Comparison of five radioimmunoassays and enzyme bioassay for measurement of digoxin in blood.

Digoxin-containing sera from 86 patients were analyzed for this drug by bioassay and radioimmunoassay. Each serum was analyzed in duplicate by six procedures: inhibition of Na+-K+-dependent ATPase and five "kit" radioimmunoassays from four different commercial sources. Mean values for two of the radioimmunoassays differed significantly from those for the bioassay. One radioimmunoassay mean value was significantly different from the other five mean values. We conclude that normal values for digoxin radioimmunoassay should be determined for each kit, and should not be adopted from published data.