Digoxin Measurements following Plasma Ultrafiltration in Two Patients with Digoxin Toxicity Treated with Specific Fab Fragments

The use of digoxin-specific antigen-binding Fab fragments in cases of severe digoxin toxicity is now well recognized as an effective method of treatment.l-? However, the high cost of this treatment restricts its use to especially severe cases of digoxin overdose or toxicity. Although there is very limited experience in most laboratories of the measurement of digoxin in plasma following intoxication, it is known that conventional digoxin immunoassay techniques are of little use following the administration of Fab tragments.t! It is the measurement of the pharmacologically active 'free' digoxin fraction during treatment with Fab fragments that is useful in assessing the effectiveness of the Fab fragments in reducing the concentration of free digoxin. Early studies utilized slow equilibrium dialysis techniques for the measurement of free digoxin concentraticns.l-i More recently, single case studies demonstrating the application of a simple and rapid ultrafiltration technique for the measurement of free digoxin in intoxicated patients have been described.l? We present two further cases of digoxin intoxication in which Fab fragments were used in therapy, and free digoxin concentrations were measured in plasma ultrafiltrate.