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Warfarin Dosing Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Anna Szczepanek, PhD
Based on research by
Holbrook A, Schulman S, Witt DM, Vandvik PO, Fish J, Kovacs MJ, Svensson PJ, Veenstra DL, Crowther M, Guyatt GH. Evidence-based management of anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines.; Chest; February 2012See 2 more sources
Undas A, Zawilska K Vitamin K antagonists (pol.); Interna Szczeklika 2021; 2021Witt DM, Nieuwlaat R, Clark NP, Ansell J, Holbrook A, Skov J, Shehab N, Mock J, Myers T, Dentali F, Crowther MA, Agarwal A, Bhatt M, Khatib R, Riva JJ, Zhang Y, Guyatt G. American Society of Hematology 2018 guidelines for management of venous thromboembolism: optimal management of anticoagulation therapy; Blood Advances; 2018
Last updated: Jan 18, 2024


The warfarin dosing calculator serves as a great help in the Coumadin dose adjustment necessary to meet the therapeutic INR requirements. πŸ’Š

Our warfarin calculator will not only tell you how to deal with the decreased or increased INR β€” it'll also leave you with life-saving recommendations that can be used in the event of massive bleeding in patients on warfarin.

❗ This tool cannot be used as a substitute for clinical knowledge, reasoning and/or consultation with a medical specialist. Always consult your doctor.

What is warfarin?

Warfarin is one of the most common blood-thinning medications and is sold under the brand name Coumadin. It is taken orally. Warfarin prevents the formation of blood clots, lowering the chances of strokes. It is also used in the treatment of pulmonary thromboembolism or deep vein thrombosis.

The most serious side effect of warfarin is bleeding. That's why it's crucial to know how to calculate warfarin dosing.

πŸ™‹ Want to know more about pulmonary embolism and/or vein thrombosis? Check the Geneva score calculator or the Padua score calculator.

What are the warfarin monitoring rules?

Warfarin monitoring includes frequent INR checks β€” they are performed by a blood test, which requires a blood draw.

A patient must undergo this procedure depending on their previous INR; this period usually ranges from 1 day to 4 weeks. πŸ“…

πŸ’‘ There are other types of blood-thinning drugs called anticoagulants that do not require INR testing. These include:

  • Injections with low-molecular-weight heparin (LMWH);
  • Dabigatran (Pradaxa) taken orally; and
  • Rivaroxaban (Xarelto) and apixaban (Eliquis), taken orally.

How to use the Coumadin calculator?

The INR warfarin dose calculator will supply you with ACCP & ASH recommendations, along with the calculated dose alteration β€” all you need to efficiently use our Coumadin calculator are the following 4 steps.

  1. Enter your patient's latest INR level.

  2. Assess the presence of any serious bleeding.
    You may choose between no bleeding, serious bleeding, or life-threatening bleeding.

  3. Calculate the total warfarin dose per day or week.

  4. Chose the target INR level.

    • 2–3 β€” recommended for atrial fibrillation, deep vein thrombosis, pulmonary embolism, or cardiomyopathy.
    • 2.5–3.5 β€” recommended for mechanical valves.

Hey, you're done! Our warfarin calculator has already displayed the results. What's gonna be your next step? Maybe you'd like to check the drug half-life calculator or the dosage calculator? πŸ’Š

How to calculate warfarin dosing adjustment?

It doesn't matter if you want to take matters into your own hands, or if you want to double-check the warfarin dosing calculator β€” we're here for you anyways! The math behind our calculations of the Coumadin dose adjustment is truly elementary:

  1. Choose your therapeutic INR range and refer to the INR warfarin chart to check recommendations for your patient's actual INR level.

  2. Get to know your patient's daily warfarin dose.

  3. Calculate your patient's weekly warfarin dose.
    You can achieve it by simply adding all their doses taken during the week.

  4. Adjust the dose to the needed percentage.
    Let's say that our patient's INR is 4 with the target being 2–3; we'd want to decrease the weekly dose of 70 mg by 20%.

πŸ’‘ 100% = 1, so
20% = 0.2
In this case, our target dose will be equal to 100% βˆ’ 20% = 80%, so
1 βˆ’ 0.2 = 0.8.

  1. Use the equation featured in our warfarin dosing calculator:

    Weekly adjusted dose = Actual dose Γ— 0.8

    Weekly adjusted dose = 70 mg Γ— 0.8

    Weekly adjusted dose = 56 mg

  2. Well, that's it. πŸŽ‰ You may finish off by calculating the new dose for every single day of therapy.

    Daily adjusted dose = Weekly adjusted dose / Number of days in a week with warfarin treatment

(not 7, since some patients don't take warfarin every day, but, e.g., every other day)

FAQ

How to read the INR warfarin chart?

In the table below, we gathered all the data used in our Coumadin dosing calculator in one place β€” let's make the process as easy as possible!

  1. Determine your patient's target INR and find the corresponding column in the table.
  2. In that column, find the patient's actual INR.
  3. Find the recommended dosage adjustments on the far right.

Target INR 2–3 

Target INR 2.5–3.5

Adjustments

< 1.5

< 2.0

Consider increasing maintenance dose by 5–20%.

Consider a single booster of 1.5–2Γ— the daily maintenance dose.

Schedule the next appointment in 3–7 days.

1.5–1.7

2.0–2.3

Consider increasing maintenance dose by 5–15%.

Consider a single booster of 1.5–2Γ— the daily maintenance dose.

Schedule the next appointment in 3–7 days.

1.8–1.9

2.3–2.4

If the two previous INRs were in range, you might consider not making any adjustments to the dose.

Consider increasing maintenance dose by 5–10%.

Consider a single booster of 1.5–2Γ— the daily maintenance dose.

Schedule the next appointment in 3–7 days. 

2.0–3.0

2.5–3.5

Desired range.

3.1–3.2

3.6–3.7

If the two previous INRs were in range, you might consider not making any adjustments to the dose.

Consider omitting one dose or decreasing maintenance dose by 5–10%.

Schedule the next appointment in 3–7 days. 

3.5–3.9

4.0–4.4

Consider omitting one dose or decreasing maintenance dose by 5–15%.

Schedule the next appointment in 1–3 days.

 >4.0

 >4.5

Hold warfarin or decrease maintenance dose by 5–20%.

Schedule the next appointment in 1 day.

Always check your patient's compliance and look for any temporary changes that could affect their INR levels. Always consult ACCP & ASH guidelines when in doubt.

What to do in case of an increased INR?

The procedure in increased INR varies depending on the desired INR level, presence of bleeding, and the magnitude of the increase. According to INR adjustment charts, possible plans of action in a patient without serious bleeding may include:

  • Omitting one dose; and
  • Decreasing maintenance dose, by either 5–10%, 5–15% or 5–20%.

We must always keep in mind the circumstances that may influence the INR levels, such as changes in diet, medications, or a patient's wellbeing (diarrhea, vomiting). Warfarin has multiple interactions with food and other drugs; the patient must be educated when initializing the treatment.

What to do in a case of a decreased INR?

The procedure in decreased INR varies depending on the desired INR level, presence of bleeding, and the magnitude of the decrease. According to INR adjustment charts, possible plans of action in a patient without serious bleeding may include:

  • Administering single booster of 1.5–2Γ— the daily maintenance dose; and
  • Increasing maintenance dose, by either 5–10%, 5–15% or 5–20%.

Always check for any changes in diet, drugs, general health of the patient. Check their compliance and inquire about any missed doses.

Łucja Zaborowska, MD, PhD candidate
Current INR
Bleeding
No significant bleeding
Total warfarin dose
mg / 
wk
Target INR
2–3
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