Commutability of Possible External Quality Assessment Materials for Cardiac Troponin Measurement

Background

The measurement of cardiac troponin is crucial in the diagnosis of myocardial infarction. The performance of troponin measurement is most conveniently monitored by external quality assessment (EQA) programs. The commutability of EQA samples is often unknown and the effectiveness of EQA programs is limited.

Methods

Commutability of possible EQA materials was evaluated. Commercial control materials used in an EQA program, human serum pools prepared from patient samples, purified analyte preparations, swine sera from model animals and a set of patient samples were measured for cTnI with 4 assays including Abbott Architect, Beckman Access, Ortho Vitros and Siemens Centaur. The measurement results were logarithm-transformed, and the transformed data for patient samples were pairwise analyzed with Deming regression and 95% prediction intervals were calculated for each pair of assays. The commutability of the materials was evaluated by comparing the logarithmic results of the materials with the limits of the intervals. Matrix-related biases were estimated for noncommutable materials. The impact of matrix-related bias on EQA was analyzed and a possible correction for the bias was proposed.

Results

Human serum pools were commutable for all assays; purified analyte preparations were commutable for 2 of the 6 assay pairs; commercial control materials and swine sera were all noncommutable; swine sera showed no reactivity to Vitros assay. The matrix-related biases for noncommutable materials ranged from −83% to 944%. Matrix-related biases of the EQA materials caused major abnormal between-assay variations in the EQA program and correction of the biases normalized the variations.

Conclusion

Commutability of materials has major impact on the effectiveness of EQA programs for cTnI measurement. Human serum pools prepared from patient samples are commutable and other materials are mostly noncommutable. EQA programs should include at least one human serum pool to allow proper interpretation of EQA results.     

Requirements for reference (calibration) laboratories in laboratory medicine

In addition to reference measurement procedures and reference materials, reference or calibration laboratories play an integral role in the implementation of measurement traceability in routine laboratories. They provide results of measurements using higher-order methods, e.g. isotope dilution mass spectrometry and may assign values to materials to be used for external quality assessment programs and to secondary reference materials. The requirements for listing of laboratories that provide reference measurement services include a statement of the metrological level or principle of measurement, accreditation as a calibration laboratory according to ISO 15195 and the participation in a proficiency testing system (regular inter-laboratory comparisons) for reference laboratories. Ring trials are currently conducted for thirty well-defined measurands and the results are made available to all laboratories. Through the use of reference laboratory services that are listed by the Joint Committee for Traceability in Laboratory Medicine there is the opportunity to further promote traceability and standardisation of laboratory measurements.     

Traceability, reference systems and result comparability

The standardisation of measurements is of high priority in laboratory medicine, its purpose being to achieve closer comparability of results obtained using routine measurement procedures. At present, there is international cooperation in developing reference measurement systems (reference methods, reference materials, and reference laboratory networks) for analytes of clinical significance. These reference systems will reduce, wherever possible, measurement uncertainty and promote the comparability of results. The implementation of measurement traceability through the reference system provides one of the most important tools that supports the standardisation process in laboratory medicine. It aims to achieve result comparability regardless of the measurement procedure (test kit) and the clinical laboratory where analyses are carried out. The aim of this review is to discuss some concepts related to the achievement of standardisation by the implementation of a metrologically-correct measurement system and to provide some examples that illustrate the complexity of this approach and the impact of these activities on patient care.     

Harmonisation of Measurement Procedures: how do we get it done?

Clinical laboratory measurement results must be comparable among different measurement procedures, different locations and different times in order to be used appropriately for identifying and managing disease conditions. Harmonisation in the broad sense is the overall process of achieving comparability of results among clinical laboratory measurement procedures that measure the same measurand. The term standardisation is used when comparable results among measurement procedures are based on calibration traceability to SI using a reference measurement procedure of the highest available order. When there is no higher order reference measurement procedure available, and it is unlikely that one can be developed, the term harmonisation refers to any process for achieving comparable results among measurement procedures for an individual measurand.This review explains calibration traceability and focuses on the principles of harmonisation for those measurands for which a reference measurement procedure does not exist. We discuss the value of harmonisation, the importance of commutable reference materials, the barriers to harmonisation that exist today, and conclude with a discussion of a current global effort to improve the state of harmonisation.     

