A sensitive enzyme immunoassay for the measurement of low concentrations of IgA.

A microtitre plate enzyme immunoassay (EIA) for determining low concentrations of IgA is described and validated for serum and plasma products. The measuring range of the assay was 3.3-150 micrograms/l. The predilution requirement of samples was matrix dependent, ranging from 1/16 for serum to 1/2 for 4% albumin. Dilute protein solutions required no predilution. The limits of detection were 50 micrograms/l for serum, 25 micrograms/l for intravenous immunoglobulin, 13 micrograms/l for 20% albumin, 7 micrograms/l for 4% albumin and 3 micrograms/l for washing solutions of blood cell components. Interassay coefficients of variation over the range of 3.4 mg to 1.5 g IgA/l ranged from 3.8 to 5.7%. Respective values for two low-level sera, containing 309 and 512 micrograms IgA/l, were 15.5% and 11.1%. Comparison of the EIA with a commercial radial immunodiffusion (RID) method showed that the results of the two assays correlated well ([EIA] = 0.877 x [RID] + 0.401 mg/l, r = 0.996, n = 20). This assay is also suitable for the large-scale screening of blood donors.