On the use of trimethylchlorosilane in methanol for methylation of thyroxine prior to perfluoroacylation and isotope dilution-gas chromatography/mass spectrometry.
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Acylation↗
Biomedical subjects
D Stöckl
Publications and source records attributed to D Stöckl.
Isotope dilution-liquid chromatography/electrospray ionization-tandem mass spectrometry for the determination of serum thyroxine as a potential reference method.
A new candidate reference method is presented for the determination of thyroxine in serum. The method is based on isotope dilution-liquid chromatography/tandem mass spectrometry using electrospray for ionization. The internal standard used was 13C6-thyroxine, sample pretreatment consisted of protein precipitation and a two-step liquid/liquid extraction procedure, HPLC was performed on a C-18 column with an eluent containing methanol/water/formic acid (60:40:0.1, by volume), and finally thyroxine and its isotopically labeled analogue were measured in the selected reaction monitoring mode for the transitions m/z 777.7--> 731.7 and m/z 783.7--> 737.7, respectively. The detection limit for thyroxine was 6 pg, the within-run coefficient of variation was 1.1%. The samples were measured in six-fold: in duplicate on three independent days. The mean overall coefficient of variation of the method was 1.6%. The new method was evaluated by measuring nine control sera previously determined by an existing ID-GC/MS method. The differences between the results of the two methods ranged from-1.6% to +3.3%, with a mean of +0.2%.
Calibration↗
Standardization of ionized magnesium by correlation with total magnesium.
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Blood Chemical Analysis↗
Assessment of the state-of-the-art trueness and precision of serum total-calcium and glucose measurements in Finnish laboratories--the QSL-Finland study.
The purpose of the QSL-Finland study was to assess the state-of-the-art trueness and precision of serum total-calcium and glucose measurements in Finnish clinical laboratories. For this purpose, 21 hospitals and clinical institutes were selected. They measured six single donation sera, the total-calcium (t-calcium) and glucose content of which had been determined by ion chromatography and isotope dilution-gas chromatography-mass spectrometry (ID-GC-MS) reference methods. The results were interpreted in light of specifications for imprecision, bias and total error of routine methods that have been proposed in the past. The data revealed that the performance of t-calcium and glucose methods is generally acceptable in Finnish clinical laboratories. This study did not lead to a separation of laboratories according to accreditation. In consequence, it seems that accreditation, in its present form, cannot substitute dedicated quality assurance practices.
Blood Glucose↗
Intrinsic and routine quality of serum total potassium measurement as investigated by split-sample measurement with an ion chromatography candidate reference method.
We evaluated the intrinsic quality of eight routine test systems for the measurement of serum total potassium (K+), as well as the routine quality of four of these systems, using a group of 60 single-donation serum samples that had been certified with an ion chromatography reference method. The intrinsic quality of the tests was evaluated by analysis of the sera in the manufacturers' application laboratories under strict internal quality control. The routine quality was evaluated by analysis of the same sera in five (per system) routine laboratories under daily working conditions. The results of the study were interpreted in light of the most stringent specifications derived from the biological variation of K+, which require limits of 6.3% for total error and 1.6% for systematic error. The study revealed that the intrinsic quality of all systems was excellent. None of the test systems yielded a substantial number of results outside the 6.3% total error limit, and only one test system exceeded the 1.6% systematic error limit. The majority of the routine laboratories reproduced the manufacturers' intrinsic quality. In particular, most laboratories satisfied the 6.3% total error limit. However, several laboratories exceeded the 1.6% systematic error limit. Generally, there was a considerable difference in quality between the participating laboratories. This showed that the major problems for serum K+ analysis (for samples with no unusual matrices and with concentrations within the reference interval) are at the routine laboratory.
Adult↗
Modern quality management misunderstood?
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Chemistry, Clinical↗
Validity of linear regression in method comparison studies: is it limited by the statistical model or the quality of the analytical input data?
We compared the application of ordinary linear regression, Deming regression, standardized principal component analysis, and Passing-Bablok regression to real-life method comparison studies to investigate whether the statistical model of regression or the analytical input data have more influence on the validity of the regression estimates. We took measurements of serum potassium as an example for comparisons that cover a narrow data range and measurements of serum estradiol-17beta as an example for comparisons that cover a wide data range. We demonstrate that, in practice, it is not the statistical model but the quality of the analytical input data that is crucial for interpretation of method comparison studies. We show the usefulness of ordinary linear regression, in particular, because it gives a better estimate of the standard deviation of the residuals than the other procedures. The latter is important for distinguishing whether the observed spread across the regression line is caused by the analytical imprecision alone or whether sample-related effects also contribute. We further demonstrate the usefulness of linear correlation analysis as a first screening test for the validity of linear regression data. When ordinary linear regression (in combination with correlation analysis) gives poor estimates, we recommend investigating the analytical reason for the poor performance instead of assuming that other linear regression procedures add substantial value to the interpretation of the study. This investigation should address whether (a) the x and y data are linearly related; (b) the total analytical imprecision (s(a,tot)) is responsible for the poor correlation; (c) sample-related effects are present (standard deviation of the residuals >> s(a,tot)); (d) the samples are adequately distributed over the investigated range; and (e) the number of samples used for the comparison is adequate.
