Initiating Warfarin (Coumadin) Therapy
Start warfarin at 5 mg daily (or 2-4 mg in elderly patients ≥75 years), avoid loading doses, and monitor INR 2-4 times weekly initially until stable therapeutic range is achieved. 1, 2
Starting Dose
- Standard dose: 5 mg daily for most patients 2
- Reduced dose: 2-4 mg daily for elderly patients (≥75 years require approximately 1 mg/d less than younger patients) 3, 2
- Avoid loading doses (such as the outdated 10 mg × 3 days regimen) as they cause excessive anticoagulation in one-third of patients 4
- The INR will not rise appreciably in the first 24 hours with a 5 mg dose, except in rare patients who ultimately require very small maintenance doses (0.5-2.0 mg) 2
INR Monitoring Schedule
Initial phase (first 2-4 weeks):
- Monitor INR 2-4 times per week immediately after warfarin initiation 2
- First INR check typically on day 3-4, then adjust based on response
- One alternative protocol uses weekly INR checks (days 8,15,22) for atrial fibrillation patients starting at 3 mg daily, though this is less intensive 5
Stabilization phase:
- Once INR is stable in therapeutic range, gradually lengthen intervals between tests
- Maximum interval: 4-6 weeks for patients with consistently stable INR values 2
Bridging with Heparin
When to bridge:
- Start warfarin concurrently with heparin or LMWH for acute venous thromboembolism 6
- Continue heparin/LMWH for at least 5 days AND until therapeutic INR (≥2.0) is achieved 1, 6
- It is unnecessary to continue heparin beyond 5 days if patient is adequately anticoagulated with warfarin 4
High-risk scenarios requiring bridge therapy:
- Patients at high risk of thromboembolism (mechanical heart valves, recent VTE) should receive therapeutic-dose LMWH (100 U/kg SC q12h) or IV heparin when warfarin is interrupted perioperatively 3
- For cancer patients with VTE, consider starting with 3-6 months of LMWH due to survival advantage 6
Target INR Ranges
- Most indications (VTE, atrial fibrillation, bileaflet mechanical aortic valve): INR 2.5 (range 2.0-3.0) 1
- Higher-risk mechanical valves or recurrent thromboembolism: INR 3.0 (range 2.5-3.5) 1
- INR >4.0 provides no additional benefit and significantly increases bleeding risk 1
Dose Adjustments for Enzyme-Inducing Drugs
For CYP450-inducing anticonvulsants (carbamazepine, phenytoin, phenobarbital, oxcarbazepine):
- Carbamazepine: Expect significant interaction requiring frequent INR monitoring and warfarin dose escalation (mean weekly dose increased from 33 mg to 37 mg in studies) 7, 8
- Oxcarbazepine: Requires closer INR monitoring though interaction may be less pronounced than carbamazepine 8, 7
- Phenytoin/Phenobarbital: Require closer INR monitoring; full enzyme induction occurs over 2-4 weeks and persists 2-4 weeks after discontinuation 8
Practical approach:
- When initiating enzyme-inducing anticonvulsants, increase warfarin monitoring frequency to weekly initially
- Anticipate need to increase warfarin dose by approximately 50% for strong inducers like carbamazepine 8
- The dose/INR ratio will increase significantly (from ~13 mg/INR to ~18 mg/INR) 7
Dose Adjustment Principles
- Single slightly out-of-range INR: No dose change needed; recheck INR within 1-2 weeks (watchful waiting) 3, 9
- Stable dosing adjustments: Alter total weekly dose by 5-20% when changes are needed 2
- INR 1.5-1.99 or 3.01-3.49 without bleeding: Watchful waiting is as effective as dose changes 9
Critical Pitfalls to Avoid
- Do not use loading doses - they cause excessive anticoagulation without faster therapeutic benefit
- Do not stop heparin prematurely - continue until INR ≥2.0 for at least 24 hours AND minimum 5 days of overlap
- Patients <60 kg are at higher risk for both low TTR with warfarin and poor adherence, requiring more intensive monitoring 10
- Elderly patients bleed more even at therapeutic INR levels; start with lower doses 3
- Multiple bleeding risk factors are additive - age ≥65, prior stroke/GI bleed, renal insufficiency, anemia, concurrent antiplatelet agents 3