Does Coumadin (warfarin) come in a 1.25 mg tablet, and for an 81‑year‑old man with prior cerebrovascular accident (CVA), atrial fibrillation (AF), and remote deep‑vein thrombosis (DVT) who is taking 1 mg daily with an INR that fell from 2.9 to 1.9, how should the dose be adjusted?

Coumadin Tablet Strengths and Dose Adjustment for Subtherapeutic INR

Available Coumadin Tablet Strengths

Coumadin does NOT come in a 1.25 mg tablet. Available strengths are 1 mg, 2 mg, 2.5 mg, 3 mg, 4 mg, 5 mg, 6 mg, 7.5 mg, and 10 mg 1. For this 81-year-old patient currently taking 1 mg daily with an INR that dropped from 2.9 to 1.9, precise dose adjustment requires using the existing tablet strengths or splitting tablets.

Immediate Dose Adjustment Strategy

Increase the weekly warfarin dose by 10-20% (approximately 1-2 mg increase per week) and recheck INR within 3-5 days 2. For a patient taking 1 mg daily (7 mg weekly), a 10-20% increase translates to 0.7-1.4 mg additional per week 2.

Practical Dosing Options:

• Option 1: Increase to 1.5 mg daily (using 1 mg + half of 1 mg tablet) = 10.5 mg weekly (50% increase)
• Option 2: Alternate 1 mg and 2 mg daily = 10.5 mg weekly
• Option 3: Give 2 mg on 2 days per week, 1 mg on 5 days = 9 mg weekly (more conservative 28% increase)

The most practical approach is alternating 1 mg and 2 mg tablets daily, achieving a weekly dose of 10.5 mg 2, 1.

Critical Investigation Required Before Dose Adjustment

Medication Changes

• New enzyme-inducing drugs (rifampin, carbamazepine, phenytoin, barbiturates) can lower INR by increasing warfarin metabolism 2
• Antibiotics are the most common cause of unexpected INR fluctuations in elderly patients 3

Dietary Factors

• Increased vitamin K intake from leafy green vegetables or nutritional supplements can lower INR 2, 3
• Poor oral intake or reduced caloric consumption affects warfarin absorption 3

Patient Adherence

• Missed doses or medication non-adherence is common in elderly patients and must be assessed 2

Clinical Status

• Malabsorption issues or gastrointestinal changes affect warfarin metabolism 2
• Recent illness, fever, or diarrhea can alter INR 3
• Changes in liver or renal function modify warfarin clearance 3, 2

Monitoring Strategy for This High-Risk Patient

Recheck INR within 3-5 days after dose adjustment because the full anticoagulant effect takes several days to manifest 2. This patient has multiple high-risk features requiring closer monitoring:

• Age 81 years: Elderly patients (≥75 years) have increased bleeding risk even at therapeutic INR levels 2
• Prior CVA: History of stroke increases both thrombotic and bleeding risk 3
• Atrial fibrillation with CHA₂DS₂-VASc ≥2: Requires therapeutic INR 2.0-3.0 for stroke prevention 3
• Remote DVT: Adds to thrombotic risk profile 3

Monitoring Schedule:

• Days 3-5: Recheck INR after dose increase 2
• If INR remains subtherapeutic: Increase dose by another 10-20% and recheck in 3-5 days 2
• Once therapeutic (2.0-3.0): Check INR 2-3 times weekly for 1-2 weeks 3
• After stability for 1 month: Extend to monthly monitoring 3

Special Considerations for Elderly Patients

Elderly patients (≥75 years) exhibit greater-than-expected PT/INR response to warfarin and typically require lower maintenance doses (approximately 1 mg/day less than younger patients) 2. This patient's current 1 mg daily dose is already at the very low end, suggesting marked warfarin sensitivity 2.

More frequent monitoring is essential because elderly patients experience more INR fluctuations, particularly during illness or medication changes 3, 2.

Bridging Anticoagulation Decision

Bridging with LMWH or unfractionated heparin is NOT routinely recommended for subtherapeutic INR in stable outpatients without acute thrombotic events 2.

Consider bridging ONLY if:

• Mechanical heart valve (this patient does not have one) 2
• Recent VTE within 3 months (patient's DVT is remote) 2
• Acute arterial thromboembolism (not present) 2
• INR expected to remain subtherapeutic >48 hours with very high thrombotic risk 2

For this patient with remote DVT, prior CVA, and atrial fibrillation, bridging is NOT indicated because the thrombotic risk during a few days of subtherapeutic INR does not outweigh the bleeding risk of dual anticoagulation 2.

Common Pitfalls to Avoid

• Do not use loading doses: They increase bleeding risk without achieving therapeutic INR faster 4, 1
• Do not make large dose adjustments: Changes should alter weekly dose by 5-20% 1
• Do not assume tablet splitting creates 1.25 mg: Use available strengths (1 mg, 2 mg, 2.5 mg) 1
• Do not delay monitoring: Elderly patients require more frequent INR checks 3, 2
• Do not ignore medication reconciliation: New drugs are the most common cause of INR changes 3, 2