Certification of an enzyme reference material for alkaline phosphatase (CRM 371)

We have produced a batch of lyophilized alkaline phosphatase (AP) for use as an enzyme reference material. The enzyme was partly purified from pig kidney to a specific activity of 400 U/mg of protein and is essentially free from contaminating enzyme activities. The kinetic properties of the preparation are very close to those of the enzyme present in human serum. The partly purified AP was lyophilized in a matrix containing bovine serum albumin (40 g/L), MgCl2, ZnCl2 and NaCl. The vial-to-vial variability with respect to the catalytic concentration of the final product was 0.008. The predicted annual relative loss of activity was less than 0.01% at -20 degrees C and 0.04% at 4 degrees C. This material was certified using the IFCC proposed method. The certification procedure involved 19 laboratories throughout the world. The certified alkaline phosphatase catalytic concentration in the reconstituted material was 254 U/L with a 0.95 confidence interval of +/- 6 U/L.