Generation of a new cystatin C-based estimating equation for glomerular filtration rate by use of 7 assays standardized to the international calibrator.

BACKGROUND: Many different cystatin C-based equations exist for estimating glomerular filtration rate. Major reasons for this are the previous lack of an international cystatin C calibrator and the nonequivalence of results from different cystatin C assays. METHODS: Use of the recently introduced certified reference material, ERM-DA471/IFCC, and further work to achieve high agreement and equivalence of 7 commercially available cystatin C assays allowed a substantial decrease of the CV of the assays, as defined by their performance in an external quality assessment for clinical laboratory investigations. By use of 2 of these assays and a population of 4690 subjects, with large subpopulations of children and Asian and Caucasian adults, with their GFR determined by either renal or plasma inulin clearance or plasma iohexol clearance, we attempted to produce a virtually assay-independent simple cystatin C-based equation for estimation of GFR. RESULTS: We developed a simple cystatin C-based equation for estimation of GFR comprising only 2 variables, cystatin C concentration and age. No terms for race and sex are required for optimal diagnostic performance. The equation, [Formula: see text] is also biologically oriented, with 1 term for the theoretical renal clearance of small molecules and 1 constant for extrarenal clearance of cystatin C. CONCLUSIONS: A virtually assay-independent simple cystatin C-based and biologically oriented equation for estimation of GFR, without terms for sex and race, was produced.

First certified reference material for cystatin C in human serum ERM-DA471/IFCC.

The IFCC Working Group for the Standardisation of Cystatin C (WG-SCC), in collaboration with the Institute for Reference Materials and Measurements (IRMM), announces the availability of the new certified reference material ERM-DA471/IFCC. The material was characterised using a pure protein primary reference preparation (PRP) as calibrant. The PRP was prepared from recombinant cystatin C, and its concentration measured using dry mass determination. The characterisation of ERM-DA471/IFCC was performed by particle enhanced immuno-nephelometry, particle enhanced immuno-turbidimetry, and enzyme amplified single radial immuno-diffusion. The certified cystatin C mass concentration in ERM-DA471/IFCC, if reconstituted according to the specified procedure, is 5.48 mg/L, the expanded uncertainty (k=2) being 0.15 mg/L.

Standardizing plasma protein measurements worldwide: a challenging enterprise.

The need for harmonizing laboratory results is particularly intense in the field of quantitative protein assays in consideration of the clinical impact of specific protein measurements and their relevance in monitoring disease. We report the efforts made by the Committee on Plasma Proteins of the IFCC Scientific Division to achieve worldwide comparability in plasma protein results. We focus on the production of reference materials and the methods applied throughout their production process. Particularly, the recent characterization of ERM-DA470k/IFCC and ERM-DA472/IFCC has demonstrated that it is possible to reproduce the earlier established procedures and thereby maintain standardization. Plasma protein reference materials have had a substantial impact in improving the harmonization of patient protein results that should translate into better patient care.

Characterization of the new serum protein reference material ERM-DA470k/IFCC: value assignment by immunoassay.

BACKGROUND: The availability of a suitable matrix reference material is essential for standardization of the immunoassays used to measure serum proteins. The earlier serum protein reference material ERM-DA470 (previously called CRM470), certified in 1993, has led to a high degree of harmonization of the measurement results. A new serum protein material has now been prepared and its suitability in term of homogeneity and stability has been verified; after characterization, the material has been certified as ERM-DA470k/IFCC. METHODS: We characterized the candidate reference material for 14 proteins by applying a protocol that is considered to be a reference measurement procedure, by use of optimized immunoassays. ERM-DA470 was used as a calibrant. RESULTS: For 12 proteins [α(2) macroglobulin (A2M), α(1) acid glycoprotein (orosomucoid, AAG), α(1) antitrypsin (α(1)-protease inhibitor, AAT), albumin (ALB), complement 3c (C3c), complement 4 (C4), haptoglobin (HPT), IgA, IgG, IgM, transferrin (TRF), and transthyretin (TTR)], the results allowed assignment of certified values in ERM-DA470k/IFCC. For CRP, we observed a bias between the lyophilized and liquid frozen materials, and for CER, the distribution of values was too broad. Therefore, these 2 proteins were not certified in the ERM-DA470k/IFCC. Different value transfer procedures were tested (open and closed procedures) and found to provide equivalent results. CONCLUSIONS: A new serum protein reference material has been produced, and values have been successfully assigned for 12 proteins.

Protein standardization V: value transfer. A practical protocol for the assignment of serum protein values from a Reference Material to a Target Material.

We present a practical protocol for the assignment of values to serum proteins in a Target Material using a Reference Material. This protocol is based on the model of Direct Value Transfer between serum matrices and is intended to improve the value assignment of commercial calibrators using the Reference Material CRM 470 (now labeled ERM-DA 470) or similar reference materials. The procedure describes the general as well as the practical principles involved in the value assignment (with examples). The practical transfer protocol is based on multiple assays of 6 dilutions of the Reference Material and 6 dilutions of the Target Material. The transfer protocol requires several measurements a day repeated on several days, an important prerequisite being that all reconstitutions and dilutions are controlled by weighing thus reducing uncertainty in the transfer. In open systems that allow the use of the Reference Material as calibrator and the Target Material as samples, the proportionality of the two materials (the presence or absence of matrix effects) can now be directly assessed by evaluating a single regression plot. If no matrix effects are found, the regression line will pass through zero with a slope equal to the ratio of the concentrations of the two materials. In closed systems, the dedicated commercial calibrator has to be used as such; the Reference Material and the Target Material are now assayed as samples against this calibrator. Two regression plots are therefore obtained; if no matrix effects are present among the two materials and the calibrator, both the Reference and Target Materials will show zero intercepts, and the ratio of the two slopes will equal the ratio of the concentrations.

A particle-enhanced turbidimetric immunoassay for quantitative determination of orosomucoid in urine: development, validation and reference values.

Increased urinary orosomucoid excretion rate (UOER) is an independent predictor of cardiovascular and all-cause mortality in patients with type 2 diabetes, as demonstrated by a conventional, immunoturbidimetric method. We wanted to optimize the method by developing a fully automated, particle-enhanced turbidimetric (PET) immunoassay with a lower detection limit, to allow assessment of orosomucoid in urine in healthy individuals and patients. A micro-particle-based immunoreagent was prepared for a PET immunoassay. The calibration was traceable to the certified reference material (CRM 470) for specific human serum proteins. We studied 69 healthy adults (28 men and 41 women) to establish reference values for the new assay. The detection limit of orosomucoid in urine was found to be 0.05 mg/l, about 20 times lower than for the conventional assay. Within-run imprecision [CV%, (level)] was 6.7% (0.23 mg/l), 1.0% (1.08 mg/l) and 1.0% (4.69 mg/l). Total imprecision [CV%, (level)] was 10.4% (0.23 mg/l), 3.9% (1.08 mg/l) and 3.4% (4.69 mg/l). Reference values [median (2.5-97.5 percentiles)] for UOER were 0.36 (0.07-2.04) microg/min and for urinary orosomucoid/creatinine ratio 0.04 (0.009-0.17) mg/mmol. We describe a fully automated, transferable, sufficiently precise, high-sensitivity assay for orosomucoid in urine and present reference values traceable to CRM 470.