Such a performance, especially when using whole blood specimens, is largely a consequence of the inherently fluorescent and stable Eu-chelate employed in the system. Correlation to commercial assays was excellent for all three analytes, as was between-sample matrix correlation using the AIO assays. The presented assay concept enabling a simple automation is particularly suited for point-of-care applications, where the performance characteristics are fully comparable to state-of-the-art central laboratory immunoassays.