Calibration in three dimensions: optimizing a two-parameter calibration technique to extend the range of an immunoturbidimetric urinary albumin assay into antigen excess.

I describe a general technique, called "two-parameter calibration," which allows precise determination of analyte from non-monotonic calibration plots and the calibration of immunoturbidimetric assays in antigen excess. Using three-dimensional calibration plots and relative-sensitivity curves, two optimal parameters may be selected from a number of possible options by using criteria presented here. Choosing two different values of delta At1-t2, the change in absorbance from time t1 to t2--as the reaction parameters in an immunoturbidimetric assay for albumin, I have optimized the choice of time interval for two-parameter calibration and extended the working range of the assay by three- to fourfold. The albumin assay shows excellent agreement of observed and expected values (r = 0.996) and also with results of a routine kinetic dye-binding method used on diluted plasma samples (r = 0.970).