DigiFab dosing (digoxin immune Fab) in digoxin poisoning

Clinical background

Digoxin has a narrow therapeutic window, and toxicity occurs both in acute overdose and as a complication of chronic treatment, particularly in older patients with impaired renal function and interacting drugs. The symptoms are non-specific (nausea, visual disturbance, confusion) and there is no serum digoxin value that alone determines whether toxicity is present. The decision to give digoxin-specific antibody fragments (Fab) must therefore rest on a combination of the history, the serum digoxin, the ECG and the electrolytes, and once the decision has been made the dose must be calculated quickly and accurately. DigiFab dosing aims to calculate the number of vials needed to bind the circulating amount of digoxin, either from a steady-state serum digoxin concentration or from a known ingested amount.

Calculating the DigiFab dose

The method of calculation depends on what information is available.

From the serum digoxin concentration:

$$\text{vials} = \frac{\text{serum digoxin (ng/mL)} \times \text{body weight (kg)}}{100}$$

This formula rests on equimolar dosing: each vial binds 0.5 mg of digoxin, and the volume of distribution of digoxin is assumed to be 5 L/kg. The formula therefore gives the number of vials required to neutralise the total body load of digoxin. The serum digoxin concentration must be at steady state, that is, taken at least 6 hours after the last dose during oral therapy; otherwise free digoxin is overestimated and the dose becomes too high [2].

From a known ingested amount:

$$\text{vials} = \frac{\text{amount ingested (mg)} \times 0{.}8}{0{.}5}$$

The factor 0.8 corresponds to the oral bioavailability of digoxin. Each vial binds 0.5 mg, hence the denominator of 0.5. This method is relevant in acute overdose where the amount ingested is known but a steady-state concentration is not available.

Both formulas are rounded up to whole vials.

The underlying evidence is the multicentre study published by Antman and co-workers in 1990, in which 150 patients with potentially life-threatening digitalis toxicity were treated with digoxin-specific Fab fragments [1]. The Fab dose was calculated as the amount corresponding to the body's digoxin or digitoxin content, estimated from the history or the serum concentration. The cohort comprised chronic therapy (50 per cent), accidental overdose (10 per cent) and suicide attempts (39 per cent), with ages ranging from newborn to 94 years. Of 148 evaluable patients, 80 per cent had a complete response, 10 per cent improved and 10 per cent had no response. The median time to an initial response was 19 minutes, and 75 per cent had some response by 60 minutes. No allergic reactions were observed. The commonest adverse effects were rapidly developing hypokalaemia and worsening of heart failure.

Interpretation in practice

The calculator gives a number of vials, but its interpretation differs between acute and chronic toxicity.

An important observation from the ATOM-6 study is that titrated dosing with repeated low doses (1 to 2 vials) based on clinical assessment led to a total dose of only 25 to 35 per cent of that calculated, with no deaths [3]. This suggests that the full calculated dose may be excessive in acute overdose in a stable patient. In chronic toxicity the position is different: tissue stores release digoxin continuously, and inadequate dosing has been associated with death [2].

After Fab has been given, the serum digoxin becomes uninterpretable with standard immunoassays, since the Fab-digoxin complex cross-reacts with the antibodies in the assay. The total serum digoxin rises sharply and reflects bound, not free, digoxin. Measuring free (toxic) digoxin requires ultrafiltration, which is not routine in most laboratories [4]. Clinical assessment of arrhythmias and electrolytes is therefore the practical marker of treatment effect.

Validation and performance

There are no randomised controlled trials of Fab in digoxin toxicity. A systematic review from 2000 identified no RCTs and found that the evidence rests mainly on case series, with high consistency regarding efficacy in severe acute intoxication [5]. The expert consensus published in 2025 undertook a systematic review of 34,587 publications over six decades, of which 114 were included, and produced 33 consensus statements [6]. The consensus emphasises that the type of exposure (acute, acute-on-chronic, chronic) and the timing of ingestion must be taken into account in dosing decisions.

In Manini's case-control study of chronic digoxin toxicity, 26 patients were analysed (13 who died, 13 who survived) [2]. Fab dosing was defined as adequate if it followed the formula (serum digoxin × weight) / 100, rounded up. Among the seven patients who died despite an adequate dose, 86 per cent had presented with the combination of bradycardia and hyperkalaemia, indicating that even correctly dosed Fab does not save every patient with advanced chronic toxicity. Hyperkalaemia (potassium ≥5.0 mmol/L) was associated with a 36-fold increase in the odds of death (OR 36.7; 95 per cent CI 3.2 to 412), with an AUC of 0.76 (95 per cent CI 0.55 to 0.98) for predicting death.

A retrospective study of 278 patients from a poison control centre showed that empirical dosing (6 to 10 vials) resulted in a higher dose than that calculated in 36 per cent of patients with acute poisoning and 9 per cent of those with chronic poisoning [7]. The mean additional cost per patient of overdosing amounted to up to USD 50,589, which underlines the value of formula-based dosing.

Limitations

The formula based on the serum digoxin concentration presupposes steady state. A sample taken within 6 hours of the last dose overestimates free digoxin and leads to an excessive calculation. In acute overdose, steady state is not reached until 6 to 8 hours after ingestion, so concentration-based dosing early in the course may be misleading. In these cases, calculation from the amount ingested is preferable if the history is reliable.

The history in suicide attempts is often unreliable. The patient may report the wrong amount, and concurrent ingestion of other drugs can affect the pharmacokinetics of digoxin. Where the amount ingested is uncertain and no steady-state concentration is available, empirical dosing remains the only option.

The formula takes no account of impaired renal function. The Fab-digoxin complex is eliminated renally, and in severe renal failure elimination may take days to weeks, with a risk of recurrent toxicity as the complex dissociates. Patients with end-stage renal failure may require repeat dosing or even dialysis of the Fab-digoxin complex.

The calculator applies to digoxin. In digitoxin toxicity, the volume of distribution and protein binding differ, and the formula cannot be used without adjustment.

References

1. Antman EM et al. Treatment of 150 cases of life-threatening digitalis intoxication with digoxin-specific Fab antibody fragments. Final report of a multicenter study. Circulation 1990. PMID: 2188752
2. Manini AF et al. Prognostic utility of serum potassium in chronic digoxin toxicity: a case-control study. Am J Cardiovasc Drugs 2011. PMID: 21619380
3. Chan BS et al. Clinical experience with titrating doses of digoxin antibodies in acute digoxin poisoning. (ATOM-6). Clin Toxicol (Phila) 2022. PMID: 34424803
4. Parant F et al. Antidigoxin Fab fragments and digoxin monitoring: a challenge for the biologist. Ann Biol Clin (Paris) 2003. PMID: 14711605
5. González Andrés VL. Systematic review of the effectiveness and indications of antidigoxin antibodies in the treatment of digitalis intoxication. Rev Esp Cardiol 2000. PMID: 10701323
6. Hack JB et al. Expert Consensus on the Diagnosis and Management of Digoxin Toxicity. Am J Med 2025. PMID: 39265879
7. Nordt SP et al. Assessment of Digoxin-Specific Fab Fragment Dosages in Digoxin Poisoning. Am J Ther 2016. PMID: 25379735