Fluoroimmunoassay of digitoxin in serum

This fluoroimmunoassay for digitoxin in serum involves use of a sheep antiserum to digitoxin coupled to magnetizable solid-phase particles and fluorescein-labeled 3-O-succinyl digitoxigenin as tracer. Sodium salicylate blocks binding of the drug by binding proteins, and endogenous fluorophores and other interfering components in serum samples are reliably and completely removed at the separation and wash steps, which are facilitated by magnetic sedimentation. The method is sufficiently sensitive, precise, and specific for application to routine monitoring of digitoxin therapy, and results correlate closely (r = 0.992) with those of an established radioimmunoassay.