Serum digoxin levels in nontoxic patients obtained 6 to 24 hours after administration of a maintenance dose of digoxin were measured by means of a radioimmunoassay method. There were 31 infants, 33 children, and 24 adults studied; 19 digitalis-toxic patients were also investigated.

The mean serum digoxin concentration in the nontoxic infants was 2.8 (±1.9) ng/ml, a level significantly greater than that obtained in the older children or adults, 1.3 (±0.4) ng/ml and 1.3 (±0.6) ng/ml, respectively (P < .01). In the infant group, similar dosage schedules resulted in a higher serum concentration when the digoxin was administered intramuscularly rather than orally, but differences were also related to the age of the patients within the infant group. Sixty-five percent of the infants receiving standard maintenance therapy had digoxin levels > 2.0 ng/ml, a range which is commonly associated with toxicity in adults. whereas only 18% of the children had serum concentrations above this value.

Mean serum digoxin levels in the digitalis toxic infants was 4.4 ng/ml and in the toxic children 3.4 ng/ml. Eleven of these 15 patients had received digoxin intramuscularly. The mean serum level measured in digitalis-toxic adults was 2.9 ng/ml.

Serum digoxin levels may be of value (1) as an adjunct in the diagnosis of toxicity, (2) in the early evaluation of accidental ingestion, (3) in the assessment of the adequacy of digitalization, and (4) in the management of digitalization of patients with renal insufficiency.

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