Comparison of methods for estimating digoxin dosing regimens

Five methods of predicting serum digoxin concentrations (SDCs), used in conjunction with three equations for estimating creatinine clearance, were compared. Radioimmunoassay was used to determine the nadir SDCs in 79 patients (38 men, 41 women) meeting predetermined study criteria. All patients, whose ages ranged from 33 to 94 years, had steady-state digoxin levels, were on oral digoxin, and were free from thyroid dysfunction, malabsorption syndromes, dehydration, and renal failure. Three equations, involving combinations of age, lean body weight, and serum creatinine, were used to estimate creatinine clearance (Clcr). These Clcr values were then used in each of five equations to estimate SDCs. The estimated SDCs were compared to actual SDCs in the 79 patients, and the effects of changing the fraction of dose absorbed and the method of determining Clcr were analyzed. No substantial difference in predictive reliability was evident among the methods studied. Poor correlations existed between observed and calculated SDCs (r less than 0.70), and these correlations were not significantly affected by gender, fraction of dose absorbed, or method of Clcr estimation. Higher correlations were found for the oldest and youngest age groups. Severe limitations are associated with the use of the formulas studied.