Serum digoxin determination in outpatients--need for standardization.

1 Blood samples were taken from 30 outpatients for serum digoxin analysis before and after 5 min-4 h of rest in the supine position. 2 The digoxin concentration increased significantly during rest and a steady-state concentration was reached after approximately 2 h of rest. 3 The mean increase in serum digoxin after 2 h of rest was 23% with a considerable range, i.e. 0-75% (0-0.6 nmol/l). 4 On the analogy of a previous finding of a decrease in serum digoxin concentration during exercise in healthy subjects ingesting digoxin, the present results suggest that everyday physical activity affects the serum digoxin concentration. Standardized rest in the supine position prior to collecting blood samples from outpatients is therefore necessary for reliable serum digoxin determinations.