Determination of free digoxin concentrations in serum for monitoring Fab treatment of digoxin overdose.

A rapid method for assessing the free digoxin concentration in the serum of digoxin-overdosed patients receiving treatment with digoxin-specific Fab fragments has been developed. For this method, a protein-free ultrafiltrate is prepared from the patient's serum, and the digoxin in the ultrafiltrate (free digoxin) is measured by fluorescence polarization immunoassay. Both the inaccuracies associated with measurements of total digoxin by immunoassay in the presence of Fab and the long turnaround time associated with measurements of free digoxin by equilibrium dialysis were avoided. Good correlation was observed between measurements of free digoxin by this ultrafiltration technique and by equilibrium dialysis. The ultrafiltration method was used to evaluate the concentrations of free digoxin in a digoxin-overdosed patient treated with Fab at our hospital. In retrospect, the results suggest that her hospital stay could have been shortened by a timely appreciation of her increased concentration of free digoxin. Using the ultrafiltration method, one can determine free digoxin concentrations quickly, conveniently, and accurately in the clinical laboratory. This procedure therefore should be a valuable aid in monitoring the efficacy and adequacy of Fab treatment.