Kinetic turbidimetric method for the immunochemical quantification of immunoglobulins, including samples with excess antigen.

Here we describe a kinetic approach for quantification of the immunoglobulins (IgG, IgA, IgM) in all regions of the immunoprecipitin curve. We use centrifugal mixing and report results for maximum-velocity, two-point, and multipoint curve-fitting methods as well as the use of rate coefficients obtained from the curve-fitting process to differentiate among regions of excess antibody, equivalence, and excess antigen. We show that it is possible to quantify each immunoglobulin over a concentration range from a large excess of antibody to moderate excesses of antigen with a single set of measurements made on a single dilution of each sample. Results for standard additions of the immunoglobulins to pooled sera have relative standard deviations (coefficients of variation) in the range of 1% to 3%, with analytical recoveries in the range of 95% to 106%. Correlations among determined and reported values in individual sera are quite good, with slopes ranging from 0.86 to 1.03 and no intercepts differing from zero by more than two standard deviation units. Concentrations quantified in 14 pathological sera by the proposed method correlated well with concentrations quantified by a fluorescence immunoassay method.