Is the determination of serum digoxin concentration useful for the diagnosis of digitalis toxicity?

In a prospective symptom-oriented study, patients with (n = 81) or without (n = 206) digoxin toxicity were not discernible on their serum digoxin concentration (SDC) because of a large overlap between toxic and non-toxic groups. There was, however, a significant difference between the mean values of the groups (p < 0,01). Serum creatinine, the presence or absence of ischemic heart disease, and/or chronic pulmonary heart disease were significantly different (both p < 0,025) between toxic and non-toxic groups. It is concluded that a decision on digitalis toxicity should be made by a synopsis of the influencing factors in the individual case. Dosage, serum creatinine and cardiac status seem to be the most important factors to be taken into account. Lack of agreement between SDC and digoxin effects could be demonstrated in differences in "T 1/2" between SDC and the normalisation of prolonged PQ-time after digoxin withdrawal. The inability of the SDC to describe all alterations important for digitalis effects and side effects seems to be that it does neither mirror the glycoside concentration at the receptor site, nor the changes of receptor affinity, nor the changes of (Na+ + K+)-ATPase activity.