Production and certification of an enzyme reference material for creatine kinase isoenzyme 2 (CRM 608)

Current concepts in standardization of cardiac marker immunoassays

Cardiac markers are measured by a number of different immunoassays using specific antibodies directed to the respective antigens. Lacking assay standardization, different results from different assays measuring the same marker may be obtained and this problem may cloud interpretations of reported data. Presently, there are no reference procedures for cardiac markers; certified reference materials should still be established and, at least for cardiac troponins, the analyte in the patients' blood is significantly different from newly synthesized protein. It is therefore clear that the problems of cardiac marker standardization will not be quickly solved. A number of projects are, however, underway under the auspices of the IFCC and other organizations. The aim of this opinion is to reflect on some concepts related to the implementation of a metrologically correct measurement system, giving practical examples on how these concepts can be applied to immunoassays measuring cardiac markers.

IFCC Primary Reference Procedures for the Measurement of Catalytic Activity Concentrations of Enzymes at 37°C. Part 7. Certification of Four Reference Materials for the Determination of Enzymatic Activity of γ-Glutamyltransferase, Lactate Dehydrogenase, Alanine Aminotransferase and Creatine Kinase according to IFCC Reference Procedures at 37°C

This paper is the seventh in a series dealing with reference procedures for the measurement of catalytic activity concentrations of enzymes at 37 degrees C and the certification of reference preparations. Other parts deal with: Part 1. The Concept of Reference Procedures for the Measurement of Catalytic Activity Concentrations of Enzymes; Part 2. Reference Procedure for the Measurement of Catalytic Concentration of Creatine Kinase; Part 3. Reference Procedure for the Measurement of Catalytic Concentration of Lactate Dehydrogenase; Part 4. Reference Procedure for the Measurement of Catalytic Concentration of Alanine Aminotransferase; Part 5. Reference Procedure for the Measurement of Catalytic Concentration of Aspartate Aminotransferase; Part 6. Reference Procedure for the Measurement of Catalytic Concentration of Gamma-Glutamyltransferase. A document describing the determination of preliminary reference values is also in preparation. The certification of the catalytic activity concentrations as determined by the recently elaborated IFCC primary reference methods at 37 degrees C of four enzyme preparations, namely IRMM/IFCC 452 (gamma-glutamyltransferase), IRMM/IFCC 453 (lactate dehydrogenase 1), IRMM/IFCC 454 (alanine aminotransferase) and IRMM/IFCC 455 (creatine kinase) is described. Homogeneity data were derived from previous results. Stability was assessed using recently obtained data as well as data from previous stability studies. The collaborative study for value assignment was performed under a strict quality control scheme to ensure traceability to the primary reference method. Uncertainty of the materials was assessed in compliance with the Guide to the Expression of Uncertainty in Measurement. The certified values obtained at 37 degrees C are 1.90 microkat/l +/- 0.04 microkat/l (114.1 U/l +/- 2.4 U/l), for gamma-glutamyltransferase, 8.37 microkat/l +/- 0.12 microkat/l (502 U/l +/- 7 U/l), for lactate dehydrogenase 1, 3.09 microkat/l +/- 0.07 microkat/l (186 U/l +/- 4 U/l), for alanine aminotransferase and 1.68 microkat/l +/- 0.07 microkat/l (101 U/l +/- 4 U/l), for creatine kinase. The materials are intended for internal quality control as well as for the evaluation of test systems as required by recent European Union legislation. Furthermore, the materials can be used to transfer accuracy from a reference method to a routine procedure provided the procedures exhibit the same analytical specificity and the certified materials are commutable.

Certification of the mass concentration of creatine kinase isoenzyme 2 (CK-MB) in the reference material BCR 608

We describe the certification of a mass concentration value in the already prepared creatine kinase-2 reference material (BCR 608). Creatine kinase-2 was purified from human heart. The purified enzyme was diluted in order to measure its protein concentration by the Doetsch method. A protein concentration value of 124.30+/-13.17 mg/l was assigned to the stock solution of purified creatine kinase-2. This stock solution was diluted in 25 mmol/l piperazine-N,N'-bis[2-ethanesulfonic acid] (PIPES) pH 7.2, containing 2 mmol/l ADP, 5 mmol/l 2-mercaptoethanol, 154 mmol/l sodium chloride and 50 g/l human albumin to obtain a stable liquid standard of known creatine kinase-2 mass concentration (80.36 microg/l). This standard was then used to recalculate the creatine kinase-2 mass concentration measured in the BCR 608 material by immunoassay. The mass concentration of creatine kinase-2 in samples of reconstituted BCR 608 was certified to be 93.30+/-9.65 microg/l.

