Digoxin Unit Converter
Digoxin Unit Converter
Convert digoxin between ng/mL, µg/L and nmol/L, with the separate heart-failure and atrial fibrillation targets and the six-hour sampling rule that makes a level interpretable.
Digoxin converter
Mass ⇄ molarDigoxin 1.1 ng/mL, drawn 8 hours after the dose
Formula and conversion factor
ng/mL = nmol/L ÷ 1.28051
- 1.281
- derived from the molecular mass of digoxin, 780.94 Da (1000 ÷ 780.94)
- µg/L
- numerically identical to ng/mL — a report in either can be read straight across
- ≥ 6 hours post-dose
- distribution into tissue is slow; a level drawn earlier reflects drug still in transit and is meaningless
Worked example
Digoxin 1.1 ng/mL, drawn 8 hours after the dose
1.1 × 1.28051 = 1.41 nmol/L
= 1.1 µg/L
Within the atrial fibrillation range (0.5 – 2.0 ng/mL), but above the heart-failure target (0.5 – 0.9 ng/mL)
Conventional and SI thresholds
| ng/mL (= µg/L) | nmol/L | |
|---|---|---|
| Heart failure target | 0.5 – 0.9 | 0.64 – 1.15 |
| Atrial fibrillation range | 0.5 – 2.0 | 0.64 – 2.56 |
| Toxicity increasingly likely | > 2.0 | > 2.56 |
What raises the level, and what raises the risk at a given level
| Factor | Effect |
|---|---|
| Renal impairment | Raises the level — digoxin is cleared renally |
| Amiodarone, verapamil, quinidine, clarithromycin | Raise the level, often substantially |
| Hypokalaemia | Potentiates toxicity at any level |
| Hypomagnesaemia | Potentiates toxicity at any level |
| Hypercalcaemia | Potentiates toxicity at any level |
Reading a digoxin level against the clock
Digoxin is reported in ng/mL in most laboratories and in nmol/L where SI units are used; ng/mL and µg/L are numerically identical, so a report in either can be read straight across. The molar conversion uses a molecular mass of 780.94, giving a factor of 1.281 from ng/mL to nmol/L. Confusing the two shifts a level by roughly a quarter, which is enough to move an apparently therapeutic result into the toxic range or the other way.
A digoxin level is only interpretable if the time it was taken relative to the dose is known. Digoxin distributes slowly out of plasma into tissue, and that distribution takes hours, so a sample drawn early is high and meaningless — it reflects drug still in transit rather than the concentration at the site of action. Sample at least six hours after the dose, and preferably immediately before the next one. A number reported without a timing should be repeated, not interpreted.
The target differs by indication. In atrial fibrillation the conventional range is 0.5 to 2.0 ng/mL, but in heart failure the target is lower, 0.5 to 0.9 ng/mL, because post-hoc analysis of the DIG trial associated higher serum digoxin concentrations with increased mortality. The lower heart-failure target is not a laboratory quirk — it reflects an outcome signal, and a level of 1.5 ng/mL in a patient taking digoxin for heart failure warrants a dose reduction even in the absence of symptoms.
A level inside the range does not exclude digoxin toxicity. Hypokalaemia, hypomagnesaemia and hypercalcaemia all potentiate digoxin at the sodium-potassium ATPase, so toxicity can occur at an entirely therapeutic concentration, and the electrolytes should be reviewed alongside every level. Concentrations rise with renal impairment, since digoxin is cleared renally, and with amiodarone, verapamil, quinidine and clarithromycin. Toxicity is diagnosed clinically — nausea, visual disturbance, confusion, bradyarrhythmias — and treated on that basis rather than on the number alone.
Frequently asked questions
How do I convert digoxin from ng/mL to nmol/L?
Multiply by 1.281, derived from digoxin’s molecular mass of 780.94 Da. A level of 1.1 ng/mL is 1.41 nmol/L. Note that ng/mL and µg/L are the same number.
When should a digoxin level be taken?
At least 6 hours after the dose, and ideally immediately before the next one. Distribution into tissue is slow, so a level drawn earlier is high and reflects drug still in transit rather than concentration at the site of action. A level with no recorded timing cannot be interpreted.
Why is the heart-failure target lower than the atrial fibrillation range?
Post-hoc analysis of the DIG trial associated higher serum digoxin concentrations with increased mortality in heart failure. The target is therefore 0.5 – 0.9 ng/mL rather than the 0.5 – 2.0 ng/mL range used for rate control in atrial fibrillation.
Can digoxin toxicity occur with a therapeutic level?
Yes. Hypokalaemia, hypomagnesaemia and hypercalcaemia all potentiate digoxin at the sodium-potassium ATPase, so toxicity can occur at an entirely normal concentration. Toxicity is a clinical diagnosis, supported by the level rather than defined by it.
Which drugs raise digoxin levels?
Amiodarone, verapamil, quinidine and clarithromycin are the classic culprits, and renal impairment raises levels because digoxin is cleared renally. Any of these should prompt an early level rather than waiting for the next routine one.
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References
- The Digitalis Investigation Group. The effect of digoxin on mortality and morbidity in patients with heart failure. N Engl J Med. 1997;336(8):525–533.
- Rathore SS, Curtis JP, Wang Y, Bristow MR, Krumholz HM. Association of serum digoxin concentration and outcomes in patients with heart failure. JAMA. 2003;289(7):871–878.
- Joint Formulary Committee. British National Formulary. London: BMJ Group and Pharmaceutical Press.