Spurious underestimation of results for digoxin radioimmunoassay with a commercial kit

Estimates of digoxin in serum with a commercial redioimmunoassay kit (Schwarz/Mann) occasionally result in more counts in the patient's "bound" fraction than in the "bound" fraction for the zero standard. This phenomenon has been investigated in 26 serum samples obtained from patients who were ingesting known amounts of digoxin. The problem appears to relate to environmental differences in at least the zero standard and affects results when serum digoxin concentrations are less than 0.8 microgram/liter. Apparently dose estimates are more accurate with other methods or by calculating results after omitting the zero standard.