Comparative In Vivo Evaluation of a Radioimmunoassay and a Chromatographic Assay for the Measurement of Digoxin in Biological Fluids

Abstract

The concentrations of digoxin in plasma and urine samples obtained from three healthy male volunteers, who received 1.2mg of labeled digoxin perorally and intravenously, were simultaneously measured by a commercially available radioimmunoassay (RIA) and by a combined column thin-layer chromatographic assay (CA). The CA method, previously shown to assay digoxin specifically, was also used to monitor the individual digoxin metabolites. The results of this investigation showed that digoxin was significantly metabolized, particularly after peroral administration. The lower level of sensitivity of the RIA in plasma was 0.4ng/mL. There were highly significant positive linear correlations between the values of the following parameters of digoxin as obtained by the RIA and CA methods: the concentrations in plasma and urine, the AUCs, and the cumulatively excreted amounts in urine. The two assays did not give completely identical results either with plasma or urine; the slopes of the regression lines deviated from unity in a significant number of cases. However, there was no relationship between the magnitude of the slopes of the regression lines and the extent of metabolism. It was concluded that the commercially available RIA evaluated was specific for digoxin and that the presence of digoxin metabolites did not affect the determinations.