Clinical value of serum digoxin assays in outpatients: Improvement by the standardization of blood sampling

Everyday physical activity previously has been shown to affect serum digoxin concentrations. Standardized rest in the supine position increases outpatient serum digoxin levels 0% to 75%. The present study comprising 56 outpatients treated with digoxin was undertaken to elucidate the clinical importance of a standardized period of rest before collection of the blood sample. Blood samples were taken about 24 hours after the latest dose, before and after 2 hours of rest in the supine position. A careful clinical examination, including electrocardiogram (ECG) findings, systolic time intervals, and chest x-ray studies, was performed to identify adverse effects/intoxication or failure of digitalis treatment. Signs of failure of digitalis treatment occurred in 12% of the patients, with a serum digoxin concentration of 0.68 ± 0.15 (mean ± SD) nmol/L before rest and 0.85 ± 0.22 nmol/L after rest. Eleven percent showed signs of adverse effects/intoxication, with serum digoxin concentrations of 1.70 ± 0.70 nmol/L before rest and 2.08 ± 0.80 nmol/L after rest. The serum digoxin concentrations of the adequately treated patients (77%) were 1.02 ± 0.35 nmol/L before rest and 1.28 ± 0.41 nmol/L after rest. The importance of standardized rest before blood sampling is illustrated by the fact that only one third of the patients without signs of adverse effects/intoxication or failure of digitalis treatment had serum digoxin concentrations within the therapeutic range most commonly used (1.2 to 2.6 nmol/L) without supine rest. If allowed to rest in the supine position before blood sampling, approximately 60% of the adequately treated patients had serum digoxin concentrations within this range. An optimal correlation between clinical status and serum digoxin concentration was reached when standardized rest and a therapeutic range of 1.0 to 2.1 nmol/L was employed.