Traceability in clinical enzymology

The primary goal of standardisation for measurements of catalytic concentrations of enzymes is to achieve comparable results in human samples, independent of the reagent kits, instruments and laboratory where the assay is carried out. In order to pursue this objective, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) has established reference systems for the most important clinical enzymes. These systems are based on three requirements: a) reference measurement procedures that are extensively evaluated and carefully described; b) certified reference materials; and c) a network of reference laboratories operating in a highly controlled manner. Using these reference systems and the manufacturer's standing procedures, industry can assign traceable values to commercial calibrators. Clinical laboratories, which use routine procedures with validated calibrators to measure human specimens, can finally obtain values which are traceable to higher-order reference procedures. These reference systems constitute the structure of the traceability chain to which the routine methods can be linked via an appropriate calibration process, provided that they have a comparable specificity (i.e. they are measuring the same quantity).     

Standardization of insulin and C-peptide – A status report

ObjectiveIt is important that assays used for insulin and C-peptide testing are metrologically traceable and meet predefined quality specifications. Realizing this is the mission statement of the Insulin Standardization Work Group and the C-Peptide Standardization Committee.Materials and methodsBoth groups adopt the traceability concept as described by the International Organization for Standardization. It is based on a higher-order reference measurement system, defining the measurand and système internationale d’unités, and comprising a primary calibrator (highly purified standard materials with certified property values in mass units of milligram), trueness-based reference measurement procedures (ReMPs) and secondary calibrators. Both groups developed already isotope dilution-mass spectrometry candidate ReMPs, investigated different options of secondary calibrators and negotiated with the World Health Organization to obtain the required certified primary calibrators. Meanwhile, the first studies have been done to look into the feasibility of using a reference measurement system for the purpose of establishing traceability of immunoassays, while judging on their quality of performance.ResultsThe studies showed that native sera, assigned with reference measurement values, are the best secondary calibrators. They also demonstrated the feasibility of establishing traceability of immunoassays from a method comparison study with a candidate ReMP and use of the relationship of the results as a recalibration basis.ConclusionBoth groups aim at completing the establishment of the reference measurement system. They intend to provide the in-vitro diagnostic manufacturers with a panel of single-donor sera assigned with ReMP values for recalibration of their assays and to follow-up this process by periodic surveillance.     

Reference materials and reference measurement procedures: an overview from a national metrology institute

An outline of the processes involved in both certified clinical reference material production and clinical reference measurement procedure development at the National Institute of Standards and Technology (NIST), the national metrology institute of the United States, is presented. The role that NIST and other national metrology institutes play in the metrological traceability of certified reference material is discussed. Highlighted are the challenges associated with the development of reference measurement systems for complex clinical analytes, such as proteins, and examples of existing efforts in this area are given. Examples of recent international collaborations in developing certified reference materials for analytes such as cardiac troponin I, brain natriuretic peptide, and serum creatinine demonstrate the close cooperation that national metrology institutes must have with the clinical community to establish complete reference measurement systems.     

Introduction to standardization of laboratory results

A fundamental goal of laboratory medicine is that laboratory results will be comparable or standardized and be independent of the laboratory where the testing was performed. Routine measurement procedures that are traceable to the same system of reference standards should produce numerical values for clinical samples that are comparable regardless of time, place, or laboratory generating the result. Standardization of laboratory measurements is key to providing accurate and reliable results from investigational studies and for optimal patient care.     

Design considerations for proteomic reference materials

In order to improve the repeatability, comparability, and accuracy of MS-based proteomic measurements, there has been considerable international effort to develop appropriate reference materials. Although the majority of reference materials are developed to support measurement quality of routine assays, the development of reference materials for a diverse and changing research field such as proteomics represents unique challenges. In order to define common measurement components and common features of typical proteomic samples, the metrology underpinning proteomics must be considered due to the diversity and changing nature of the field. Reference materials can then be designed around common aspects in order to produce reference materials with the broadest applicability. Reference materials are needed to support both qualitative and quantitative proteomic measurements, involving different design considerations. Consensus and validated statistical approaches to describe the confidence in qualitative measurement, such as protein identification, needs to be established. Common sources of measurement bias also need to be considered in proteomic reference material design.     

