The effect of oral spironolactone and intravenous canrenoate-K on the digoxin radioimmunoassay.

Twenty patients receiving a constant maintenance therapy of digoxin preparations were selected, and daily plasma digoxin concentrations were measured using the clinical assays gammacoat 125I-digoxin radioimmunoassay. A single intravenous dose of canrenoate-K was given to 10 patients, thus producing after 5 min. a rapid rise of plasma digoxin concentrations, which varied from 121.1 to 214.3% of the baseline value (100%). In 3 of these patients, plasma digoxin concentrations were measured serially over a period of 8 hrs. and yielded digoxin levels of 81.5, 112.5 and 122.5% of the baseline value. The other 10 patients received an oral dose of 100 mg spironolactone twice a day, and plasma digoxin determinations were carried out daily. Up to the fifth day, a continuous rise of average digoxin levels was found (on day 5, 181.0%) as long as digoxin and spironolactone were continued. From the sixth to the ninth day of therapy, average mean values remained fairly constant (on days 6-9, 168.9, 173.6, 170.8, 172.5% respectively). Contradictory results have been obtained when the interference of spironolactone, canrenone or canrenoate-K on the radioimmunoassay was studied [1, 2]. This study is a further contribution for evaluating the specifity a commercially available 125J-labeled digoxin kit.