Importance of the definition of catalytic properties for the commutability of an enzyme reference material: example of lipase

Abstract

Assays for pancreatic lipase activity in serum need the use of calibrators, but there is a lack of a reference method and of a reference material. Under routine conditions, comparability of lipase catalytic activities was found to be poor when lipase was determined in 50 patients’ specimens by a turbidimetric (Boehringer) and a colorimetric (Sigma) assay. Mean values of the results differed by a ratio of 2.39. Catalytic properties of three materials, two commercial calibrators and a home-purified preparation of human pancreatic lipase (HPL), have been compared in titrimetry at constant pH. Optimal common conditions were defined for the titration of lipase activity in the three materials. When using these titres for each calibrator, comparability was greatly improved (ratio = 1.25). This result indicates that a significant part of between-method discrepancy was due to the lack of a reference method for the titration of lipase calibrators. The inter-method behavior of each material was compared to that of patients’ specimens. Using HPL as calibrator, the comparability of patients’ results was still dramatically improved (ratio = 1.01). This study shows the importance of verifying the similarity of catalytic properties of an enzyme reference material to those of the corresponding enzyme in human specimens, in order to ensure its commutability. The use of validated commutable calibrators, traceable to certified reference materials, is an opportunity to improve the inter-method comparability of results in clinical enzymology.