Warfarin Dose Adjustment for INR 1.0
Increase the warfarin dose by 15% per week (approximately 2.3 mg daily, rounded to 2.5 mg daily) and recheck INR within 1 week. 1
Dose Adjustment Algorithm
The 2018 European Heart Rhythm Association provides a validated dosing algorithm specifically for this scenario:
- INR <1.5: Increase dose by 15% per week 1
- INR 1.6-1.9: Increase by 10% per week 1
- INR 2.0-2.9: No change 1
- INR 3.0-3.9: Decrease by 10% per week 1
Calculation for This Patient
Current dose: 2 mg daily = 14 mg/week 1
15% increase: 14 mg × 1.15 = 16.1 mg/week 1
This translates to approximately 2.3 mg daily, which should be rounded to a practical dose of 2.5 mg daily (17.5 mg/week) given available tablet strengths. 1
Monitoring Strategy
- Recheck INR within 1 week after dose adjustment, as the patient is still in the initiation phase and far from therapeutic range 1
- During warfarin initiation, INR should be monitored at least weekly until stable therapeutic levels are achieved 1
- Once stable in therapeutic range (2.0-3.0), INR monitoring can be extended to monthly 1
Important Considerations
Why Not Bridge with Heparin?
Bridging is not routinely recommended for a single subtherapeutic INR in stable patients. 1 The CHEST guidelines found no significant difference in thromboembolic events between bridged and non-bridged patients with subtherapeutic INRs (0.3% event rate, 95% CI 0%-1.9%). 1 However, this patient is only 1 week into therapy and never achieved therapeutic anticoagulation, so clinical judgment regarding stroke risk (CHADS₂-VASc score) should guide whether bridging is needed. 1
Target INR Range
The therapeutic INR target for atrial fibrillation is 2.0-3.0, with an optimal target of 2.5 to maximize time in therapeutic range. 2 This range provides maximum protection against ischemic stroke while minimizing bleeding risk. 2 Lower INR targets (1.6-2.6) provide only approximately 80% of the protection and are not recommended. 2
Common Pitfalls to Avoid
- Avoid excessive dose increases: While the patient is significantly subtherapeutic, increasing by more than 15-20% weekly can lead to overshooting the target and subsequent INR instability 1, 3
- Don't use loading doses: Loading doses (10 mg) can cause excessive INR elevation and are not recommended, especially in elderly or frail patients 3
- Consider patient factors: Age, frailty, renal insufficiency, and concurrent medications may warrant more conservative dosing adjustments 1
- Ensure medication adherence: Verify the patient has been taking the medication as prescribed before making dose adjustments 1
Time to Therapeutic Effect
With a 5 mg initial dose, steady-state anticoagulation typically takes 5-7 days. 4 Since this patient started at only 2 mg, achieving therapeutic INR will take longer, making weekly monitoring essential until the target range is reached. 1