What warfarin (coumarin) dose adjustment is needed for a patient with a low International Normalized Ratio (INR) of 1.8 while on warfarin 4 mg for 5 days/week and 3 mg for 2 days/week?

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Last updated: June 5, 2025 • View editorial policy

From the Guidelines

For a patient with a low INR of 1.8 while on warfarin 4 mg for 5 days/week and 3 mg for 2 days/week, I recommend increasing the warfarin dose by approximately 10% per week, as suggested by the 2018 European Heart Rhythm Association practical guide 1. This could be achieved by changing to 4.4 mg for 5 days/week and 3.3 mg for 2 days/week. After implementing this change, the INR should be rechecked in 5-7 days to assess the response. The goal INR for most indications is 2.0-3.0, and the current value of 1.8 indicates slightly subtherapeutic anticoagulation, increasing the risk of thrombotic events. The dose adjustment is modest because the current INR is only slightly below the therapeutic range, and warfarin has a narrow therapeutic window. Overcorrection could lead to an excessive INR and bleeding risk. Factors that might be contributing to the low INR should also be considered, including dietary changes with increased vitamin K intake, missed doses, or drug interactions that decrease warfarin's effect. Ensure the patient maintains consistent vitamin K intake and adheres to the prescribed regimen. Some studies suggest that for INRs between 1.7 and 3.3, warfarin doses may not need to be changed 2, but given the patient's current INR of 1.8, a dose adjustment is warranted to achieve therapeutic anticoagulation. Key considerations in managing warfarin therapy include:

  • Regular INR monitoring to adjust the dose as needed
  • Patient education on the importance of consistent vitamin K intake and adherence to the prescribed regimen
  • Awareness of potential drug interactions that may affect warfarin's efficacy.

From the FDA Drug Label

The dosage and administration of warfarin sodium tablets must be individualized for each patient according to the particular patient’s PT/INR response to the drug. The dosage should be adjusted based upon the patient’s PT/INR. The dose of warfarin should be adjusted to maintain a target INR of 2.5 (INR range, 2.0 to 3.0) for all treatment durations.

The patient's current INR is 1.8, which is below the target range of 2.0 to 3.0. To adjust the dose, the patient's current warfarin dose of 4 mg for 5 days/week and 3 mg for 2 days/week should be increased.

  • The exact increase in dose cannot be determined from the provided information, but the dose should be adjusted to achieve a target INR of 2.5.
  • It is recommended to consult the prescribing doctor to determine the best course of action for adjusting the warfarin dose.
  • The patient should be closely monitored for any signs of bleeding or other adverse effects after the dose adjustment. 3

From the Research

Warfarin Dose Adjustment for Low INR

To adjust the warfarin dose for a patient with a low International Normalized Ratio (INR) of 1.8 while on warfarin 4 mg for 5 days/week and 3 mg for 2 days/week, consider the following:

  • The patient's current INR is below the therapeutic range, which may increase the risk of thromboembolism 4.
  • According to the study on management and dosing of warfarin therapy, adjusting a steady-state warfarin dose depends on the measured INR values and clinical factors, and most changes should alter the total weekly dose by 5% to 20% 5.
  • The dose does not need to be adjusted for a single INR that is slightly out of range, but since the patient's INR is 1.8, which is below the therapeutic range, a dose adjustment may be necessary.

Considerations for Dose Adjustment

When adjusting the warfarin dose, consider the following:

  • The patient's current dose is 4 mg for 5 days/week and 3 mg for 2 days/week, which may need to be increased to achieve a therapeutic INR.
  • The study on management and dosing of warfarin therapy suggests that the INR should be monitored frequently, especially after initiation of warfarin or dose adjustments 5.
  • Other factors that may affect the patient's INR, such as diet, medications, and kidney function, should also be considered 6.

Potential Dose Adjustment

Based on the study on management and dosing of warfarin therapy, a potential dose adjustment could be:

  • Increase the weekly dose by 5-20%, which would result in a new dose of 4.2-4.8 mg for 5 days/week and 3.2-3.6 mg for 2 days/week.
  • Monitor the patient's INR closely after the dose adjustment to ensure that it reaches the therapeutic range.

References

Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

Research

Epidemiology of subtherapeutic anticoagulation in the United States.

Circulation. Cardiovascular quality and outcomes, 2009

Research

Management and dosing of warfarin therapy.

The American journal of medicine, 2000

Research

Influence of kidney function on risk of supratherapeutic international normalized ratio-related hemorrhage in warfarin users: a prospective cohort study.

American journal of kidney diseases : the official journal of the National Kidney Foundation, 2015

Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.