Testosterone and estradiol assays: current and future trends

Sex steroid measurements for the investigation of endocrine disorders have been fraught with accuracy and imprecision problems since the advent of high throughput, direct assays almost 10 years ago on automated analyzers. Results from testosterone and estradiol measurements at the low end of detectability have suffered the most and there are few automated systems that can accurately measure these steroids in women, children and hypogonadal males on a routine basis. With the advent of mass spectrometry coupled to either gas chromatography or liquid chromatography, an improved approach to the measurement of these steroids has developed that shows promise for accurately and precisely measuring testosterone and estradiol in all patient populations including women and children. These mass spectrometry based methods for the sex steroids have been established as higher order reference method procedures that will resolve the issues of low end sensitivity measurements for these steroids, provide for appropriate standardization and reference materials and align most laboratories in hospital and reference laboratories to generate results that are inter-changeable between laboratories and methods.     

质量平衡法及其在标准物质定值中的应用进展

为保证不同地区、不同时间测量结果的可比性，测量结果需溯源至适当的、规定的参考标准。对于化学、生物、工程、物理学领域的材料和样品测量，该参考标准为标准物质。由此可见，标准物质的定值对物质的检测及定量是十分重要的。标准物质（reference material，RM）是一种足够均匀的、具有一种或多种相对容易确定的特性值的材料或物质，可用于给材料赋值、评价测量方法及校准测量仪器等。质量平衡法作为标准物质的定量方法之一，是一种常用的纯度测量方法，将水分、灰分、挥发组分、无机元素等杂质的含量从100%中扣除，再根据主要组分在有机组分中的百分比来确定物质纯度。质量平衡法具有较高准确度，能够溯源到国际单位制中的质量单位，且若使用基准方法测量样品中的主成分及各部分杂质以完成整个质量平衡法的测量，质量平衡法则有望成为新的基准方法。基于此，对质量平衡法原理及质量平衡法在标准物质的研制中的应用进行了介绍，并对近期质量平衡法在标准物质中的最新应用进行了总结，以期探索质量平衡法在标准物质研制中的更多可能。     

Determination of total arsenic concentrations in nails by inductively coupled plasma mass spectrometry

The analysis of trace elements in biological samples will extend our understanding of the impact that environmental exposure to these elements has on human health. Measuring arsenic content in nails has proven useful in studies evaluating the chronic body burden of arsenic. In this study, we developed methodology with inductively coupled plasma-mass spectrometry (ICP-MS) for the determination of total arsenic in nails. We assessed the utility of the washing procedures for removing surface contamination. Four types of preanalysis treatments (water bath, sonication, water bath plus sonication, and control) after sample decomposition by nitric acid were compared to evaluate the digestion efficiencies. In addition, we studied the stability of the solution over 1 wk and the effect of acidity on the arsenic signal. Arsenic content in the digested solution was analyzed by using Ar-N₂ plasma with Te as the internal standard. The results suggest that washing once with 1% Triton Χ-100 for 20 min for cleaning nail samples prior to ICP-MS analysis is satisfactory. Repeated measurement analysis of variance revealed that there was no significant difference among the various sample preparation techniques. Moreover, the measurements were reproducible within 1 wk, and acidity seemed to have no substantial influence on the arsenic signal. A limit of detection (on the basis of three times the standard deviation of the blank measurement) of 7 ng As/g toenail was achieved with this system, and arsenic recoveries from reference materials (human hair and nails) were in good agreement (95–106% recovery) with the certified/reference values of the standard reference materials. ICP-MS offers high accuracy and precision, as well as highthroughput capacity in the analysis of total arsenic in nail samples.     

Overview, conclusions, and recommendations of the IPCS collaborative study on complex mixtures.

