Estimate a warfarin starting dose in mg per week and per day from age, height, weight, race and interacting drugs, with optional CYP2C9 and VKORC1 genes.
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Calculator Functions
The calculator estimates the steady warfarin dose a person is likely to need for a target INR of 2 to 3, in mg per week and as an average in mg per day. It is written for prescribers and students. Select a function to open it in the calculator.
| Function | Use it when | You enter |
|---|---|---|
| Clinical estimate (no genotype) | No CYP2C9 or VKORC1 test result is available | Age, height, weight, race, and whether the person takes an enzyme inducer or amiodarone |
| Genotype-guided estimate | A CYP2C9 or VKORC1 result is available | The same details plus the VKORC1 and CYP2C9 genotypes, either of which can stay at Not known |
Height can be entered in inches or centimeters and weight in pounds or kilograms. Race starts at Mixed or not specified and both drug questions start at No. The result also lists the age group, height and weight the model used. To apply a prescribed percentage change to a dose already being taken, use the Coumadin Adjustment Calculator.
Warfarin Starting Dose Formulas
D = W ÷ 7
| W | Estimated stable warfarin dose (mg per week) |
| D | Average dose (mg per day) |
| R | Square root of the weekly dose, which is what the model predicts. It is the sum of the terms in the function below |
| A | Age in decades, rounded down: 1 for ages 10 to 19, 6 for ages 60 to 69, and 9 for 90 and over |
| H | Height (cm) |
| M | Body weight (kg) |
Both functions come from the International Warfarin Pharmacogenetics Consortium (IWPC) model [1]. A starting dose is chosen from the expected maintenance dose [3], which is why an estimate of the steady dose is used when warfarin is started.
Clinical estimate (no genotype)
The race and drug terms are in the Clinical column of the table below. Each one is added only when it applies.
Genotype-guided estimate
VKORC1 means the −1639G>A variant (rs9923231), and CYP2C9 covers the *2 and *3 variants. A gene left at Not known has its own term, so this function still works with one result.
| Term | Clinical | Genotype-guided |
|---|---|---|
| Race: White | 0 | 0 |
| Race: Asian | −0.6752 | −0.1092 |
| Race: Black or African American | +0.4060 | −0.2760 |
| Race: mixed or not specified | +0.0443 | −0.1032 |
| Taking carbamazepine, phenytoin or rifampin | +1.2799 | +1.1816 |
| Taking amiodarone | −0.5695 | −0.5503 |
| VKORC1 G/G | Not used | 0 |
| VKORC1 A/G | Not used | −0.8677 |
| VKORC1 A/A | Not used | −1.6974 |
| VKORC1 not known | Not used | −0.4854 |
| CYP2C9 *1/*1 | Not used | 0 |
| CYP2C9 *1/*2 | Not used | −0.5211 |
| CYP2C9 *1/*3 | Not used | −0.9357 |
| CYP2C9 *2/*2 | Not used | −1.0616 |
| CYP2C9 *2/*3 | Not used | −1.9206 |
| CYP2C9 *3/*3 | Not used | −2.3312 |
| CYP2C9 not known | Not used | −0.2188 |
Limits of This Estimate
- The result is a starting point for a prescriber, not a dose to take. Warfarin dosing must be individualized from each person’s INR response, with the INR checked daily after the first dose until it is stable [3]. Nobody should start or change warfarin from this number.
- The model was built from 5,052 patients with a target INR of 2 to 3. On patients it had not seen, its average miss was 8.5 mg per week with genotypes and 9.9 mg per week without, a little over 1 mg a day [2].
- It is less predictable for people who need more than 70 mg per week, and it does not account for vitamin K intake, smoking or interacting drugs other than the four listed [2].
- When genotypes are not known, the U.S. label gives a usual starting dose of 2 to 5 mg once daily, advises lower doses for elderly or debilitated patients, and does not recommend routine loading doses [3]. An estimate far outside that range deserves a second look at the inputs.
Sources
- International Warfarin Pharmacogenetics Consortium, Estimation of the Warfarin Dose with Clinical and Pharmacogenetic Data, N Engl J Med 2009, Supplementary Appendix 1, Sections S1e and S1f: both dose equations and how each term is coded
- International Warfarin Pharmacogenetics Consortium, Estimation of the Warfarin Dose with Clinical and Pharmacogenetic Data, N Engl J Med 2009;360:753-764: the patients the model was built from, its accuracy and its limits
- U.S. Food and Drug Administration, COUMADIN (warfarin sodium) prescribing information, 2017, Sections 2.1, 2.3 and 2.4: INR-based dosing, usual starting dose and loading doses
