Quantifying non-compliance in patients receiving digoxin--a pharmacokinetic approach.

Non-compliance has a major influence on the successful outcome of a therapeutic regimen. It also unnecessarily increases the costs of health care. In a study involving 137 outpatients receiving digoxin 55 patients (40%) were found to be non-compliant. Patients who experienced communication problems and who lacked a meaningful relationship with their doctor showed a marked deterioration in compliance. An applied pharmacokinetic approach was used to predict the serum digoxin concentration for each patient. The creatinine clearance was determined and the degree of severity of heart failure was assessed. Total body clearance was then calculated. The predicted concentration was also calculated and compared with the measured digoxin concentration enabling an objective assessment of compliance. Twenty-four of the non-compliant patients who had subtherapeutic levels of digoxin (less than 0.8 ng/ml) had signs of cardiac failure. Eighteen of these patients were receiving additional medication (1.7 +/- 0.5 items) for the treatment of cardiac failure.