External Quality Assessment of Immunoassays for Specific Proteins in Serum; 18 Months' Experience in the United Kingdom

Abstract

Five surveys of immunoglobulin A, G and M assay have been carried out in 200 UK laboratories; assays of complement components C3 and C4 and α₁-antitrypsin (α₁-AT) by 70 laboratories were also included in the fourth and fifth surveys. Clarified liquid human serum was used for specimens, two pairs of specimens related by dilution being used in each survey. For immunoglobulins, no inter-method bias was noted although one calibrant gave significantly different mean values; nephelometry, as used by one method group (largely comprising Beckman ICS users), and radial immunodiffusion gave the best inter-laboratory agreement for IgA and IgM, while IgG showed little method dependence. Inter-laboratory precision was better for C4 than for C3 and α₁-AT, which indicates the importance of calibrant differences for the latter two; there were too few participants to allow reliable conclusions about methods to be drawn.