Warfarin Dose Adjustment for Subtherapeutic INR
For a 64-year-old woman on warfarin 4 mg daily with a target INR of 2.5–3.5 and a current INR of 1.7, increase the weekly warfarin dose by approximately 10–20% and recheck the INR within 3–7 days.
Immediate Dose Adjustment
Increase the weekly warfarin dose by 10–20% because the current INR of 1.7 is significantly below the target range of 2.5–3.5, indicating inadequate anticoagulation. 1
For a patient taking 4 mg daily (28 mg weekly), a 10–20% increase translates to adding 2.8–5.6 mg to the weekly total, which means increasing to approximately 30–33 mg per week (4.3–4.7 mg daily average). 1
The specific indication requiring a target INR of 2.5–3.5 (rather than the standard 2.0–3.0) suggests either a mechanical prosthetic heart valve or acute myocardial infarction with mural thrombus, both of which carry high thromboembolic risk when anticoagulation is subtherapeutic. 2
Monitoring Schedule
Recheck the INR within 3–7 days after the dose increase to assess response, as this interval allows sufficient time for warfarin's pharmacodynamic effect to manifest while preventing prolonged subtherapeutic anticoagulation. 1, 3
Continue weekly INR monitoring until the INR stabilizes within the target range of 2.5–3.5 for at least two consecutive measurements. 1, 3
Once stability is achieved for 2–3 weeks, reduce monitoring frequency to 2–3 times per week for 1–2 weeks, then to weekly checks for the first month. 1
After one month of consistent therapeutic INRs, extend monitoring intervals to every 1–2 months, with a maximum interval of 4–6 weeks. 1, 3
Investigation of Contributing Factors
Before implementing the dose increase, identify and address potential causes of the subtherapeutic INR:
Review all medications for new additions or discontinuations, particularly enzyme-inducing drugs (e.g., carbamazepine, rifampin, phenytoin) that accelerate warfarin metabolism and lower INR. 4
Assess dietary changes, specifically increased intake of vitamin K–rich foods (green leafy vegetables) or vitamin K supplements, which antagonize warfarin's effect. 1, 3
Evaluate medication adherence, as missed doses are a common cause of subtherapeutic INR. 1
Check for gastrointestinal issues such as diarrhea or malabsorption that could reduce warfarin absorption. 1
Consider intercurrent illness that may affect warfarin metabolism or absorption. 1
Thromboembolic Risk Considerations
The risk of thromboembolism is greatest when INR is below 2.0, making prompt correction essential in this patient with a high-risk indication requiring INR 2.5–3.5. 5
For patients with mechanical prosthetic heart valves (a common indication for INR 2.5–3.5), subtherapeutic anticoagulation carries substantial risk of valve thrombosis and systemic embolization. 2
For patients with acute myocardial infarction and mural thrombus (another indication for INR 2.5–3.5), inadequate anticoagulation increases the risk of systemic embolization. 2
Common Pitfalls to Avoid
Do not make excessive dose adjustments based on a single subtherapeutic INR; a 10–20% weekly dose increase is appropriate and avoids overshooting the target range. 1, 3
Do not delay dose adjustment in patients with high-risk indications requiring INR 2.5–3.5, as prolonged subtherapeutic anticoagulation increases thromboembolic risk. 2
Do not administer loading doses or make daily dose changes exceeding 20% of the weekly total, as this can cause excessive INR fluctuation. 3
Do not extend monitoring intervals until the INR has been stable within the target range for at least one month. 1, 3
Age-Related Considerations
Although this patient is 64 years old, elderly patients (generally >65 years) typically require lower maintenance doses (2–4 mg daily rather than 5 mg) due to increased warfarin sensitivity. 2, 1
The current dose of 4 mg daily is already at the lower end of typical maintenance dosing, suggesting this patient may have moderate warfarin sensitivity. 2, 1
Monitor closely for supratherapeutic INR after dose increases in elderly patients, as they demonstrate exaggerated INR responses to dose changes. 1