The International Programme on Chemical Safety (IPCS) sponsored an international collaborative study to examine the variability associated with the extraction and bioassay of standard reference materials (SRMs) that are complex environmental mixtures provided by the U.S. National Institute of Standards and Technology (NIST). The study was also intended to evaluate the feasibility of establishing bioassay reference values and ranges for the SRMs. Twenty laboratories from North America, Europe, and Japan participated in the study. As part of the mandatory core protocol, each laboratory extracted the organic material from two particulate samples and bioassayed these extracts. A coal tar polycyclic aromatic hydrocarbon (PAH) solution and two mutagenic control compounds were also subjected to bioassay without prior extraction by the participating laboratories. The bioassay used was the Salmonella/microsomal plate incorporation assay. For the optional portion of the study, a laboratory was free to use the SRMs for any type of exploratory research. The primary purpose of the required portion of the study was to estimate the intra- and inter-laboratory variability in mutagenic potencies of the test materials and to determine whether or not the NIST mixtures could be used as reference materials by others performing the Salmonella assay. Repeatability (intra-laboratory variance) of the bioassay results ranged from 16% to 88% depending on the SRM and the bioassay conditions (tester strain and metabolic activation), whereas reproducibility (inter-laboratory variance) ranged from 33% to 152%. Between-laboratory variability was the main source of variation accounting for approximately 55-95% of the total variation for the three environmental samples. Variation in the mutagenic potency of the control compounds was comparable, with the exception of 1-nitropyrene for which the reproducibility ranged from 127% to 132%. In summary, NIST SRMs provided useful materials for an international inter-laboratory study of complex mixtures. By establishing both intra- and inter-laboratory variance for the mutagenicity results for these materials, the usefulness of these SRMs as reference materials for the Salmonella bioassay was established, critical procedures within the bioassay protocol were identified, and recommendations for future efforts were delineated.     

Correction of timing errors in photomultiplier tubes used in phase-modulation fluorometry

The measurement of fluorescence lifetimes is known to be hindered by the wavelenght-dependent and photocathode area-dependent time response of photomultiplier tubes. A simple and direct method is described to minimize the effects in photomultiplier tubes for phase-modulation fluorometry. Reference fluorophores of known lifetime were used in place of the usual scattering reference. The emission wavelenghts of the reference and sample were matched by either filters or a monochromator, and the use of a fluorophore rather than a scatter decreases the differences in spatial distribution of light emanating from the reference and sample. Thus photomultiplier tube artifacts are minimized. Five reference fluorophores were selected on the basis of availability, ease of solution preparation, and constancy of lifetime with temperature and emission wavelenght. These compounds are p-terphenyl, PPO, PPD, POPOP and dimethyl POPOP. These compounds are dissolved in ethanol to give standard solutions that can be used over the temperature range from ?55 to +55°C. Purging with inert gas is not necessary. The measured phase and modulation of the reference solution is used, in conjunction with the known reference, lifetime, to calculate the actual phase and modulation of the exictation beam. The use of standard fluorophores does not require separate experiments to quantify photomultiplier effects, and does not increase the time required for the measurement of fluorescence lifetimes. Examples are presented which demonstrate the elimination of artifactual photomultiplier effects in measurements of the lifetimes of DADH (0.4 ns) and indole solutions quenched by iodide. In addition, the use of these reference solutions increases the accuracy of fluorescence lifetime measurements ranging ranging to 30 ns. We judge this method to provide more reliable lifetime measurements by the phase and modulation method. The test solutions and procedures we describe may be used by other laboratories to evaluate the performance of their phase fluorometers.     

The Joint Committee for Traceability in Laboratory Medicine (JCTLM): a global approach to promote the standardisation of clinical laboratory test results

Clinical laboratories are moving towards global standardisation to produce equivalent test results across space and time. Standardisation allows use of evidence-based medicine, eliminates the need of method-specific reference intervals, decision levels and cut-offs, and can be achieved by application of metrological principles. For example, in vitro diagnostics (IVD) manufacturers can make kit calibrators traceable to internationally recognised reference materials and reference methods.The first step towards standardisation is to identify appropriate reference materials and methods. This has been undertaken by a new international consortium, the Joint Committee for Traceability in Laboratory Medicine (JCTLM), formed in 2002. It brings together experts representing the clinical laboratory profession, government agencies, and manufacturers, to promote international comparability, reliability, and equivalence of measurement results in clinical laboratories for the purpose of improving healthcare. Through the efforts of the JCTLM, manufacturers are able to assign values to kit calibrators with consistency using appropriate higher order reference materials and methods, and traceability flowcharts, according to ISO Standards to ensure accuracy of test results and to promote assay performance harmonisation. Users of assay kits can assess suitability of calibrators on the basis of acceptable reference materials and/or methods identified by the JCTLM. The JCTLM exemplifies the dynamic nature of clinical laboratory medicine, the inherent spirit of cooperation among professionals in this scientific field, and the international desire to strive for the highest level of clinical laboratory practice for the benefit of patients.     

