How to adjust warfarin dose with current INR of 1.6 and target INR of 3?

Warfarin Dose Adjustment for INR 1.6 with Target INR 3.0

Increase the weekly warfarin dose by 10-20% and recheck INR in 3-5 days, as the current INR of 1.6 remains substantially below the target of 3.0 despite 10 days of therapy. 1

Rationale for Dose Increase

• Your patient received a total of 22 mg over 5 days (average 4.4 mg/day), yet achieved an INR of only 1.6 when targeting 3.0, indicating significant under-dosing 1
• The INR should have risen more substantially after this loading period, suggesting the patient requires a higher maintenance dose 2
• Most dose adjustments should alter the total weekly dose by 5-20%, and given the substantial gap between current (1.6) and target (3.0) INR, a 15-20% increase is appropriate 2

Specific Dosing Algorithm

Recommended approach:

• Calculate current weekly dose: 22 mg over 5 days = approximately 30.8 mg/week if continued at 4 mg daily
• Increase by 15-20%: New weekly dose should be 35-37 mg/week
• Practical dosing: Give 5 mg daily for the next 5-7 days 1, 2
• Recheck INR in 3-5 days (not 1-2 weeks, given the significant underdosing) 1

Monitoring Strategy

• The INR should be monitored 2-4 times per week immediately after dose adjustments until therapeutic range is achieved 2
• Once stable in therapeutic range (2.5-3.5 for target INR 3.0), gradually lengthen intervals up to maximum 4-6 weeks 2
• For a target INR of 3.0, the acceptable therapeutic range is typically 2.5-3.5 1, 3

Critical Context: Why Target INR 3.0?

• A target INR of 3.0 (range 2.5-3.5) is typically reserved for mechanical heart valves (particularly caged ball/disc valves or mitral position) or recurrent thromboembolism despite standard anticoagulation 3
• Standard VTE treatment targets INR 2.5 (range 2.0-3.0), not 3.0 1, 4
• Verify the indication for this higher target, as most conditions require only INR 2.0-3.0 4, 3

Common Pitfalls to Avoid

• Do not make small incremental changes (e.g., 0.5 mg increases) when the INR is this far from target—this prolongs time to therapeutic anticoagulation and increases thrombotic risk 2
• Do not use loading doses or large boluses at this stage, as steady-state dosing is more predictable 2
• Do not wait 1-2 weeks to recheck when significantly subtherapeutic—this is only appropriate for stable patients with minor INR fluctuations 1
• Avoid the temptation to give vitamin K or hold doses when INR is subtherapeutic; bridging with heparin is not routinely needed for a single low INR 1

Alternative Consideration

If the patient has risk factors for warfarin resistance (malabsorption, high vitamin K intake, certain medications, genetic polymorphisms), consider:

• Reviewing medication list for drug interactions
• Assessing dietary vitamin K intake
• Ensuring medication adherence 2
• If INR remains refractory after appropriate dose escalation, consider pharmacogenetic testing or alternative anticoagulation strategies