Fluoroimmunoassay of digoxin in serum

A heterogeneous (solid-phase separation) fluoroimmunoassay for digoxin in serum was developed employing antibodies coupled to magnetisable cellulose/iron oxide particles and a fluorescein-labelled digoxin derivative as tracer. Intrinsic fluorophores and other potentially interfering components of serum samples were reliably and completely removed at the separation and wash steps of the assay procedure which were facilitated by magnetic sedimentation. In order to attain adequate sensitivity (detection limit 0.2 micrograms/l (0.26 nmol/l) serum digoxin), a sample volume of 500 microliters was necessary. Assay results for patients' specimens correlated well with those obtained using established charcoal--separation (r = 0.96) and magnetisable solid-phase (r = 0.95) radioimmunoassays. The feasibility of a "stat" adaptation of the fluoroimmunoassay that involved only two standards (0.5 and 4 micrograms/l digoxin) was demonstrated. The stat method would be suitable for the assay of urgent or single specimens.