Interlaboratory standardization of enzyme results: the Richmond project

Three concentrations of proficiency monitoring material and two concentrations of secondary standard calibrating material were prepared and stored frozen. The materials were prepared in buffer containing amylase from human saliva, aspartate aminotransferase from human liver, creatine kinase from human muscle, human serum albumin (20 g/L), and cofactors. The proficiency monitoring material was assayed by 10 methods in nine laboratories for 15 days to establish baseline performance. Each laboratory then used the secondary standard calibrating material to calibrate their instruments' responses to that of a standardization method, and repeated the assay of the proficiency monitoring material for 15 days. For amylase before calibration, between-laboratory mean values for the three concentrations of proficiency monitoring material were 29% lower than the standardization method, and the between-laboratory CV was 28%. After calibration the mean amylase values were 4% lower and the CV was 6%. For aspartate aminotransferase, the pre-calibration between-laboratory mean values were 24% higher than the standardization method (CV 14%) but only 3% higher (CV 6%) after calibration. CK activity deteriorated at storage temperatures above -70 degrees C. This study demonstrates that, by using a common secondary standard, laboratories can improve calibration of enzyme results.