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D Stöckl

Publications and source records attributed to D Stöckl.

At least 55 records · Page 3Linked to original sources
PubMed1996

Analytical specifications of reference methods compilation and critical discussion (from the members of the European EQA-Organizers Working Group B).

We present a compilation of published data for accuracy, precision, and measurement design for analytes that, currently, are of major interest for European reference laboratories. These data are compared with recent recommendations for performance of reference methods to be used within networks of European reference laboratories. In addition, we review the literature on reference methods and related topics.

Blood Glucose↗
PubMed1996

Characterization and classification of external quality assessment schemes (EQA) according to objectives such as evaluation of method and participant bias and standard deviation. External Quality Assessment (EQA) Working Group A on Analytical Goals in Laboratory Medicine.

Within the scope of this paper, the Working Group has attempted to place external quality assessment (EQA) within the whole context of quality management in laboratory medicine. First, the objectives of EQA schemes are defined and current EQA schemes evaluated. In most schemes, the objectives are not defined a priori and do not allow the definition of the origin of unacceptable individual results from participants. There is an ongoing trend for making traditional EQA schemes more interesting for the participants. Analysis of the factors involved in analytical quality allow the definition of the essential analytical tasks of educational EQA schemes. Beside these quality control tasks, educational EQA also includes quality assurance elements. EQA today has not only an important role to play in the assessment of each participant's performance but also in the assessment of the method. Efficiency of the schemes and educational impact can be improved by appropriate scheme designs according to objectives. After this theoretical approach, some practical examples of problem related EQA designs are given.

Clinical Laboratory Techniques↗
PubMed1996

Determination of reference method values by isotope dilution-gas chromatography/mass spectrometry: a five years' experience of two European Reference Laboratories.

We report on the cooperation of two European Reference Laboratories for the determination of reference method values in serum based materials intended for use in internal accuracy control and external quality assessment. Reference method values were determined by isotope dilution-gas chromatography/mass spectrometry for aldosterone, cortisol, oestradiol-17 beta, progesterone, testosterone, thyroxine, theophylline, cholesterol, creatinine, glucose, total triacylglycerols and uric acid. All determinations were done in parallel in the two laboratories, independently and within certain time constraints. The general measurement design consisted of duplicate measurement of each sample on three different occasions. In each laboratory, rigorous internal quality control was performed according to predefined analytical quality specifications. This was done using certified reference materials. If not available, control materials targeted before by the two laboratories were utilized. Here we present the results of the cooperation during five years. We discuss the precision and accuracy achieved, the between-laboratory agreement and the total analytical error. For the hormones and theophylline, the mean overall coefficient of variation for both laboratories (calculated from the measurements on three days) was always < 2%, for the substrates < 1%. For all substances, the method bias (estimated from several measurement series over the five years) was < 1%, and the average deviation of the results between the two laboratories was < 1.2%. The maximum total analytical error was in all cases < 3%. These data demonstrate that current reference methodology is able to guarantee a stable level of high quality of performance, and that reference laboratories of today are capable of providing, in due time, adequate service in the framework of accuracy-based harmonization of methods in routine laboratory medicine.

Aldosterone↗
PubMed1996

Proposed guidelines for the internal quality control of analytical results in the medical laboratory.

The factors involved in analytical quality relate to definition of quality, creation of quality, and control of quality, and errors arise from external and internal sources as well as from permanent and variable factors. Further, the two main types of error are classified as systematic and random errors. Internal quality control (IQC) systems can only operate on the variable factors which are related to batch-to-batch variations (external factors) and to the performance in the laboratory (internal factors). In creating an adequate internal control system, several problems are faced: (i) quality of control materials, (ii) types and frequency of possible errors, (iii) number and types of control materials, (iv) number of replicates of the control, (v) probability of error detection, (vi) probability of false rejection, (vii) consequences of reject signals, (viii) trouble-shooting systems, and (ix) prevention of errors among many other conditions. Gaussian distributions of control results are assumed and the statistical control rules are evaluated in relation to probability of false rejections, Pfr, and probability of error detection, Ped, for the different rules. Combinations of low Pfr and high Ped are obtained by combining results from e.g. four measurements of the same control sample by use of mean and range rules. Further, it is not possible to establish a common control system which can be used for all quantities and analytical procedures; on the contrary, each procedure should have its particular efficient IQC system. These aspects are discussed and a number of guidelines for statistical control rules and problem related internal quality control are presented.

Guidelines as Topic↗
PubMed1995

Ion chromatography as potential reference methodology for the determination of total sodium and potassium in human serum.

