A Method for Measuring Plasma Levels of Digitalis Glycosides

A method is described for measuring plasma levels of cardiac glycosides, utilizing their inhibitory effect on the rubidium-86 uptake of human red cells. As little as 0.05 m&mgr;g. of digoxin/ml. of plasma produces detectable inhibition, but due to the variable effect of other substances in human plasma the practical limit of the method is 1.0 m&mgr;g./ml., about 20 times as sensitive as the duck embryo heart or the C14-labeled technics.Patients on digitalis leaf were found to have much higher circulating levels than patients on digoxin (average value 7.5 m&mgr;g./ ml. of digoxin equivalent for digitalis leaf, 0.05 m&mgr;g./ml. for digoxin). Patients toxic on digoxin averaged 6 m&mgr;g./ml.Of patients not taking digitalis, 7 per cent had significant levels of digitalis-like substances in their plasma; these were mostly individuals with metastatic carcinoma, severe fluid and electrolyte disturbances, or heart failure. No drug (other than cardiac glycosides) gave consistent high levels by this test.