Rx
Interactive Tool
Warfarin Dosing Calculator
Select new-start initiation or maintenance adjustment. The calculator applies the UW Madison algorithm exactly — enter the inputs, receive a specific dose recommendation with next-step instructions.
⚕ Clinical Disclaimer
This calculator implements the UW Madison Warfarin Dosing Protocol and is intended as a clinical decision support tool for trained providers. Always apply clinical judgment. Verify INR values; if elevated INR was obtained via POC device, repeat venipuncture is required to confirm before acting on INR ≥3.1.
Warfarin Dosing Calculator
UW Madison Protocol · New Start & Maintenance
New Start / Initiation
Maintenance Adjustment
Φ
Clinical Context
Warfarin in 2026 — When and Why We Still Use It
DOACs have largely replaced warfarin for most indications — AFib, DVT/PE, and VTE prophylaxis. However, warfarin remains the only oral anticoagulant for patients with mechanical heart valves (especially mitral position) and for those with moderate-to-severe mitral stenosis of rheumatic origin. It also remains a practical option in patients with significant renal impairment (CrCl <15–25) or in patients where cost or adherence to twice-daily dosing favors once-weekly dosing regimens.
Indications Where Warfarin Remains Preferred or Required
| Indication | INR Target | Notes |
|---|---|---|
| Mechanical aortic valve (low-risk) | 2.0 – 3.0 | DOACs contraindicated in mechanical valves |
| Mechanical mitral valve | 2.5 – 3.5 | Higher thrombogenic risk — use upper range |
| Mechanical valve + AFib or prior thromboembolism | 2.5 – 3.5 | Most conservative target |
| Moderate-severe mitral stenosis (rheumatic) | 2.0 – 3.0 | DOACs not indicated; warfarin only |
| AFib (DOAC not tolerated or unavailable) | 2.0 – 3.0 | Eliquis preferred when accessible |
| Antiphospholipid syndrome (high-risk triple positive) | 2.0 – 3.0 (or 2.5–3.5) | DOACs have shown inferior outcomes in triple-positive APS |
| Bioprosthetic valve (first 3–6 months) | 1.5 – 2.0 or 2.0–3.0 | Some guidelines allow DOAC after 3 months; practice-specific |
High-Sensitivity Patients — Use Reduced Starting Dose (2.5 mg)
Start at 2.5 mg (not 5 mg) in: age ≥75 · weight <50 kg · CrCl <30 · hepatic impairment · malnourished · congestive heart failure · Asian heritage · concurrent amiodarone or fluconazole · recent major surgery.
Key Drug Interactions — The Most Clinically Significant
Increase INR (↑ anticoagulation effect): Amiodarone · Fluconazole / azoles · Metronidazole · Trimethoprim-sulfamethoxazole · Ciprofloxacin · Clarithromycin · Omeprazole · Statins (moderate) · Aspirin / NSAIDs (bleeding risk)Decrease INR (↓ anticoagulation effect): Rifampin · Carbamazepine · Phenytoin · St. John's Wort · Vitamin K (diet — leafy greens, kale, spinach)
Rule of thumb: Any new antibiotic or antifungal in a warfarin patient = recheck INR in 3–5 days.
1
Initiation Protocol
New Start Dosing — Days 1 Through 6
Standard vs. High-Sensitivity Starting Dose
Standard patients: Start at 5 mg daily on Day 1.High-sensitivity patients (age ≥75, low weight, renal/hepatic impairment, HF, amiodarone): Start at 2.5 mg daily on Day 1.
Days 2–3: First INR Check After Initiation
| INR Value | Recommended Daily Dose |
|---|---|
| < 1.5 | 5 – 7.5 mg daily |
| 1.5 – 1.9 | 2.5 – 5 mg daily |
| 2.0 – 2.5 | 2.5 mg daily |
| > 2.5 | Hold and recheck INR the next day |
Days 4–6: Second INR Check (2–3 days after first check)
| INR Value | Recommended Daily Dose |
|---|---|
| < 1.5 | 7.5 – 10 mg daily |
| 1.5 – 1.9 | 5 – 10 mg daily |
| 2.0 – 3.0 | 2.5 – 5 mg daily |
| > 3.0 | Hold warfarin — recheck INR in 1–2 days |
Tablet Strengths — Standard Warfarin Colors
1
1 mg (pink/tan)
2
2 mg (lavender)
2.5
2.5 mg (green)
3
3 mg (blue)
4
4 mg (blue)
5
5 mg (peach/salmon)
6
6 mg (teal)
7.5
7.5 mg (yellow)
10
10 mg (white/denim)
2
Maintenance Protocol
INR-Guided Dose Adjustments
All maintenance adjustments are expressed as a change to the weekly dose, not the daily dose. Calculate the total weekly dose, apply the percentage adjustment, then redistribute evenly (or with one higher-dose day for odd amounts).
⚠ POC Device Verification Rule
If the INR result is above the therapeutic range and was obtained via a point-of-care fingerstick device, a repeat venipuncture is required to verify the INR before making any dose adjustment for INR ≥ 3.1. POC devices are less reliable at the high end of the range.