The potential of ion chromatography to serve as a new reference method principle for the determination of total sodium and potassium in human serum was investigated. Sample pretreatment consisted of acidic dilution and filtration and detection was based on conductivity. Methods for the separate and simultaneous determination of both analytes were investigated. Further, the influence of calibration (using either a single-point calibration or a standard curve) on method imprecision, inaccuracy and analysis time was examined. The best method performance was achieved by separate analysis using single-point calibration with bracketing analysis scheme. For this variant, the mean total coefficient of variation for sodium and potassium was 1.0% and the mean method bias was -0.2% for sodium and 2% for potassium, as determined with three control materials from the National Institute of Standards and Technology. Our results are comparable to those of reference methods based on flame atomic emission spectrometry. Therefore, we consider ion chromatography as a valuable reference methodology for the determination of total sodium potassium in human serum.

Chromatography, Liquid↗
PubMed1995

Evaluation of 2-iminoimidazolidin-4-one and thymine as respective internal standards for normal-phase and reversed-phase high-performance liquid chromatographic determination of creatinine in human serum.

We evaluated two internal standards for HPLC determination of creatinine in human serum after ultrafiltration: 2-iminoimidazolidin-4-one for normal-phase HPLC on aluminium oxide, and thymine for C18 reversed-phase HPLC. Detection of 2-iminoimidazolidin-4-one was done at the same wavelength as that used for creatinine, i.e. 240 nm. For thymine, the wavelength was switched to 280 nm. The suitability of the selected compounds to serve as an internal standard in the described measurement procedures, including ultrafiltration of serum, was evaluated from the precision and accuracy obtained. The method based on normal-phase HPLC with 2-iminoimidazolidin-4-one showed an imprecision expressed as R.S.D. ranging from 0.8 to 3.4% (mean: 2.1%) and an inaccuracy, calculated from the deviations from target values determined by isotope-dilution gas chromatography-mass spectrometry, ranging from -1.3 to +1.8% (mean: +0.4%). For the reversed-phase HPLC procedure with thymine, the imprecision ranged from 0.3 to 1.3% (mean: 1.0%) and the inaccuracy from +0.1 to +3.9% (mean: +1.7%). The occasional observation of interferences with 2-iminoimidazolidin-4-one limited the application of the normal-phase method to a certain extent.

Chromatography, High Pressure Liquid↗
PubMed1995

Candidate reference method for determining serum creatinine by isocratic HPLC: validation with isotope dilution gas chromatography-mass spectrometry and application for accuracy assessment of routine test kits.

We present a candidate Reference Method for determining creatinine in serum, based on isocratic HPLC. The chromatographic column, with alkaline-treated aluminum oxide as stationary phase, is eluted with an equivolume mixture of methanol/acetonitrile containing 70 mL/L aqueous NaOH (10 mmol/L), and ultraviolet absorbance is detected at 240 nm. We investigated the ruggedness of the method and validated its performance with isotope dilution gas chromatography-mass spectrometry (ID GC-MS) for a set of 22 patients' sera and 10 commercially available lyophilized control materials. The mean deviation from ID GC-MS was +0.1% (range, -2.2% to +2.2%). The between-day CV, calculated from six independent measurements performed on three different days, was 0.9% (range 0.2% to 1.9%); the within-run CV was 0.8%. The total error of the method was < 3%. These performance characteristics make the method suitable for target-setting of quality-control materials and accuracy assessment of routine test kits. For the latter application, done with split-sample measurements of a panel of 83 patients' specimens, we used the Boehringer Mannheim enzymatic creatinine PAP test on the Hitachi 911, the Kodak Ektachem single-slide enzymatic creatinine test on the Ektachem 700, the Merck Jaffé kinetic assay on the Mega analyzer, and the Roche Jaffé kinetic test on the Cobas Mira.

Aluminum Oxide↗
PubMed1995

Desirable routine analytical goals for quantities assayed in serum. Discussion paper from the members of the external quality assessment (EQA) Working Group A on analytical goals in laboratory medicine.

The aim of the Working Group was to describe guidelines for deriving desirable analytical goals in laboratory medicine. First, a literature review is given of the different approaches used until now, and some of the most important studies are presented in detail. These approaches are then discussed critically, and the analytical goals proposed by the group are outlined with respect to monitoring and diagnostic testing. The group recommends that, most realistically, analytical quality specifications be biologically based. For diagnostic testing, the aim is achievement of accuracy, allowing the use of common reference intervals when populations are homogeneous for a given quantity. For monitoring (within an individual laboratory and performed with the same instrument), analytical performance should aim at stable operation and low imprecision compared with the within-subject biological variation. Method accuracy is also very important for the comparability of results from different laboratories or instruments.

Blood Chemical Analysis↗
PubMed1995

Analytical quality specifications for reference methods and operating specifications for networks of reference laboratories. discussion paper from the members of the external quality assessment (EQA) Working Group B1) on target values in EQAS.

The aim of the Working Group was to describe guidelines for the establishment of networks of reference laboratories. The need for such networks to achieve an accuracy-based uniform measurement system with traceability of results of analytical systems/test-kits to the true value is outlined. Criteria for analytical quality specifications, which are related to the ultimate purpose of the reference method and thereby to the objectives of the networks, are emphasized. The group recommends the use of two models: one based on specifications for routine methods, which are dictated by the biological variations of the respective analytes, the second respecting the analytical state-of-the-art of reference methodology. Further, the group presents operating specifications for networks that guarantee the continuous performance of reference method measurements whilst maintaining a uniform and stable level of quality.