Chromatography, Ion Exchange↗
Practical aspects of suppressed ion chromatography for cations in human serum.
In spite of the good accuracy and precision of ion chromatographic methods for the determination of mono- and divalent cations in human serum, the major drawback with these methods were problems with the membrane suppressor's performance. Here, we describe experiments undertaken to solve these problems. We address in particular the use of histidine-sulfuric acid eluents, sample purification with OnGuard-A cartridges and chromatographic "front-cut" for divalent cations. The latter two adaptations, resulting in removal of the anionic species from the sample, were successful in solving the observed suppressor problems. The eluent substitution, moreover, allowed us to switch from the chemical to the electric suppression mode. We believe that these adaptations will allow secure and robust determination of cations in human serum samples with ion chromatography.
Cations↗
State-of-the-art of serum total calcium measurement as investigated by split-sample measurement with an ion chromatography candidate reference method.
We evaluated bias and inaccuracy of four frequently used routine test systems for serum total calcium, using a candidate reference method based on ion chromatography. The mean biases and 95% confidence intervals that we observed were 0.0 +/- 0.59% for Johnson & Johnson arsenazo(III), 1.3 +/- 0.62% for Beckman arsenazo(III), -0.4 +/- 0.44% for Beckman applying ion selective electrode measurement after sample dilution, and -1.9% +/- 0.42% for Boehringer o-cresolphthalein. The inaccuracy of all test systems was usually < 4.7% (calculated as deviation of singlicates from ion chromatography). Both bias and inaccuracy are discussed in the light of specifications set by expert groups or derived from the biological variation of serum total calcium. The study revealed that the intrinsic quality of commonly used test systems for serum total calcium satisfies even some of the more stringent criteria for method bias and inaccuracy.
Bias↗
13C-NMR and mass spectral data of steroids with a 17,17-dialkyl-18-nor-13(14)-ene substructure.
We present carbon-13 nuclear magnetic resonance (13C-NMR) and mass spectral data for several androstanes and estranes having a 17,17-dialkyl-18-nor-13(14)-ene substructure. These compounds are formed by a Wagner-Meerwein rearrangement of steroids bearing a tertiary 17-hydroxy group during the derivatization reaction with heptafluorobutyric anhydride. The 13C-NMR assignments are compared with those of natural products having a similar substructure. The mass spectra show characteristic fragment ions for which a fragmentation mechanism is proposed.
Carbon Isotopes↗
Current stage of standardization of measurements of specific polypeptides and proteins discussed in light of steps needed towards a comprehensive measurement system.
We present a standardization model for the measurement of specific polypeptides and proteins, which is based on an integrated development of all important elements of a reference measurement system. Generally, the model is in line with other current recommendations. However, it puts special emphasis on the definition of the analyte and on the role of reference methods for verification of the standardization process by measurement of patient specimens. Further, we discuss the needs for its implementation in the routine laboratory. In the light of this model, we investigate the current stage of standardization of routine methods for enzymes, peptide hormones, proteins, apolipoproteins, glycohaemoglobin, and tumour markers.
Apolipoproteins↗
Graphical interpretation of analytical data from comparison of a field method with reference method by use of difference plots.
Various viewpoints have been offered regarding the appropriate use of scatter plots or difference plots (bias plots or residual plots) in comparing analytical methods. In many of these discussions it seems the basic concepts of identity (within inherent imprecision) and acceptability based on analytical goals (analytical quality specifications) have been forgotten. With the increasing number of Reference Methods in laboratory medicine, these basic concepts are becoming more important in validation of field methods. Here we describe a simple and effective graphical test of these hypotheses (identity and acceptability) by use of difference plots. These plots display the underlying hypothesis before the measured differences are plotted and allow interpretation of the results according to specific criteria. We further describe simple but effective interpretations of the data, when the hypothesis is not fulfilled, by using two data sets drawn from comparisons of field methods for S-creatinine with a Reference Method for this analyte. The difference plot is a graphical tool with related simple statistics for comparison of a field method with a Reference Method, focusing on (a) identity within the inherent analytical imprecision or (b) acceptability within analytical quality specifications. Calculation of the standard deviation of the differences is an indispensable tool for evaluation of aberrant-sample bias (matrix effects).
Chemistry, Clinical↗
Candidate reference method for determining serum cortisol based on isotope dilution-gas chromatography/mass spectrometry using heptafluorobutyrilation as derivatization method.
We present a new candidate reference method for determining cortisol in serum. The method is based on isotope dilution-gas chromatography/mass spectrometry and makes use of derivatization with heptafluorobutyric anhydride and selected ion monitoring at m/z 489 and 491. A detection limit of 0.08 pmol (30 pg) was achieved. Twenty-four structurally related steroids were tested for interference and found negative. Verification of the analytical quality specifications was done from measurement in two laboratories of the certified reference materials 192 and 193 of the Bureau Communautaire de Référence and a number of commercial quality control materials. The maximum systematic error was estimated to be 1.0% and the mean imprecision was 1.0%. The total error was lower than 2.05%. The method was applied for target-setting in external quality assessment and internal accuracy control and for measurement of patient samples.
