Progress towards international reference standards for human serum proteins

Abstract

Scientists in immunodiagnostic reagent production facilities in four nations—England, Federal Republic of Germany, The Netherlands and the United States—made five candidate international reference preparations for human serum proteins from processed, freeze-dried pools of human sera. Collaborators in 12 reference laboratories in these four nations and Canada comparatively evaluated the suitability of the five candidates as standards for measuring the serum concentrations of nine proteins: albumin, alpha-1-antitrypsin, alpha-2-macroglobulin, ceruloplasmin, C3, IgA, IgG, IgM and transferrin. Criteria for accepting a candidate as a standard included an evaluation of its performance with single radial immunodiffusion, immunonephelometry and immunofluorometry and of its stability during storage at elevated temperatures. When plotted against the logarithms of the concentration, mathematical transformations of the analytical response variable were found that gave linear curves that were parallel among all five candidate standards and serum samples of the type ordinarily encountered in immunodiagnostic practice for all techniques and all proteins evaluated. The proteins studied in all five freeze-dried preparations were remarkably resistant to thermal degradation. Tests in which coded samples were subjected to elevated storage temperatures, however, did show small, but statistically significant differences in stability among preparations. The potency ratios for the proteins studied in each preparation were obtained relative to the same analytes in the most stable candidate standard. The among-collaborator Relative potency variance, which reflects the bias of the individual collaborators was small in comparison to the corresponding within-collaborator variance for all nine proteins, irrespective of the general analytical method used by each collaborator. The imprecision and bias of each collaborator's results are appraised in detail and the clinical relevance of assay errors in discussed. On the basis of the findings, one of these candidate standards has been submitted to the World Health Organization (W.H.O.) expert Committee on Biological Standards for consideration as the W.H.O. Reference Preparation for Human Serum Proteins. If it is agreed that this is a suitable preparation it would be appropriate to assign a unitage of 100 i.u. per vial for each of the activities in the protein studied and in which there has not been an assigned unitage previously.