Chemistry, Clinical↗
PubMed1994

Use of cyclodextrins for prepurification of progesterone and testosterone from human serum prior to determination with isotope dilution gas chromatography/mass spectrometry.

A new application of cyclodextrins is presented, being their use for sample purification. This new application is exemplified with a method for the determination of progesterone and testosterone in human serum based on isotope dilution gas chromatography/mass spectrometry. The whole procedure consists of a traditional solvent extraction with hexane for progesterone, respectively dichloromethane for testosterone, followed by a back-extraction of the steroids into a cyclodextrin-water solution. After washing of the aqueous phase with hexane, the steroids are finally extracted with toluene (progesterone) and dichloromethane (testosterone). Different cyclodextrins and cyclodextrin derivatives in various concentrations are investigated. A solution of 150 mmol/L hydroxypropyl beta-cyclodextrin in water is selected. The recovery of progesterone and testosterone is 70 and 80% respectively. The purification effectiveness, accuracy, and precision of the new method are equivalent to a traditional method using gel chromatography for sample purification.

Cyclodextrins↗
PubMed1994

Ion chromatography as potential reference methodology for the determination of total calcium and magnesium in human serum.

The potential of ion chromatography to serve as a new reference method principle for the determination of total calcium and magnesium in human serum was investigated. Sample pretreatment consisted of acidic dilution and filtration. Detection was based on conductivity. Mean total coefficients of variation for calcium and magnesium were respectively 0.91% and 1.20% and the mean method bias was +0.06% for calcium and -0.35% for magnesium, as determined with five control materials from the Community Bureau of Reference and the National Institute of Standards and Technology. These results were comparable to those of the proposed candidate reference methods based on flame atomic absorption spectrometry. Therefore, we consider ion chromatography as a valuable reference methodology for the determination of total calcium and magnesium in human serum.

Calcium↗
PubMed1994

Development of a new method for the determination of thyroxine in serum based on isotope dilution gas chromatography mass spectrometry.

A new gas chromatographic/mass spectrometric method in combination with isotope dilution for the determination of thyroxine in serum is described. Special attention was paid to the methylation step of thyroxine, which was investigated using methanolic HCl, dimethylformamide/dimethylacetal and diazomethane, the latter giving the best results in terms of reproducible isotope ratios. For internal standardization, (13C6)-thyroxine was dissolved in fraction V human albumin solution (70 g l-1). The internal standard-in-albumin solution was mixed with known amounts of thyroxine standard, dissolved in 0.05 M Na2HPO4 buffer at pH 11.6, to give isotope ratios of 0.75, 1.00 and 1.25. The same internal standard solution was also used for isotope dilution of the unknown serum samples. The volume of serum was adapted to give a 1:1 isotope ratio. Sample pretreatment consisted of protein precipitation and a two-step liquid/liquid extraction procedure. After methylation of unlabelled and labelled thyroxine with diazomethane and perfluoroacylation with pentafluoropropionic anhydride and heptafluorobutyric anhydride, respectively, mass spectrometric monitoring was done at m/z 951/957 and 1001/1007. Quantitative determination of thyroxine in five serum samples in duplicate, during three consecutive days, showed a mean overall imprecision of 1.0% and a deviation of +0.4% from the target value as determined by a definitive method.

Albumins↗
PubMed1994

Candidate reference method for determining serum theophylline applied to target-setting in external quality assessment and routine method evaluation.

We present a candidate reference method for the determination of serum theophylline by isotope dilution gas chromatography-mass spectrometry, using extractive alkylation in sample preparation. The maximum method bias was < 1.0%. The mean method imprecision (CV) was 0.63% (range 0.37-0.96%), calculated from the results of six measurements independently performed on 3 days. The maximum total error of the method, including the method bias and 95% confidence interval, was < 2.1%. Investigation of various substances and metabolites for interference proved the specificity of the proposed method. The method was used to evaluate three routine assays by linear regression analysis. Performance of the TDx II (Abbott) was excellent and of the Emit 2000 (Syva) assay was good. The Kodak Ektachem assay showed a high dispersion of results and a pronounced positive bias in the subtherapeutic range.

Evaluation Studies as Topic↗
PubMed1993

Development of criteria for the evaluation of reference method values.

We present a uniform concept for the requirements of reference methods (RMs) used for target-setting in the German external quality assessment (EQA) scheme. It is proposed that the total allowable error (TAE) of the RMs shall be 0.2 times the acceptance limits in the EQA-scheme. From the TAE are derived limits for maximum RM-bias, RM-imprecision, and the number of measurements.

Chemistry, Clinical↗