Effect of serum lyophilization on the rate constants of enzymatic methods for measuring cholesterol

We determined the equilibrium absorbances and rate constants for two enzymatic methods, aca (DuPont) and RA-1000 (Technicon), used in determining cholesterol in reconstituted lyophilized serum. The lyophilized materials included two serum pools, three control materials, a College of American Pathologists' survey material, and Standard Reference Material no. 909. We calibrated the reagents with aca standards for cholesterol (DuPont). The difference in the mean concentrations of cholesterol (aca - RA-1000) was -0.09 g/L overall and was not statistically significant by analysis of variance. The mean rate constant for all materials was 0.23 min-1 for the aca and 1.42 min-1 for the RA-1000, significantly different (P less than 0.001). Lyophilization causes lower results for the aca method than for the Ra-1000, because the reaction rate for the aca method is slower and has not reached equilibrium when the final absorbance reading is made.