Warfarin Dose Adjustment Using INR

Dose adjustment keeps INR in range with the smallest change in weekly dose. The adjustment depends on the INR value, the target, the cause of any deviation, and the bleeding and clot risk. Change the weekly dose, not only the daily dose, and avoid frequent large changes.

First check the cause. Missed or extra doses, new or stopped drugs, illness (vomiting, diarrhoea, fever, heart failure), diet change, alcohol and recent changes in thyroid status.

For a target INR of 2.0 to 3.0 (2.5), with no bleeding:

For a target of 2.5 to 3.5, raise each band by 0.5.

Bleeding with a raised INR. Stop warfarin and assess urgently. For serious or major bleeding give phytomenadione (vitamin K) 5 to 10 mg by slow intravenous injection with prothrombin complex concentrate 25 to 50 units/kg by INR, or fresh frozen plasma 15 mL/kg when concentrate is unavailable.

Rounding. Adjust the weekly total with the tablet strengths available (1 mg, 3 mg and 5 mg), and write the daily schedule so that the patient does not split tablets unevenly.

Temporary causes. Do not change the weekly dose for a single out-of-range result with a clear short-term cause; recheck in 3 to 7 days.

Interacting drug started. Reduce the weekly dose by 25 to 30% for amiodarone and 10 to 25% for a short antibiotic course, with INR in 3 to 5 days.

Frequency of testing. After a change, check in 1 week (INR out of range by 0.5 or less) or 2 to 3 days (large deviations), and extend to 4 weeks when stable for 3 visits.

Special groups. Older adults and low-weight patients need smaller changes (5%). Women starting contraception or becoming pregnant need review. In East Africa, vitamin K may be the only reversal agent and should be stocked in anticoagulation clinics.

Monitoring. Record the INR, weekly dose, adherence, interacting drugs and bleeding symptoms at every visit. Report time in therapeutic range quarterly.

Referral and escalation. Refer patients with time in therapeutic range under 60% despite adherence, recurrent INR over 5.0, or unexplained resistance, and consider a direct oral anticoagulant where suitable.

Stopping. Stop only for a clear reason, with a plan for the next anticoagulant or the end of treatment.