Calculator inputs
Example data table
| Scenario | Target INR | Current INR | Weekly dose | Example adjustment | Follow-up |
|---|---|---|---|---|---|
| Mildly low INR | 2.0 to 3.0 | 1.8 | 35.0 mg | About 5% to 15% increase | 7 to 14 days |
| Therapeutic INR | 2.0 to 3.0 | 2.5 | 35.0 mg | No routine change | 2 to 4 weeks |
| Mildly high INR | 2.0 to 3.0 | 3.3 | 35.0 mg | About 5% to 10% decrease | 7 to 14 days |
| High INR | 2.0 to 3.0 | 4.8 | 35.0 mg | Hold 1 dose and reduce | 3 to 7 days |
Formula used
1. Weekly adjustment
New weekly dose = Current weekly dose × (1 + percentage change ÷ 100)
Negative percentages reduce the weekly dose. Positive percentages increase it.
2. Daily average
Average daily dose = Suggested weekly dose ÷ 7
3. Practical schedule rounding
The tool rounds the weekly plan to the selected dose increment, then spreads doses across seven days while honoring one or two hold days when needed.
How to use this calculator
- Choose the goal INR range used for the patient.
- Enter the current INR and the latest weekly warfarin dose.
- Add the previous INR to let the tool judge trend direction.
- Record missed doses, medicine changes, diet changes, or illness.
- Select the bleeding status before trusting any maintenance suggestion.
- Press Calculate adjustment to show the result above the form.
- Review the recommendation, safety alerts, and seven-day dosing schedule.
- Download the summary as CSV or PDF for documentation.
FAQs
1. Does this tool replace clinician judgment?
No. It is an educational support tool for maintenance review. Bleeding, thrombosis symptoms, adherence, procedures, interacting medicines, and local protocols must still drive the final decision.
2. Why does the tool ask about missed doses?
A low INR can reflect missed doses rather than a true maintenance requirement. Increasing the weekly dose without checking adherence can overshoot the INR after routine dosing resumes.
3. Why are medicine changes important?
Many medicines alter warfarin metabolism or bleeding risk. Antibiotics, amiodarone, azoles, and enzyme inducers are classic examples. Review the medicine timeline before applying a permanent dose change.
4. What does the weekly dose percentage mean?
Warfarin maintenance changes are commonly planned as a percentage of the total weekly dose. This approach usually creates smoother adjustments than changing a single day without reviewing the full week.
5. Why can the tool suggest holding doses?
When the INR is clearly above range, a short hold may be safer than only trimming later doses. The tool limits holds to maintenance-style situations without bleeding management instructions.
6. Can I use a custom target INR?
Yes. Choose the custom option and enter the desired lower and upper limits. Confirm that the selected range matches the patient’s indication and local specialist guidance.
7. Why is the result sometimes cautious?
The tool becomes conservative when confounders are present. Missed doses, illness, medicine changes, and large diet changes can explain INR shifts without proving the maintenance dose is wrong.
8. What should happen with major bleeding or very high INR?
Treat that as urgent. The tool intentionally stops short of giving full reversal orders. Use emergency assessment, senior review, and local warfarin reversal protocols immediately.