[Serum digoxin concentration: dependence on body weight and age].

In patients of a cardiological practice, 121 digoxin serum concentrations were determined by radioimmunoassay (RIA). Some drugs were suspected of interfering with the RIA or with the pharmacokinetics of digoxin. Patients having such additional drugs or patients with elevated serum creatinine were not included. The daily maintenance dose of digoxin was roughly adjusted to body weight. Patients with 0.5 mg digoxin daily showed unexpectedly low serum digoxin levels not fully explained by the relatively high body weight. This dose group was not included in the following correlations. At a maintenance dose of 0.25 and 0.375 mg digoxin and in the age groups 40-69 years (n = 66) there was an approximately inverse proportionality between serum digoxin concentration (per 0.25 mg digoxin daily) and body weight. When all age classes from 20 to 89 years were included (n = 96), a week positive correlation between serum digoxin concentration (per 0.25 mg digoxin daily and per 69.28 kg body weight) and age was found. A similar positive correlation resulted between serum digoxin concentration (per 0.25 mg digoxin daily) and the reciprocal of the nomographically determined creatinine clearance, always within the normal serum creatinine range. Based on these correlations, two simplified formulas are presented to predict the serum concentration and therapeutic maintenance dose of digoxin. The formulas are valid for the normal serum creatinine range and for digoxin tablets of optimal bioavailability.