Faster and Easier Radioimmunoassay of Digoxin

Abstract

Abstract We describe an improved radioimmunoassay method for serum digoxin, with 125I as tracer, to eliminate the problems that endanger the validity of this assay: apparent instability of tracer, time-dependence of charcoal separation of free and bound fractions, and influence of sample-to-sample differences on results. Principal innovations leading to a more rugged and reliable test are the use of (a) a stable [125I] tyros ine-methyl-ester of digoxin, (b) ammonium sulfate to separate bound and free fractions, (c) a fresh serum pool for the standard curve, and (d) individual specimen blanks. Results compare favorably with those of the properly-controlled, original tritium method of Smith et al. [New Engi. J. Med. 281, 1212 (1969)] in terms of digoxin values, while the method excels it in convenience and speed. Between-run precision is 11% (CV) over the range of interest (mean, 2.7 µg/liter), sensitivity is 0.2 µg/liter, and recovery of added digoxin averages 102%.