Standards versus standardised methods in enzyme assay.

In a trial of the Netherlands coupled external/internal quality control program a control serum and an enzyme standard were analysed over a period of eight weeks, five times each week. Five enzymes were determined: alkaline phosphatase, creatine kinase, lactate dehydrogenase, alanine aminotransferase, and gamma-glutamyltransferase. The measured values in the serum were converted to the standards. Those laboratories using the recommended methods also submitted their non-transformed serum values. The following standardisation techniques have been compared: (a) no standardisation of methodology but use of enzyme standards; (b) standardisation of methodology; (c) standardisation of methodology combined with use of an enzyme standard. Results were submitted to analysis of variance. Standardisation of methodology did not yield smaller interlaboratory variation than the standardisation with enzyme standards. In this trial a combination of both standardisation techniques yielded generally better results. Results for gamma-glutamyltransferase indicate that standardisation of substrate may be necessary apart from the use of an enzyme standard. The preparation of stable enzyme standards is stressed.