Significance of non-steady-state serum digoxin concentrations

In order better to define the optimal time for sample collection, the authors examined the serial concentrations of digoxin in serum and urine of six hospitalized patients during the first 8 hours after administration of their maintenance doses of digoxin. Expressed as % of baseline value, mean serum digoxin concentrations 1/2, 1, 1 1/2, 2,4, 6, and 8 hours after administration of the drug were 167, 185, 228, 256, 175, 145, and 134%, respectively. Steady-state serum concentrations were not established until 6-8 hours after administration of the drug, and high serum values during the first 6 hours did not correlate with clinical and/or electrocardiographic evidence of digoxin toxicity. It is concluded that when serum digoxin levels are utilized as an index of digitalization or toxicity in patients on maintenance therapy, the blood samples should be drawn just prior to the daily dose and no sooner than 6 hours after administration of the drug.