A linear graph for digoxin radioimmunoassay.

The determination of drug or hormone concentrations by radioimmunoassay involves interpolation of values for radioisotope counts within standard curves, a technique which requires some dexterity in curve drawing and which results in some inaccuracy in practice. A number of linear transformations (Ekins, 1974) and computer programs (Burger et al, 1972; Duddleson et al, 1972; Livesey, 1974) for standard curves which overcome this difficulty have been described, but most of these are complex and consequently time-consuming for routine use. In radioi'munoassays involving saturation of the antibody-binding sites a special case exists in that the bound radioactivity is directly proportional to the specific activity of ligand in the system. Thus a graph of the ratio of radioactivity bound in the absence to that in the presence of added nonradioactive ligand is linear against the concentration of added ligand (Hales and Randle, 1963). The present work, which supplements Hales and Randle's method, describes a modified expression of this relationship which has been applied to the determination of digoxin using a commercial kit. Specially constructed graph paper, which yields linearity with standard solutions, can be used directly without data transformation.