Digoxin Immune Fab Dose Calculator
Digoxin Immune Fab Dose Calculator
Work out the number of digoxin immune Fab vials from a serum concentration or from a known ingested amount, with the empiric doses alongside — and the reason the serum digoxin becomes uninterpretable afterwards.
Digoxin immune Fab vials
Level or ingestion → vialsSerum digoxin 3.2 ng/mL in a 70 kg adult, calculating from the concentration
Two routes to the same answer
From an ingestion: total body load (mg) = amount ingested (mg) × 0.8, then vials = body load ÷ 0.5
Round up in both cases. One vial contains 40 mg of digoxin immune Fab and binds 0.5 mg of digoxin.
- ÷ 100
- the constant folds together digoxin's volume of distribution of about 5 L/kg and the 0.5 mg bound per vial, which is why a weight is needed alongside the concentration
- × 0.8
- the oral bioavailability of digoxin tablets. The ingested amount is not the body load — a fifth of a tablet dose never reaches the circulation
- ÷ 0.5
- one 40 mg vial binds 0.5 mg of digoxin. The 40 mg is the mass of Fab protein in the vial and is not the number to divide by
- round up
- vials are indivisible and the calculated requirement is always rounded up to the next whole vial; 2.24 vials means 3
Worked example
Serum digoxin 3.2 ng/mL in a 70 kg adult, calculating from the concentration
3.2 × 70 = 224
224 ÷ 100 = 2.24 vials
Rounded up to the next whole vial: 3 vials, which is 120 mg of digoxin immune Fab
By the other route, a reported ingestion of 5 mg gives a body load of 5 × 0.8 = 4 mg, and 4 ÷ 0.5 = 8 vials
The two routes answer different questions and routinely disagree; the choice between them depends on what is actually known, and on advice from a clinical toxicology service
The two routes, and the empiric doses
| Situation | Dose | Note |
|---|---|---|
| Steady-state concentration known | vials = digoxin (ng/mL) × weight (kg) ÷ 100, rounded up | The concentration must be a true post-distribution sample, at least 6 hours after the dose |
| Ingested amount known, concentration not | body load = mg ingested × 0.8, then vials = body load ÷ 0.5, rounded up | Depends entirely on a history that is frequently wrong |
| Acute toxicity with cardiac arrest, nothing known | 10 vials empirically, repeatable after 15 minutes | Empiric adult dose |
| Chronic toxicity in an adult, concentration unknown | 6 vials empirically | Empiric adult dose |
| Any | One vial contains 40 mg of Fab and binds 0.5 mg of digoxin | Dividing by 40 instead of 0.5 under-doses eightyfold |
After Fab is given, the digoxin assay stops meaning anything
| Question | Answer |
|---|---|
| Can I use a serum digoxin concentration to judge the response? | No. Routine assays measure total digoxin, bound and unbound together, so the level rises sharply after Fab and stays high for days or longer while the patient improves |
| How long before a level is interpretable again? | Not until the Fab–digoxin complexes have been cleared, which takes days and considerably longer in renal impairment |
| What is followed instead? | The clinical state, the rhythm and the serum potassium — Fab can precipitate hypokalaemia as potassium moves back into cells |
| Does a free digoxin assay help? | It answers the question a total assay cannot, but it is not routinely available and results rarely return in a useful timeframe |
Sizing a dose is not the same as deciding to give one
Digoxin immune Fab is dosed by how much digoxin has to be bound, and there are two ways to estimate that. Where a genuine post-distribution concentration is available, the number of vials is the serum digoxin in ng/mL multiplied by body weight in kilograms and divided by 100 — a constant that folds together digoxin’s volume of distribution of roughly 5 L/kg and the 0.5 mg of digoxin one vial binds. Where only the ingested amount is known, the total body load is the amount ingested multiplied by 0.8 for oral bioavailability, and the vial count is that load divided by 0.5.
Two arithmetic details cause real errors. A vial contains 40 mg of Fab protein and binds 0.5 mg of digoxin, and those numbers are not interchangeable: dividing a body load by 40 rather than by 0.5 under-doses by a factor of eighty. And the requirement is always rounded up, because a part vial does not exist — a calculated 2.24 vials is three. The two routes also disagree routinely, since they answer different questions from different information, and the concentration route is worthless if the sample was drawn inside six hours of a dose, when it reflects distribution rather than the body load.
None of that says whether Fab should be given. The decision is clinical: a life-threatening ventricular arrhythmia or bradyarrhythmia unresponsive to atropine, haemodynamic instability, or hyperkalaemia attributable to the digoxin itself, which in acute poisoning reflects poisoning of the sodium-potassium pump and is a marker of how much active drug is at the target. A concentration on its own, in a patient who is well, is not an indication. Poisoning management is directed by a clinical toxicology service, and this calculator supports that conversation rather than replacing it.
Once Fab has been given, the serum digoxin assay stops being interpretable. Routine assays measure total digoxin — bound to Fab and free together — so the concentration rises steeply after administration and remains high for days, longer in renal impairment, while the patient recovers. A high level after Fab is expected and is not evidence of a failed or inadequate dose. What is followed instead is the patient: the rhythm, the haemodynamics, and the serum potassium, which can fall as potassium moves back into cells once the pump is unblocked.
Frequently asked questions
How many vials of digoxin immune Fab are needed?
From a steady-state concentration, vials = serum digoxin in ng/mL × body weight in kg ÷ 100, rounded up. From a known ingestion, the body load is the ingested amount in mg × 0.8, and the vial count is that load ÷ 0.5, again rounded up. Whether to give any is a clinical decision.
What are the empiric doses when nothing is known?
Ten vials in acute toxicity with cardiac arrest, repeatable after 15 minutes, and 6 vials in chronic toxicity in an adult where the concentration is unknown. These are the published figures for those specific situations, not a general starting dose.
How much digoxin does one vial bind?
0.5 mg. The vial contains 40 mg of digoxin immune Fab protein, which is a different number entirely — dividing a body load by 40 instead of 0.5 under-doses by a factor of eighty.
Why does the serum digoxin go up after giving Fab?
Because routine assays measure total digoxin, bound and free together, and the Fab-bound drug is redistributed into the plasma. Levels read high for days, longer in renal impairment, and a high level after Fab is expected rather than evidence of continuing toxicity.
What decides whether to give digoxin immune Fab?
The clinical picture, not a number: life-threatening arrhythmia, haemodynamic instability, or hyperkalaemia attributable to the digoxin. A raised concentration in a well patient is not by itself an indication. The decision is taken with a clinical toxicology service.
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References
- Hassan SA, Goyal A. Digoxin immune Fab. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2024.
- Antman EM, Wenger TL, Butler VP Jr, Haber E, Smith TW. Treatment of 150 cases of life-threatening digitalis intoxication with digoxin-specific Fab antibody fragments. Circulation. 1990;81(6):1744–1752.
- Chan BS, Buckley NA. Digoxin-specific antibody fragments in the treatment of digoxin toxicity. Clin Toxicol (Phila). 2014;52(8):824–836.