A new turbidometric digoxin immunoassay on the ADVIA 1650 analyzer is free from interference by spironolactone, potassium canrenoate, and their common metabolite canrenone.

Spironolactone and potassium canrenoate (aldosterone antagonist diuretics) are often used with digoxin in clinical practice. It has been well documented in the literature that spironolactone, potassium canrenoate, and their common metabolite canrenone cross-react with the fluorescence polarization immunoassay (FPIA) for digoxin and falsely elevate measured serum digoxin concentrations. Recently a new turbidometric assay for digoxin became commercially available from Bayer Diagnostic for application on the ADVIA 1650 Chemistry analyzer. We studied the potential interference of these compounds in this new digoxin assay. Aliquots of drug-free serum were supplemented with therapeutic and above-therapeutic concentrations of spironolactone, canrenone, and potassium canrenoate, and apparent digoxin concentrations were measured. We observed apparent digoxin concentrations with the FPIA digoxin assay as expected but observed no apparent digoxin levels with the new turbidometric immunoassay. When serum pools prepared from patients receiving digoxin were supplemented with these compounds in concentrations expected in serum in patients receiving these medications, we observed falsely elevated digoxin levels with the FPIA digoxin assay, but no statistically significant change was observed with the new turbidometric assay. We conclude that the new turbidometric assay for digoxin is free from interference by spironolactone, potassium canrenoate, and their common metabolite canrenone.