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(Circulation. 1965;31:228.)
© 1965 American Heart Association, Inc.

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A Method for Measuring Plasma Levels of Digitalis Glycosides

JEROLD M. LOWENSTEIN M.D.¹

¹ From the Isotope Laboratory, Presbyterian Medical Center and Children's Hospital, San Francisco, California.

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 mµg. 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 mµg./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 mµg./ ml. of digoxin equivalent for digitalis leaf, 0.05 mµg./ml. for digoxin). Patients toxic on digoxin averaged 6 mµg./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.

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