Therapeutic drug monitoring of digoxin: help or hindrance?

A major role of therapeutic drug monitoring services is to advise on the dose of a drug which would be required to bring the concentration in the blood to within specific 'therapeutic' limits. Monitoring digoxin usage constitutes a substantial part of the work load. We have examined the potential variability in recommendations for digoxin dosages based on a series of serum digoxin measurements in each of 80 out-patients. In over a third of cases a dose, which seemed to be optimal on the basis of the first assay result, would have produced concentrations outside the conventional therapeutic range when the measurement was repeated. This was despite careful supervision of medication and the timing of the blood sample in relation to its administration. In routine practice the apparent variability in dose requirements would be far greater. Objectives of therapeutic drug monitoring services remain the same, the way forward would seem to lie in their taking on a heavy commitment to counsel and supervise patients before repeated blood sampling. However, effort and resources might be better channelled into producing simple prescribing aids relating the risk of cardiotoxicity directly to the size of the maintenance dose and the individual's renal function.