Context Cystatin C is becoming an increasingly popular biomarker for estimating glomerular filtration rate, and accurate measurements of cystatin C concentrations are necessary for accurate estimates of glomerular filtration rate. Objective To assess the accuracy of cystatin C concentration measurements in laboratories participating in the College of American Pathologists CYS Survey. Design Two fresh frozen serum pools, the first from apparently healthy donors and the second from patients with chronic kidney disease, were prepared and distributed to laboratories participating in the CYS Survey along with the 2 usual processed human plasma samples. Target values were established for each pool by using 2 immunoassays and ERM DA471/IFCC international reference material. Results For the normal fresh frozen pool (ERM-DA471/IFCC–traceable target of 0.960 mg/L), the all-method mean (SD, % coefficient of variation [CV]) reported by all of the 123 reporting laboratories was 0.894 mg/L (0.128 mg/L, 14.3%). For the chronic kidney disease pool (ERM-DA471/IFCC–traceable target of 2.37 mg/L), the all-method mean (SD, %CV) was 2.258 mg/L (0.288 mg/L, 12.8%). There were substantial method-specific biases (mean milligram per liter reported for the normal pool was 0.780 for Siemens, 0.870 for Gentian, 0.967 for Roche, 1.061 for Diazyme, and 0.970 for other/not specified reagents; and mean milligram per liter reported for the chronic kidney disease pool was 2.052 for Siemens, 2.312 for Gentian, 2.247 for Roche, 2.909 for Diazyme, and 2.413 for other/not specified reagents). Conclusions Manufacturers need to improve the accuracy of cystatin C measurement procedures if cystatin C is to achieve its full potential as a biomarker for estimating glomerular filtration rate.
BACKGROUND:Determining the stability of stored samples for assays that were not available at the time of original collection is problematic. To assess sample stability for a relatively new assay of glycated albumin (GA), we first measured GA in fresh samples and in samples stored for 19 -23 years. We then compared the regression of the contemporaneous glycohemoglobin (Hb A 1c ) values against the GA results from fresh vs stored samples, reasoning that similar slopes and intercepts would provide strong, albeit indirect, support for the stability of the stored samples for GA measurements.
Dear editor:1,5-anhydroglucitol (1,5-AG) is a monosaccharide, originating mainly from foods and closely resembling glucose in structure. Blood concentrations of 1,5-AG are thought to reflect glucose excursions and short-term (1-2 week) glycemic control in the diabetes patient (1-5). The attractiveness of 1,5-AG for use in diabetes care is that it may capture additional information on glycemic excursions not reflected in values of hemoglobin A1c, the most common measure of glucose control. Only one commercial assay for 1,5-AG (GlycoMark™) is approved and marketed for clinical use in the U.S. This assay is available at major laboratories and is reimbursed by some insurers including Medicare. We sought to directly compare the performance of the GlycoMark assay (GlycoMark, Inc) to the Determiner-L 1,5-AG assay manufactured by Kyowa Medex (Tokyo, Japan). We also assessed the effect of a single freeze-thaw cycle on the GlycoMark assay value.We utilized stored serum specimens obtained from a subsample of participants in the Atherosclerosis Risk in Communities (ARIC) Study in 2005-06 (6). All samples were stored at −70°C until time of measurement. To assess the impact of a freeze-thaw cycle, we conducted measurements using the GlycoMark assay on separate days (4 months apart) using identical methodology. At the time of the second GlycoMark measurement, we simultaneously conducted measurements using the Determiner-L Kyowa Medex assay in the same samples so as to have head-to-head data on the comparative performance of these 2 brands.These enzymatic colorimetric assays were implemented on the Roche Modular P800 (Roche Diagnostics Corp., Indianapolis, IN) with materials from GlycoMark, Inc and Kyowa Medex. The GlycoMark assay is a 2-step, end point enzymatic method consisting of a 2-reagent system. In the first step, the specimen is incubated with a reagent mixture (glucokinase, pyruvate kinase, phosphoenol pyruvate) that converts endogenous glucose to glucose-6-phosphate. The second reagent (pyranose oxidase) oxidizes 1,5 AG, producing hydrogen peroxide. Peroxidase catalyzes the formation of a colored end product from 4-aminoantipyrine and TOOS (N-ethyl-N-(2-hydroxy-3-sulfopropyl)-3-methylaniline sodium © 2010 Elsevier B.V. All rights reserved. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptClin Chim Acta. Author manuscript; available in PMC 2012 April 11. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript dehydrate), with the absorbance measured at 546 nm. In the Kyowa Medex assay 1,5-anhydro-D-glucitol (1,5AG) is first c...
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