An 81‑year‑old patient on warfarin (Coumadin) who was on 5 mg, held after an international normalized ratio of 3.5, received 2 mg resulting in an INR of 4.0, and now has an INR of 2.9—what warfarin dose should be given today?

Warfarin Dosing Recommendation for INR 2.9 After Recent Supratherapeutic Values

Resume warfarin today at 2.5 mg daily (approximately 50% reduction from the original 5 mg dose), then recheck INR in 3–5 days to confirm stability within the therapeutic range of 2.0–3.0.

Rationale for This Specific Dose

Your 81-year-old patient demonstrates a clear pattern of warfarin sensitivity:

• Initial dose of 5 mg → INR 3.5 (supratherapeutic)
• Reduced to 2 mg → INR 4.0 (further elevation despite dose reduction)
• After 2 days held → INR 2.9 (now therapeutic)

The American Geriatrics Society recommends reducing the weekly warfarin dose by 20–30% when resuming therapy after a supratherapeutic INR to prevent recurrence 1. In elderly patients (>65 years), warfarin sensitivity is heightened and lower maintenance doses (<5 mg daily) are typically required 1, 2.

The 2.5 mg dose represents a 50% reduction from the original 5 mg, which is appropriate given that even the 2 mg dose (60% reduction) still produced an INR of 4.0 3. This patient's response pattern indicates she requires a maintenance dose in the 2–3 mg daily range.

Why Not Hold Another Day?

• INR 2.9 is within the therapeutic range (2.0–3.0) for most indications including atrial fibrillation 1, 4
• Holding warfarin at INR 2.9 risks subtherapeutic anticoagulation and increased thrombotic risk, particularly dangerous in elderly patients 5
• The American College of Cardiology specifically states that warfarin withholding is appropriate only when INR is modestly elevated (>5.0) in the absence of bleeding 1
• Bleeding risk does not become clinically significant until INR exceeds 5.0, and the risk increases exponentially above that threshold 1, 4

Why Not Give the Full 5 mg?

Returning to 5 mg would likely reproduce the supratherapeutic INR of 3.5 seen initially 3. The patient has demonstrated clear dose-response sensitivity, requiring a substantially lower maintenance dose 1, 2.

Critical Monitoring Plan

• Recheck INR in 3–5 days after resuming at 2.5 mg 5
• If INR remains 2.0–3.0, continue 2.5 mg daily and recheck weekly for 2–3 weeks 1
• If INR <2.0, increase to 3 mg daily; if INR >3.5, hold one dose and restart at 2 mg daily 3, 5
• Once stable for 1 month, extend monitoring to every 1–2 months 1

Factors to Investigate Before Next Dose

Identify what caused the initial INR elevation to prevent recurrence 3, 5:

• New medications or antibiotics (most common culprit in elderly patients) 1, 3
• Dietary changes in vitamin K intake (decreased green vegetables, nutritional supplements) 1, 3
• Intercurrent illness (fever, diarrhea, decreased oral intake affecting absorption) 1, 5
• Changes in liver or renal function (elderly patients are particularly susceptible) 1
• Medication adherence issues (confusion about dosing, duplicate doses) 3

Common Pitfalls to Avoid

• Do not administer vitamin K at INR 2.9 — vitamin K is reserved for INR >5.0 without bleeding or any INR with active bleeding 3, 6. Giving vitamin K at this level will induce warfarin resistance and make re-anticoagulation extremely difficult 3.

• Do not make excessive dose reductions (>30% weekly change) as this can lead to subtherapeutic anticoagulation 5. However, in this elderly patient with demonstrated sensitivity, the 50% reduction is justified by the clinical response pattern.

• Do not withhold warfarin for an INR within therapeutic range (2.0–3.0), as this increases thrombotic risk without reducing bleeding risk 6, 5.

Special Considerations for This 81-Year-Old Patient

Advanced age (>65–75 years) is a critical factor that amplifies bleeding risk at any given INR level 3. However, age alone does not require dose reduction when INR is therapeutic 6. The American Geriatrics Society emphasizes that elderly patients typically require lower maintenance doses (often 2–4 mg daily rather than 5 mg) due to altered pharmacokinetics and reduced protein binding 1, 2.

The patient's exaggerated response to both 5 mg and 2 mg doses confirms she is a "warfarin-sensitive" individual, likely requiring a final maintenance dose of 2–3 mg daily 7, 2. This sensitivity pattern is common in elderly patients and necessitates cautious dose titration with frequent INR monitoring 1, 8.