Digoxin and Phenytoin analyses as part of consultations in clinical pharmacology: a study on the use of drugs.

The clinical use of digoxin and phenytoin analyses (in 405 and 152 patients, respectively) furnished by our clinical pharmacological laboratory during a five-month period was evaluated prospectively from request forms. Of first-time analyses of digoxin, 12, 38, and 50% fell above, within, and below our recommended range of 1.3-2.6 nmoles/liter, respectively. This was a significant change towards lower values compared to an earlier study. The average daily dose of digoxin was 0.22 mg, and 94% of the doses ranged between 0.13 and 0.25 mg. Thirty percent of the patients on digoxin were reinvestigated once or more, and a greater percentage of the concentrations was then within the recommended range. Mean plasma concentrations of digoxin increased significantly with age, even though the stated daily digoxin dose tended to decrease. Data from a drug surveillance study showed that 10 of 32 patients had a significant change in plasma digoxin concentration after admission to hospital, indicating deviations in compliance with the dosage regimen prior to hospitalisation. Sixty-three percent of the first-time analyses of phenytoin were below our recommended therapeutic range of 40-80 mumoles/liter. This was lower than in a previous retrospective investigation (72%). Eighty-seven percent of the doses ranged between 0.2 and 0.4 g/day, and the average daily dose was 0.3 g. High plasma concentrations were noted more frequently in patients aged 60 years or more, whilst low concentrations were noted more frequently in young patients.