How should a patient's warfarin dose be adjusted with an INR of 1.5 and a target INR of 2-3?

Warfarin Dose Adjustment for Subtherapeutic INR

For a patient with an INR of 1.5 (target 2-3) who takes 1 mg daily except for 0.5 mg on Tuesday and Wednesday, increase the warfarin dose by adding 0.5 mg to the Tuesday and Wednesday doses (making them 1 mg daily for all days of the week). 1

Assessment of Current Anticoagulation Status

• The patient's INR of 1.5 is below the therapeutic range of 2.0-3.0, indicating subtherapeutic anticoagulation 1
• The current dosing regimen (1 mg daily except Tuesday and Wednesday at 0.5 mg) results in a weekly dose of 6 mg 1
• This subtherapeutic INR increases the risk of thromboembolism while the patient remains inadequately anticoagulated 1

Recommended Dose Adjustment

• For patients with previously stable therapeutic INRs who present with a single out-of-range INR of 0.5 below therapeutic range, guidelines suggest continuing the current dose and retesting in 1-2 weeks 1
• However, this patient's INR is more than 0.5 below the lower limit of the therapeutic range (2.0), warranting a dose adjustment 1, 2
• The simplest approach is to standardize the dose to 1 mg daily for all days of the week, effectively increasing the weekly dose by 1 mg (from 6 mg to 7 mg weekly) 2

Monitoring Recommendations

• After dose adjustment, the INR should be rechecked within 1 week to assess the response to the dose change 1
• Once the INR reaches the therapeutic range (2.0-3.0), monitoring frequency can be gradually decreased 1
• For stable patients, INR testing can eventually be extended to every 4 weeks, and potentially up to 12 weeks if consistently stable 1

Important Considerations

• Avoid large dose increases as they may lead to excessive anticoagulation and increased bleeding risk 3
• Small incremental changes (5-20% of weekly dose) are preferred for dose adjustments 3
• The current adjustment (adding 1 mg weekly) represents approximately a 17% increase in the weekly dose, which is within the recommended range 3
• Elderly patients may require lower warfarin doses due to increased sensitivity to anticoagulant effects 1, 2

Factors That May Affect Warfarin Response

• Medication interactions can significantly alter warfarin metabolism and effect 1
• Dietary changes, especially those affecting vitamin K intake, can influence INR values 1
• Genetic variations in CYP2C9 and VKORC1 enzymes may affect warfarin sensitivity 2
• Concurrent medical conditions, particularly liver disease, can potentiate warfarin response 2

Follow-up Plan

• Recheck INR in 7 days after implementing the dose change 1
• If the INR remains subtherapeutic, consider an additional small dose increase 2
• Once the INR is within therapeutic range, monitor weekly for 1 month, then extend to every 2-4 weeks if stable 1
• Educate the patient about the importance of medication adherence, consistent vitamin K intake, and reporting any new medications or supplements 1