Warfarin Dose Adjustment for INR 1.8
For a patient taking 6mg three times weekly (18mg total weekly) with an INR of 1.8, increase the total weekly dose by 10-15%, which translates to approximately 20-21mg per week, distributed as 7mg three times weekly or 6mg four times weekly. 1
Rationale for Dose Adjustment
The current INR of 1.8 falls just below the therapeutic range of 2.0-3.0 for most indications. 1, 2 This represents a minor deviation (≤0.5 below target), which requires a measured response rather than aggressive intervention.
Specific Dosing Algorithm
Based on established guidelines for subtherapeutic INR management:
- INR 1.6-1.9: Increase weekly dose by 10% 1
- Current weekly dose: 18mg (6mg × 3 days)
- 10% increase: 18mg + 1.8mg = 19.8mg weekly (round to 20mg)
- Practical distribution options:
- Option 1: 7mg on three days per week (21mg total)
- Option 2: 6mg on four days per week (24mg total) - if more aggressive approach needed
- Option 3: Alternate 6mg and 7mg doses to achieve ~20mg weekly
Follow-Up Monitoring
- Recheck INR in 1-2 weeks after dose adjustment 3, 1
- For a single subtherapeutic INR with previously stable values, some guidelines suggest continuing the current dose and retesting, but given the INR is below 2.0, a modest increase is warranted 3
- Once stable in therapeutic range, monitoring intervals can extend to 4-6 weeks 4
Important Considerations
No Bridging Therapy Required
Heparin bridging is NOT recommended for a single subtherapeutic INR, as it increases bleeding risk without clear benefit. 3 Even in high-risk patients with mechanical heart valves, thromboembolic event rates remain very low (0.4%) without bridging. 3
Patient-Specific Factors to Assess
Before implementing dose adjustment, verify:
- Medication adherence: Confirm patient is actually taking prescribed doses 3
- Dietary changes: Recent increases in vitamin K intake (leafy greens) can lower INR 4
- New medications: Drug interactions are common with warfarin 4
- Gastrointestinal issues: Diarrhea or decreased oral intake can affect INR 5
- Intercurrent illness: Acute medical conditions may alter warfarin metabolism 6
Common Pitfalls to Avoid
- Avoid overreacting to a single slightly low INR with excessive dose increases 3, 1
- Do not use loading doses or make changes >20% of weekly dose for minor deviations 4
- Avoid unnecessary bridging therapy which increases bleeding risk 3
- Do not adjust dose if this is the first subtherapeutic reading and patient has been previously stable - consider retesting first 3
Special Populations
If this patient has a mechanical heart valve (particularly mitral position or older valve types), target INR may be 2.5-3.5 rather than 2.0-3.0, making this INR of 1.8 more significantly subtherapeutic and potentially warranting a 15% dose increase instead of 10%. 2, 7