IMPORTANCE OF THE DEFINITION OF CATALYTIC PROPERTIES FOR THE COMMUTABILITY OF AN ENZYME REFERENCE MATERIAL - EXAMPLE OF LIPASE

Assays for pancreatic lipase activity in serum need the use of calibra tors, but there is a lack of a reference method and of a reference mat erial. Under routine conditions, comparability of lipase catalytic act ivities 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. Cataly tic properties of three materials, two commercial calibrators and a ho me-purified preparation of human pancreatic lipase (HPL), have been co mpared in titrimetry at constant pH. Optimal common conditions were de fined for the titration of lipase activity in the three materials. Whe n using these titres for each calibrator, comparability was greatly im proved (ratio 1.25). This result indicates that a significant part of between-method discrepancy was due to the lack of a reference method f or the titration of lipase calibrators. The inter-method behavior of e ach material was compared to that of patients' specimens. Using HPL as calibrator, the comparability of patients' results was still dramatic ally improved (ratio = 1.01). This study shows the importance of verif ying the similarity of catalytic properties of an enzyme reference mat erial to those of the corresponding enzyme in human specimens, in orde r to ensure its commutability. The use of validated commutable calibra tors, traceable to certified reference materials, is an opportunity to improve the inter-method comparability of results in clinical enzymol ogy.