Determination of Therapeutic and Toxic Serum Digoxin Concentrations by Radioimmunoassay

A sensitive (0.2 ng per milliliter), precise (standard deviation 3 to 4 per cent), and specific radioimmunoassay for serum digoxin concentration has been developed. Levels are determined by measurement of the extent to which digoxin in the patient's serum competes with tritium-labeled digoxin, added in vitro, for digoxin-specific antibody binding sites. Mean values in nontoxic patients with normal renal function receiving 0.25 or 0.50 mg per day were 1.1 and 1.4 ng per milliliter respectively; the ranges fell within relatively narrow limits. Patients with cardiac arrhythmias attributed to digoxin toxicity had a mean level of 3.3 ng per milliliter, and little overlap with the nontoxic group (p less than 0.001). A determination can be completed in one hour, and may provide useful information to the clinician faced with the difficult problem of evaluating his patient's state of digitalization.