Digital microspectrofluorometry by multichannel scaling and single photon detection

Synopsis A careful analysis of the requirements for microspectrofluorometric measurements has been made. General problems encountered in the measurement of light intensity and spectral distributions are considered, together with some special factors arising when measuring the fluorescence emitted by single cells. Criteria for evaluating microspectrofluorometric apparatus are given, and the relative merits of analog and digital methods are compared.A fully digital instrument using single-photon detection and a multichannel analyser is described. Automatic procedures for obtaining photodecomposition-time curves and emission spectra are illustrated in detail. Other possible applications of the instrument are also outlined. The experimental results reported show that microspectrofluorometric measurements of high sensitivity are possible, as light intensities down to a few photons per second can be measured.     

An Inverse Analysis Approach to the Characterization of Chemical Transport in Paints

The ability to directly characterize chemical transport and interactions that occur within a material (i.e., subsurface dynamics) is a vital component in understanding contaminant mass transport and the ability to decontaminate materials. If a material is contaminated, over time, the transport of highly toxic chemicals (such as chemical warfare agent species) out of the material can result in vapor exposure or transfer to the skin, which can result in percutaneous exposure to personnel who interact with the material. Due to the high toxicity of chemical warfare agents, the release of trace chemical quantities is of significant concern. Mapping subsurface concentration distribution and transport characteristics of absorbed agents enables exposure hazards to be assessed in untested conditions. Furthermore, these tools can be used to characterize subsurface reaction dynamics to ultimately design improved decontaminants or decontamination procedures. To achieve this goal, an inverse analysis mass transport modeling approach was developed that utilizes time-resolved mass spectroscopy measurements of vapor emission from contaminated paint coatings as the input parameter for calculation of subsurface concentration profiles. Details are provided on sample preparation, including contaminant and material handling, the application of mass spectrometry for the measurement of emitted contaminant vapor, and the implementation of inverse analysis using a physics-based diffusion model to determine transport properties of live chemical warfare agents including distilled mustard (HD) and the nerve agent VX.     

Introduction of environmentally degradable parameters to evaluate the biodegradability of biodegradable polymers

Environmentally Degradable Parameter ((Ed)K) is of importance in the describing of biodegradability of environmentally biodegradable polymers (BDPs). In this study, a concept (Ed)K was introduced. A test procedure of using the ISO 14852 method and detecting the evolved carbon dioxide as an analytical parameter was developed, and the calculated (Ed)K was used as an indicator for the ultimate biodegradability of materials. Starch and polyethylene used as reference materials were defined as the (Ed)K values of 100 and 0, respectively. Natural soil samples were inoculated into bioreactors, followed by determining the rates of biodegradation of the reference materials and 15 commercial BDPs over a 2-week test period. Finally, a formula was deduced to calculate the value of (Ed)K for each material. The (Ed)K values of the tested materials have a positive correlation to their biodegradation rates in the simulated soil environment, and they indicated the relative biodegradation rate of each material among all the tested materials. Therefore, the (Ed)K was shown to be a reliable indicator for quantitatively evaluating the potential biodegradability of BDPs in the natural environment.     

Two simple volumetric methods for respiration measurements with oxygen supply to the sample

Summary Two easy and inexpensive procedures suitable especially for the measurements of aerobic respiration activity of numerous soil samples are described. Size of samples can be changed within broad limits. The first of the two methods is based on measurements of the sum of oxygen consumed during a prolonged period of time. Actual rate of O₂ consumption over short-time intervals is determined by the second method.