Changes in red blood cell electrolyte concentrations in digitalis intoxication.

The value of red blood cell (RBC) sodium/potassium ratio in diagnosing digitalis toxicity was studied in 60 adult patients. The normal ratio was established in 34 healthy volunteers and in 10 patients with heart disease not receiving digoxin (group I). During chronic digoxin therapy, RBC sodium/potassium (Na/K) ratio and plasma digoxin were measured in 50 nontoxic patients (group II), in 10 toxic patients (group III), and in 9 of these 10 toxic patients after resolution of digoxin toxicity. Red cell sodium and RBC Na/K ratio in nontoxic patients were significantly greater than in the control group. Red cell sodium and the RBC Na/K ratio in toxic patients were significantly greater than in nontoxic patients. Despite significant group differences in these variables, however, there was considerable overlap among the subjects studied. After resolution of toxicity, red cell sodium and the RBC Na/K ratio fell to values indistinguishable from those in the nontoxic group. Although there was a statistically significant positive correlation between RBC Na/K ratio and plasma digoxin level, the RBC Na/K ratio had weak predictive value (33%) in determining digoxin toxicity. Plasma digoxin was a better predictor of digoxin toxicity (60%). The combination of plasma digoxin concentration and of RBC Na/K ratio had a higher calculated predictive value (75%), and may be more useful as an indicator of digoxin intoxication.