Reference materials in clinical enzymology: preparation, requirements and practical interests.

Five enzyme materials (gamma-glutamyltransferase, alkaline phosphatase, creatine kinase, alanine aminotransferase and prostatic acid phosphatase) are currently certified using a reference method. Furthermore, feasibility studies have been performed for four other enzymes (aspartate aminotransferase, lactate dehydrogenase, amylase and lipase). They indicated that these enzymes can be purified and stabilized, but the materials have not yet been certified. This shows that the most important enzymes in clinical laboratories can be purified, and stabilized, without significant alteration of their catalytic properties. By carefully choosing a matrix, the commutability of these enzyme preparations and patients' samples between some methods, including routine methods, may be preserved. Thus, these materials can be used to calibrate the routine methods in terms of the corresponding reference methods after commutability has been verified. Current studies suggest that this objective can be reached, provided three criteria are satisfied: i) the calibrated and reference methods must be of equal specificity; ii) the enzyme calibrator should be, as closely as possible, identical to the human analyte enzyme in its native matrix (eg serum); iii) and the inter-method ratio should be constant (within the limits of experimental error) for the enzyme calibrator and for all patients' samples.