Validation of an enzyme calibrator--an IFCC guideline. International Federation of Clinical Chemistry

History of diagnostic enzymology: A review of significant investigations

This review of the history of diagnostic enzymology includes numerous important publications which were published over 70 years ago by outstanding clinical enzymologists throughout the world. The review includes the clinical enzymology of cardiovascular diseases, skeletal muscle diseases, pulmonary diseases, effusions, neurologic diseases, hematologic diseases, pregnancy, pediatric diseases, cancer, pancreatitis, hepatic diseases, bone diseases and cancer of the prostate. The advent of clinical enzymology thus occurred many years ago.

Intermethod calibration of alanine aminotransferase (ALT) and γ-glutamyltransferase (GGT) results: application to Fibrotest® and Actitest® scores

Two multi-component scores (Fibrotest

Standardization in laboratory medicine: new challenges

The primary goal of Laboratory Medicine is to provide information that is useful to assist medical decision-making and permits optimal health care. This type of information should be independently obtained of the measurement test kits and instruments, and also of the laboratory where the procedure is carried out. It is therefore important to achieve a level of comparability of laboratory results among the many measurement procedures available so that results are harmonized and interchangeable over space and time. The standardization of measurements is therefore of high priority. In recent years, numerous efforts have been made at the international level under the auspices of the IFCC and other organizations to standardize measurement results for many important analytes, e.g. enzymes, cardiac proteins, etc. The aim of this review is to discuss some concepts related to the achievement of standardization by the implementation of a metrologically correct measurement system, providing some examples on how these concepts can be applied in Laboratory Medicine.

A reference material for traceability of aspartate aminotransferase (AST) results

Standardization of aspartate aminotransferase (AST) determination is highly desirable for inter-laboratory comparison. Serum AST mean values for 20 patients suffering from viral hepatitis showed an inter-laboratory (n=13) variation of 9.4%. Part of this variation was due to two laboratories using procedures without pyridoxal-5′-phosphate. A traceable AST value was assigned to an enzyme calibrator (EC) through the appropriate International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) primary reference procedure. The EC was commutable for seven routine methods. Common calibration using the EC reduced the inter-laboratory coefficient of variation (CV=5.9%

Determination of lipase catalytic activity in two reference materials: BCR 693 and BCR 694 by titrimetry at constant pH

Because routine assays for pancreatic lipase catalytic activity are not yet standardized, between-method comparability is very poor. This is mainly due to the lack of reference materials (RMs). The aim of this study was to assign values of catalytic concentration to two human pancreatic lipase RMs, one prepared from human pancreatic juice (BCR 693), the other obtained by recombinant technology (BCR 694). Lipase catalytic activity was assayed in five experienced laboratories, using aliquots from the same lot of triolein emulsion and a standardized titrimetric procedure, optimized with regard to substrate, cofactors and pH. The accepted sets of data (n=4) gave a mean ± the corresponding uncertainty expressed as the 0.95 confidence interval of 1732±72 U/l and 1043±60 U/l for BCR 693 and 694, respectively. Transferability of the whole operating procedure proved to be quite satisfactory. The authors conclude that both RMs can be used to verify the correct implementation of the standardized measurement procedure and to assign values to secondary lipase materials (commercial calibrators, control products) which, in turn, ensures traceability to the standardized procedure in this study, and contributes to the harmonization of laboratory results according to the Directive for in vitro Diagnostic Medical Devices.

Preparation and characterization of reference materials for human pancreatic lipase: BCR 693 (from human pancreatic juice) and BCR 694 (recombinant)

There is a lack of certified reference material (CRM) for lipase catalytic activity. Consequently between-method comparability is very poor. The aim of this study was to produce two lipase CRMs, one from human pancreatic juice (BCR 693), and another using recombinant technologies (BCR 694). Lipase was purified from pancreatic juice, using column chromatography and isoelectric focusing. Recombinant lipase was produced with a transfected cell line and purified with column chromatography. Adding buffered bovine serum albumin and subsequent freeze-drying were used to stabilize both materials. A standardized titrimetric method was employed to compare their catalytic properties to those of two plasma pools of patients suffering from acute pancreatitis. About 5 kU (titrimetry, 37 degrees C) of each material were obtained. They were lyophilized without apparent modifications of their catalytic properties, which stayed identical to those exhibited by the enzyme present in patient's pools. Stability of both materials was estimated at several years when stored in a dry form at -20 degrees C. Both materials appear to have similar catalytic properties and stability and were found commutable as regards a reference method and a routine measurement procedure. An international certification campaign will be carried out to assign values to BCR 693 and BCR 694.

Establishing a reference system in clinical enzymology

The goal of standardization for measurements of catalytic concentrations of enzymes is to achieve comparable results in human samples, independent of the reagent kits, instruments and laboratory where the procedure is carried out. To pursue this objective, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) has launched a project to establish a reference system in clinical enzymology. This system is based on three hinges: a) extensively evaluated and carefully described reference procedures, b) certified reference materials and c) a network of reference laboratories operating in a highly controlled manner. The original IFCC-recommended procedures for alanine aminotransferase, aspartate aminotransferase, creatine kinase, gamma-glutamyltransferase, lactate dehydrogenase and alpha-amylase have been slightly modified to optimize them at 37 degrees C, with the definition of detailed operating procedures. A group of laboratories perform these procedures manually, with self-made reagents on carefully calibrated instruments. Partially purified and stabilized materials, prepared in the past by the Community Bureau of Reference, have been re-certified by these laboratories for alanine aminotransferase, creatine kinase, gamma-glutamyltransferase and lactate dehydrogenase activities. Using these materials and the manufacturer's standing procedures, industry can assign traceable values to commercial calibrators. Thus, clinical laboratories, which will use routine procedures with these validated calibrators to measure human specimens, can finally obtain values which are traceable to reference procedures.

Preparation of enzyme calibration materials

Standardisation in clinical enzymology needs not only reference methods but also reference materials. While single-enzyme reference enzymes have been developed, a multienzyme certified reference material (MECRM) available in high amount remains to be produced. To transfer trueness from the value of the reference system to patients' results, validated enzyme calibrators (EC) are also needed. Both the MECRM and the ECs must exhibit the same catalytic properties as the corresponding enzymes in human plasma. Moreover, commutability of these materials with patients' samples must be experimentally tested for one or a set of methods defined by an analytical specificity equal to that of the reference method. Various experimental studies have shown that the commutability of an enzyme material depends on the source of enzyme and its purification process, the matrix (including cofactors, effectors, additives, stabilisers... ) and the mode of processing of the final material. To promote intermethod calibration in clinical enzymology, a collaborative programme between the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), Institute for Reference Materials and Measurements (IRMM, Geel, Belgium) and IFCC corporate members is in progress for the development of a MECRM containing amylase, ALT, AST, ALP, CK, GGT, LDH, and lipase and exhibiting a wide and defined commutability.

Interassay calibration as a major contribution to the comparability of results in clinical enzymology

OBJECTIVE

Factors contributing to the applicability of interassay calibration of methods measuring enzyme catalytic activities are described. Also discussed are the properties essential for such a material. Similarity of specificity for the methods to be calibrated as well as commutability between the material(s) intended to be used as calibrator are the main criteria to be satisfied.

RESULT

Several examples demonstrated that interassay calibration is feasible but a multi-enzyme calibrator with a wide commutability for the most popular methods remains to be developed. This is the project of the IFCC Working Group on Calibrators in Clinical Enzymology (WG-CCE). Several experimental data are also presented that indicate that the temperature at which the reaction is carried out is not a limiting factor in the implementation of interassay calibration in clinical enzymology.