A Critical Appraisal of a Further Three New Commercial Digoxin Radioimmunoassay Kits with Reference to Cross-Reacting Substances

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Abstract
Three digoxin radioimmunoassay (RIA) kits were evaluated for performance and cross-reaction with digitoxin, spironolactone, canrenone and furosemide (Lasix-Hoechst). Effects of serum [human] protein concentrations were also been tested. The following kits were tested: The Diagnostic Products Corporation Digoxin RIA Kit (A) The Byk-Mallinckrodt SPAC Digoxin Kit (B), and The Boehringer-Mannheim Digoxin RIA Kit (C). All kits used a 125I-labeled tracer. Kit A used a conventional liquid phase system using double-antibody separation for bound and free drug, Kits B and C used a solid-phase antibody coated tube method. All kits showed a lower cross-reaction to digitoxin than quoted by the manufacturer. Cross-reaction to spironolactone (Aldactone-Boetringer-Mannheim) was less than 1.50 nmol/l at a serum concentration of 125 mg/l Aldactone in all 3 kits. The cross-reaction to canrenone was somewhat higher, 5.2 nmol/l digoxin being measured in 1 kit at a serum canrenone concentration of 125 mg/l. There was no cross-reaction with furosemide in any kit, even at a serum concentration of 5 g/l. The coated-tube assays were affected by serum albumin and globulin concentration changes, 1 kit showing a difference of over 50% binding in the range 1-20% albumin. The double-antibody kit did not show dependence on the concentration of these proteins. All kits measured digoxin with good reproducibility in the range 0.40-10.0 nmol/l.