INR Range Reference — Goal 2.0–3.0
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Maintenance Dosing — INR Goal 1.5–2.0
| INR Range | Action | Weekly Dose Change |
|---|---|---|
| ≤ 1.2 | Increase dose | +10% weekly |
| 1.3 – 1.4 | Increase dose | +5% weekly |
| 1.5 – 2.0 | ✓ No change — therapeutic | — |
| 2.1 – 3.0 | Decrease dose | −5% weekly |
| 3.1 – 4.0 * | Consider half dose today | −10% weekly |
| 4.1 – 5.0 * | Hold 1 dose | −10–20% weekly |
| 5.1 – 9.0 * | Hold 2 doses — MD order required | −10–20% weekly |
| > 9.0 | ⚠ Contact MD urgently — patient evaluation required | |
Maintenance Dosing — INR Goal 2.0–3.0 (Most Common)
| INR Range | Action | Weekly Dose Change |
|---|---|---|
| < 1.5 | Extra dose today + increase | +10–20% weekly |
| 1.5 – 1.9 | Increase dose | +5–10% weekly |
| 2.0 – 3.0 | ✓ No change — therapeutic | — |
| 3.1 – 4.0 * | Decrease dose | −5–10% weekly |
| 4.1 – 5.0 * | Hold 1 dose | −10% weekly |
| 5.1 – 9.0 * | Hold 2 doses — MD order required | −10–20% weekly |
| > 9.0 | ⚠ Contact MD urgently — patient evaluation required | |
Maintenance Dosing — INR Goal 2.5–3.5
| INR Range | Action | Weekly Dose Change |
|---|---|---|
| < 1.9 | Extra dose today + increase | +10–20% weekly |
| 1.9 – 2.4 | Increase dose | +5–10% weekly |
| 2.5 – 3.5 | ✓ No change — therapeutic | — |
| 3.6 – 4.5 * | Decrease dose | −5–10% weekly |
| 4.6 – 5.0 * | Hold 1 dose | −10% weekly |
| 5.1 – 9.0 * | Hold 2 doses — MD order required | −10–20% weekly |
| > 9.0 | ⚠ Contact MD urgently — patient evaluation required | |
* If INR is above the specified range and was obtained via POC device, repeat venipuncture is required to verify before acting.
3
Monitoring Schedule
INR Check Frequency — Initiation & Maintenance
During Initiation — INR Check Frequency
| Frequency | Continue Until |
|---|---|
| Every 2–3 days | INR within therapeutic range on 2 consecutive checks |
| Then every week | INR within therapeutic range on 2 consecutive checks |
| Then every 2 weeks | INR within therapeutic range on 2 consecutive checks |
| Then every 4 weeks | Once dose is stable — check monthly ongoing |
During Maintenance — When to Recheck INR
| Recheck Timing | Trigger |
|---|---|
| After 3–5 days | New or stopped interacting medication · Significant dietary change · Change in activity level · Intercurrent illness |
| Every 1–2 weeks | If dose was adjusted by 5–10% at last visit |
| Every 4 weeks | Maintained on same stable dose for <6 months |
| Every 6–8 weeks | Maintained on same stable dose for ≥6 months |
Practice Tip — Warfarin Clinics & Time in Therapeutic Range (TTR)
The quality metric for warfarin management is Time in Therapeutic Range (TTR). A TTR >70% is associated with outcomes approaching DOAC efficacy. Frequent monitoring, patient education on consistent Vitamin K intake, and prompt response to out-of-range INRs are the primary levers. Consider referral to an anticoagulation clinic for complex patients.
4
Emergency Management
Supratherapeutic INR & Reversal
Managing Elevated INR — Urgency by Level
| INR | Bleeding Present? | Management |
|---|---|---|
| 3.1 – 5.0 | No | Lower dose or hold 1–2 doses per protocol; recheck per schedule; ensure POC confirmed with venipuncture |
| 5.0 – 9.0 | No | Hold 2 doses; Vitamin K 1–2.5 mg oral if rapid reversal needed; MD order required; recheck in 24–48 hrs |
| > 9.0 | No | Hold all doses; Vitamin K 2.5–5 mg oral; urgent MD evaluation; recheck in 24 hrs |
| Any elevated | Yes — minor | Hold dose; Vitamin K 1–2.5 mg oral or SQ; contact MD; recheck 24 hrs |
| Any elevated | Yes — serious/life-threatening | Emergency: Hold all warfarin · Vitamin K 10 mg IV slow infusion · 4-Factor PCC (Kcentra) or FFP · ED transfer immediately · Call 760-633-7686 |
🚨 Life-Threatening Bleeding on Warfarin — Reversal Agents
4-Factor PCC (Kcentra): Preferred — rapid, complete reversal within minutes. Dose based on INR and weight.Fresh Frozen Plasma (FFP): Alternative if PCC unavailable; volume and time limitations.
Vitamin K 10 mg IV: Give alongside PCC/FFP — sustains reversal for 12–24 hours (PCC alone wears off as clotting factors consumed).
Transfer patient to ED via campus security / golf cart unless critical (then EMS). Call Scripps ER triage: 760-633-7686. Sign clinic note immediately.