Collaborative Calibration of the U. S. National and the College of American Pathologists Reference Preparations for Specific Serum Proteins

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Abstract
Under the auspices of the Ad Hoc Committee on Reference Preparations for Serum Proteins of the Standards Committee of the College of American Pathologists (CAP), and the Centers for Disease Control, Public Health Service, U. S. Department of Health and Human Services, 24 collaborators, including many experts from commercial firms that furnish the reagents and working curve calibrators actually used by most clinical laboratories in the United States, have cooperated to assign reliable values of mass and international units to 12 analytes in two freeze-dried reference preparations: The U. S. National Reference Preparation for Human Serum Proteins and the CAP Reference Preparation for Serum Proteins. The analytes estimated were: albumin, a-1-acid glycoprotein, α-1-antitrypsin, α-2-macroglobulin, ceruloplasmin, C3, C4, IgG, IgA, IgM. haptoglobin, and transferrin. Individual collaborators used single radial immunodiffusion, electroimmunodiffusion (rocket assay) rate nephelometry, end-point nephelometry, immunofluorometric assay, and enzyme immunoassay. No method-associated bias was observed in this study, but different collaborators who used the same “local” calibrator with different methods obtained results that were in closer agreement. Overall, the average among collaborator coefficient of variation of the estimated analyte concentrations was 15.9% for concentrations derived from “local” calibrators and 8.9% for concentrations that related to use of a common calibrator. The mean estimates (mass and international units) of this study provide the most practical current consensus estimates of the “true” values for these analytes in these preparations. Consequently, these mean estimates were assigned to these two generally available preparations which now can be used to provide an accuracy base to unify interlaboratory results.