Production and certification of an enzyme reference material for adenosine deaminase 1 (BCR 647)

BACKGROUND

We describe the preparation of a lyophilised reference material containing purified human adenosine deaminase 1 and the certification of its catalytic concentration.

METHODS

The enzyme was purified from human erythrocytes.

RESULTS

The enzyme was >99% pure on polyacrylamide gel electrophoresis. Only trace amounts (<0.4%) of alanine aminotransferase, aspartate aminotransferase and L-lactate dehydrogenase were detected in the purified fraction. The purified adenosine deaminase had a molar mass of 41600 g/mol and an isoelectric pH at 4.7, 4.85 and 5.0. The material was prepared by diluting the purified adenosine deaminase in a matrix containing 50 mmol/l Tris-HCl buffer pH 7.4 and 30 g/l human serum albumin; dispensing in vials and freeze-drying. The batch was homogeneous and the predicted loss of adenosine deaminase activity per year on the basis of accelerated degradation studies was 0.006% at -20 degrees C and 0.04% at 4 degrees C. The certified value for adenosine deaminase catalytic concentration in the reconstituted reference material is (2.55+/-0.09) microkat/l when measured by the method that uses adenosine as substrate and glutamate dehydrogenase as auxiliary enzyme at 37 degrees C.

CONCLUSIONS

The material can be used to verify the comparability of results from different laboratories, for intra-laboratory quality control, or for calibration of the adenosine deaminase catalytic concentration measurements.

Creatine kinase 2 mass measurement: methods comparison and study of the matrix effect

Five different commercial immunoassays for the measurement of creatine kinase isoenzyme 2 mass concentration were compared using human plasma samples covering a wide range of creatine kinase 2 concentrations. The immunoassays studied differ in the detection systems, in the specificity of the antibodies and in the calibrators used. Intermethod comparison by regression analysis showed differences in the results of creatine kinase 2 mass concentration. The following ratios were deduced from the obtained equations: Elecsys=1.10xImmulite=1.20xIMx=1.26xACS:180= 1.33 x Stratus. The commutability of different materials prepared by diluting purified human creatine kinase 2 in biological and synthetic matrices was studied using the different immunoassays in comparison with human plasma specimens. Almost all the materials tested were not commutable.

Standardization of Creatine Kinase-MB (CK-MB) Mass Assays: The Use of Recombinant CK-MB as a Reference Material

Background: The AACC assembled a committee to identify and validate a standard creatine kinase MB isoenzyme (CK-MB) material to improve the comparability of CK-MB mass assays.

Methods: Three protocols were used. In protocol I, various CK-MB materials prepared in different matrices were screened as candidate standards. In protocol II, participating manufacturers calibrated their systems with concentrates of human heart CK-MB and then tested 20 patient samples to evaluate calibration bias. In protocol III, participating manufacturers calibrated their immunoassay systems using recombinant CK-MB2 (rCK-MB2) diluted into their respective sample diluents and measured 50 samples.

Results: Candidate materials showed high recovery in stripped human serum, but bias improved only from 59% to 38%. These data led to the use of human heart CK-MB diluted in each manufacturer’s sample diluent. This strategy reduced bias from 31% to 15%. Because human heart CK-MB is difficult to provide, a lyophilized source of CK-MB2 was identified. rCK-MB2 was shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, reversed-phase HPLC, intrinsic protein fluorescence, circular dichroism, agarose gel electrophoresis, immunoreactivity studies, high and low temperature stability, and reconstituted stability to be equivalent to human heart CK-MB. Calibration of immunoassay systems with rCK-MB2 added into each respective manufacturer’s sample diluent showed a 13% between-manufacturer bias.

Conclusion: Lyophilized rCK-MB2 was determined suitable for use as a reference material for CK-MB mass assays.