Serum digoxin concentrations were measured in 101 outpatients by radioimmunoassay with radioactive iodine 125 to evaluate factors affecting these concentrations. Patient compliance, ascertained by asking how often patients missed taking digoxin, was the most important determinant of serum digoxin concentrations. "Compliant" patients had mean serum digoxin concentrations (±SD) of 1.2±0.8 ng—l; "noncompliant" patients had a mean of 0.7±0.7 ng/ml(P (P<.001). Factors affecting patient compliance were also investigated. Of patients not taking diuretics, 82% were compliant whereas 60% of patients taking both digoxin and diuretics were compliant (P <.01).
Analysis of relationships between serum concentration of digoxin and renal function, age, and dosage in outpatients should include compliance as a variable. Physicians should find out if patients are adhering to the prescribed regimen and search for correctable causes of noncompliance.