Fluorocarbons↗
Validation of candidate reference methods based on ion chromatography for determination of total sodium, potassium, calcium and magnesium in serum through comparison with flame atomic emission and absorption spectrometry.
OBJECTIVES: We report on the validation of candidate reference methods based on ion chromatography for the determination of total sodium, potassium, calcium, and magnesium in serum. The current study is a continuation of previous investigations in which we were able to show the potential of ion chromatography to serve as a reference method principle. The validation consisted of comparison of the ion chromatographic methods with generally recognized reference methods based on flame atomic emission (for sodium/ potassium) and absorption (for calcium/magnesium) spectrometry. METHODS: Sample preparation consisted of acidic dilution and pre-purification with reversed phase cartridges. Ion chromatography with suppressed conductivity was performed with polymeric stationary phases chemically modified with carboxylate and sulfonate functional groups for sodium/potassium and calcium/magnesium, respectively. Single-point calibration was used in combination with individual adaptation of serum sampling and diluting to give similar responses in samples and standards. In the study, more than 25 control sera were analyzed in parallel with the ion chromatographic methods and the traditional reference methods. The measurement protocol consisted of quadruplicate analysis of each sample per run, in 3 different runs. In each measurement series, internal quality control with certified reference materials from the National Institute of Standards and Technology and the Bureau Communautaire de Référence was applied. The performance criteria to fulfill were pre-defined (i.e., the maximum within-day and overall coefficient of variation, the confidence interval, the daily and overall deviation from the certified value and the maximum total analytical error). RESULTS: The comparison is presented as difference plot (i.e., % deviation of the results obtained with ion chromatography from those obtained with the respective traditional reference methods). The mean deviations were +0.9% for sodium, +1.0% for potassium, +/- 0.0% for calcium, and +0.1% for magnesium. With the exception of 2 values each for sodium and potassium, all deviations were within the limit of 2 times the allowed total analytical error. From the data obtained for internal quality control during the different measurement campaigns, we further estimated the long-term precision and accuracy of our methods. The overall coefficient of variation (respectively the bias) amounted to 0.7% (+0.1%) for sodium, 0.9% (-0.3%) for potassium, 1.3% (-0.1%) for calcium, and 1.0% (+/- 0.0%) for magnesium. CONCLUSION: From the precision and accuracy reached by our ion chromatographic methods, we conclude that they can be proposed as candidate reference methods for the determination of total sodium, potassium, calcium, and magnesium in serum.
Calcium↗
Beyond the myths of difference plots.
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Linear Models↗
Metrology and analysis in laboratory medicine: a criticism from the workbench.
Metrological concepts and terminology, in particular the Vocabulaire International des Termes Fondamentaux et Généraux de Métrologie and the Guide to the Expression of Uncertainty in Measurement, are discussed from the viewpoint of analytical practice. Especially addressed are terms such as error, uncertainty, accuracy, trueness, bias, and precision. Evidence is presented that the new definition of accuracy, being now a joint index of systematic and random errors, is extremely impractical and has no advantages over the old definition of accuracy. Therefore, instead of re-education of analysts, a return to the old definition is proposed. Further, arguments are presented that the error concept is still an extremely useful metrological concept and does not need to be replaced by the uncertainty concept. Rather, both concepts are complementary.
Humans↗
Accuracy-based assessment of proficiency testing results with serum from single donations: possibilities and limitations.
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Chemistry, Clinical↗
Four frequently used test systems for serum cholesterol evaluated by isotope dilution gas chromatography-mass spectrometry candidate reference method.
We evaluated the performance of four frequently used cholesterol test systems, using split-sample measurements with a panel of 79 patients' specimens and isotope dilution gas chromatography-mass spectrometry (ID GS-MS) as a comparison method. The test systems were from Beckman, a Boehringer Mannheim, Merck, and Johnson & Johnson Clinical Diagnostics, performed on the Synchron CX7, Hitachi 717, Mega, and Ektachem 250 analyzers, respectively. The liner regression data for the method comparison [ID GS-MS as independent variable (x)] were for Beckman: slope = 1.012, intercept = 0.0243 mmol/L, dispersion (S(y/x)) = 0.1303 mmol/l, and correlation coefficient (r) = 0.9867; for Boehringer Mannheim: slope = 1.002, intercept = 0.114 mmol/L, Sy/x = 0.0759 mmol/L, r = 0.9954; for Merck: slope = 1.034, intercept = -0.0613 mmol/L, Sy/x = 0.0886 mmol/L, r = 0.9941; and for Johnson & Johnson Clinical Diagnostics: slope = 1.007, intercept = 0.01 mmol/L, Sy/x = 0.15 mmol/L, and r = 0.9811. These data demonstrate excellent state-of-the-art cholesterol measurement for some of the most widely used test systems.
Blood Chemical Analysis↗