Serum digoxin by radioimmunoassay.

Using the radioimmunoassay technique for measuring serum digoxin, it was found that patients who were given 0.25 mg. digoxin orally per day had a mean serum level of 0.83 +/- 0.06 ng. per ml. In patients given 0.5 mg. daily the mean level was 1.30 +/- 0.14 ng. A higher 24-hour urinary excretion of digoxin was associated with the higher serum levels in the latter group. Individuals who exhibited electrocardiographic evidence of digoxin toxicity had a mean serum level of 2.81 +/- 0.21 ng. The majority of patients with high serum levels had evidence of impaired renal function, and it is in this clinical situation that knowledge of serum digoxin levels is likely to be most helpful in determining dose schedules.The method is specific, sensitive and reproducible. Repeated measurements on the same patient on maintenance therapy showed little variation. To obtain dependable serum levels blood should be drawn at least five hours after oral, and three hours after intravenous administration.