Effects on Radioimmunoassay of Digoxin of Varying Incubation Periods for Antigen—Antibody Reaction and Varying Periods of Adsorption by Dextran-Coated Charcoal

In radioimmunoassay of digoxin by the dextran-coated charcoal method, duration of the incubation period for the antigen-antibody reaction and time allowed for dextran-coated charcoal to adsorb the free antigen are critical factors. We compared effects of prolonged periods of adsorption on results obtained with five commercial kits. In all cases, the digoxin concentrations increased linearly with adsorption time, but the increase varied among kits. Kallestad and New England Nuclear 3H-kits and the Squibb 125I-kit showed less increase (i.e., gave relatively consistent results after various periods of adsorption); the Schwarz/Mann 125I-kit and Clinical Assays 3H-kit showed greater increases. Precautionary steps are outlined for radioimmunoassays by this technique.