Prescribing digoxin in geriatric units: the unexplained variability in dosage requirements.

Some physicians regard patients of Geriatric Units as a homogeneous population with respect to digoxin dosage requirements. Others advocate the use of pharmacokinetic models in prescribing digoxin for the elderly. Sixty in-patients of Geriatric Units were studied and the results compared with those previously obtained from 129 patients of other adult Units; all were receiving maintenance digoxin. For each patient the dose required to achieve a mean steady-state serum digoxin concentration of 1.6 nmol X l-1, the standardized dose, was calculated, assuming proportionality between the dose given and the concentration achieved. A mean of four estimates of standardized dose for each individual was used in the analysis. Threefold ranges of standardized dose covered the requirements of approximately 85% of patients both of Geriatric Units (62.5 to 187.5 micrograms per day) and of other adult Units (125 to 375 micrograms per day). The variables, serum creatinine concentration, sex, age, and body weight were of relatively little value in predicting the standardized dose for the patients in Geriatric Units. There was a sub-group of these in-patients for whom the standardized dose was extremely large.