A radioimmunoassay for digoxin in human urine

We report a digoxin radioimmunoassay in human urine employing commercially available reagents. The calibration curve has a mean correlation coefficient of -0.994 and is linear up to 16.0 micrograms/l. The average coefficients of variation between and within the runs are 3.6% and 3.5% respectively. The assay has a recovery of 101.4% and 98.9% at low and high levels of digoxin excretion respectively. Dihydrodigoxin at concentrations of 4, 8 and 16 micrograms/l has relative cross reactivities of 12.0%, 7.7% and 4.8% respectively. The assay was applied to compare within-patient and between-patient variability during treatments with Lanoxin and Lanoxicaps at steady-state. Precision, sensitivity, broad range linearity, convenience of commercially available reagents, insignificant cross reactivity of dihydrodigoxin with the digoxin antibody complex and urinary digoxin excretory data at different dilutions validate our assay for routine monitoring of urinary digoxin.