Monitoring serum digoxin concentrations during digoxin immune Fab therapy

Measurement of serum digoxin concentrations (SDCs) is used routinely in the diagnosis of digoxin toxicity. Following administration of the antidote, digoxin immune antigen binding fragments (Fab), SDC monitoring is hampered by assay-related problems because of the presence of Fab in the serum. Recent evidence has suggested several available methods to monitor free SDC during Fab therapy. This report describes the utility of monitoring free SDC following Fab administration. Free SDCs were obtained using an ultrafiltration and fluorescence polarization immunoassay system in three patients over periods of up to 204 hours after Fab administration. In each case, the decline in free SDC was temporally related to the resolution of digoxin toxicity; in one case, digoxin intoxication recurred and was associated with a rebound increase in free SDC. In addition to the therapeutic benefits, prospective monitoring of free SDC in two of the patients also may have resulted in cost savings secondary to reduced hospital stay or less use of Fab.