Standards for total serum protein assays--a collaborative study.

We have studied the standardization of total serum protein assay with the biuret reaction. Standard solutions were prepared from lyophilized preparations of human serum albumin and bovine serum albumin, with corrections made for volatile material and ash contents. These solutions and a solution of crystalline albumin standard were analyzed with a new stable biuret reagent, to establish absorptivity values (values for the absorbance of a 1 g/liter final reaction mixture). The mean values obtained were 0.302, 0.292, and 0.290 for human serum albumin, bovine serum albumin, and the crystalline albumin, respectively. We believe that the established absorptivity value will improve the accuracy of serum protein determinations. We studied the linearity of the relation between color produced and protein concentration, with use of the solutions described above and a serum pool. The color adheres to Beer's law up to the highest concentration tested: 3 g/liter for HSA and BSA, and 2.8 g/liter for serum in the final reaction mixture. The new biuret reagent has been stable for one year at room temperature. We recommend the use of bovine serum albumin as a primary standard for serum protein assays. It is inexpensive, easily available, and exhibits the best linearity in the biuret reaction.