Solid-phase immunoassay of digoxin by measuring time-resolved fluorescence.

We describe a time-resolved fluoroimmunoassay for the cardiac glycoside digoxin. The assay depends on the competitive distribution of Eu3+-labeled anti-digoxin antibodies between solid-phase-bound digoxin and the digoxin in the sample or standard. After this immunoreaction, the bound fraction of the Eu3+-label is dissociated from the solid phase, converted into a highly fluorescent beta-diketone chelate, and measured in a fluorometer with time-resolution (1230 Arcus, LKB-Wallac). Sample volume is 20 microL, the incubation lasts 30 min, and the concentration range of the assay for digoxin is from 0.25 to 4.0 ng/mL (0.32-5.1 nmol of digoxin per liter). Correlation with an in-house RIA was good (r = 0.97, n = 43). The sensitivity was equivalent to that of most RIAs: 0.2 ng/mL (0.25 nmol/L).