Pitfalls in, and new methods for, the radioimmunoassay for digoxin in urine.

Examining techniques for radioimmunoassay of digoxin in urine, we found that the apparent concentration of digoxin is affected by endogenous substances, independent of the concentration of digoxin itself. We describe a modification of the Becton Dickinson solid-phase RIA that makes it more nearly accurate and more easily performed, and obviates the endogenous interference. Digoxin concentrations from 5.0 to at least 1000 micrograms/L can be measured.